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启动Java应用

命令行的语法格式

java [options] classname [args]

java [options] -jar filename [args]

[options]:多个选项之间用空格分隔,查看 Options 章节。
[classname]:被运行类的名称。
filename:要调用的jar文件。仅与-jar选项一起使用。
[args]:传给public static void main(String[] args)方法的参数。

命令描述

命令java用于启动Java应用。通过启动JRE,加载指定类,调用类的main方法来实现这些。main方法必须是public和static的,不能有返回值,必须以String数组为参数,如下:

public static void main(String[] args) {}

命令java可以通过加载包含main方法或者继承javafx.application.Application的类来用来启动 JavaFX 应用。后一种情况中,launcher 会实例化Application类,调用init方法,然后调用start(javafx.stage.Stage)。

默认情况下,第一参数不是java命令的一个选项,是被调用class的全限定名。如果指定了 -jar 选项,他的参数是包含class和资源文件的jar文件名。启动类必须由其源代码中的manifest 的 Main-Class指出。

JRE搜索启动类(包括应用要使用的类)的路径为:引导类的路径, 已安装的扩展, 用户的类路径。

命令行选项

命令行选项可以分为以下几类:

  • Standard Options:保证所有的JVM实现都支持。它们用于常见操作,例如检查JRE的版本,设置类路径,启用详细输出等。
  • Non-Standard Options:是Java HotSpot虚拟机的通用选项,不能保证被所有JVM实现支持,并且可能会更改。这些选项以-X开头。
  • Advanced Runtime Options:不建议使用高级选项随便使用。这些是开发人员用于调整Java HotSpot虚拟机操作的特定区域的选项,这些操作通常具有特定的系统要求,可能需要对系统配置参数进行特权访问。它们也不能保证被所有JVM实现支持,并且可能会发生变化。高级选项以-XX开头。
  • Advanced JIT Compiler Options
  • Advanced Serviceability Options
  • Advanced Garbage Collection Options

布尔值用于启用关闭的选项或者关闭启用的选项,-XX选项使用加号(-XX:+OptionName)启用,使用减号(-XX:-OptionName)禁用。

对于需要参数的选项,参数可以通过空格、冒号(:)、等号(=)与选项名称分隔,或者参数可以直接跟随选项(每个选项的确切语法不同)。 如果希望指定大小(默认以字节为单位),则可以不使用后缀。或者使用后缀k或K表示千字节(KB),m或M表示兆字节(MB),g或G表示千兆字节(GB)。例如,要将大小设置为8 GB,您可以指定8g,8192m,8388608k或8589934592作为参数。如果需要指定百分比,请使用0到1之间的数字(例如,对于25%,使用0.25)。

命令行选项:标准选项(Standard Options)

所有的JVM实现都支持下面的常用选项:

-agentlib:libname[=options]
加载指定的本地代理库。在libname后,是以逗号分隔的特定库的选项列表。

比如:-agentlib:foo,那么JVM尝试在LD_LIBRARY_PATH(OS X中为DYLD_LIBRARY_PATH)描述的位置中加载libfoo.so。

下面的示例演示如何加载堆分析工具(HPROF)库,并每隔20秒获取简单的CPU信息,且栈深度为3:

-agentlib:hprof=cpu=samples,interval=20,depth=3

下面的例子演示了如何装载Java调试线协议(JDWP)库,并监听端口8000套接字连接,在主类加载前暂停JVM:

-agentlib:jdwp=transport=dt_socket,server=y,address=8000

有关「本地代理库」的详细信息,请参阅以下内容:
java.lang.instrument包描述:http://docs.oracle.com/javase/8/docs/api/java/lang/instrument/package-summary.html
JVM工具接口中的代理命令行选项http://docs.oracle.com/javase/8/docs/platform/jvmti/jvmti.html#starting

-agentpath:pathname[=options]

加载由绝对路径名指定的本地代理库。此选项相当于-agentlib使用库的完整路径和文件名。

-client

选择Java HotSpot Client VM。在Java SE开发工具包(JDK)的64位版本会忽略这个选项,而是使用 Server JVM。

对于默认的JVM选择,请参阅 Server-Class Machine Detection:


http://docs.oracle.com/javase/8/docs/technotes/guides/vm/server-class.html

-Dproperty=value
设置系统属性值。

-d32

运行在32位的环境中的应用。

如果一个32位环境未安装或不被支持,那么就会报一个错误。

默认情况下,应用程序运行在32位的环境,除非使用64位系统。

-d64

运行在64位的环境中的应用。

如果在64位环境中未安装或不被支持,那么就会报一个错误。

默认情况下,应用程序运行在32位的环境,除非使用64位系统。

目前仅在Java HotSpot的服务器虚拟机支持64位运算。

-server 隐含使用 -d64。

-client 会在使用 -d64是忽略。在将来的版本中将改变。

-disableassertions[:[packagename]…|:classname]
-da[:[packagename]…|:classname]

禁用断言。默认情况下,断言在所有的包和类中都是禁用的。

不带任何参数,-disableassertions(-da)禁用所有包和类的断言。

如果packagename以…结尾时,则禁用在指定的包和子包的任何断言。

如果参数为…,则禁用当前工作目录下未命名的包断言。如果指定类名,该禁用指定类断言。

在-disableassertions(-da)选项适用于所有的class loader和system class(不具有类加载器)。有一个例外:如果选项没有设置参数,那么它不适用于system class。这可以用来禁用除了系统类以外的所有类的断言。该-disablesystemassertions选项可以在所有的system class中禁用断言。

要明确启用特定package或class的断言,可使用-enableassertions(-ea)选项。这两种选项都可以在同时使用。例如,要运行MyClass应用,想启用com.wombat.fruitbat(以及任何子包)的断言,但禁用com.wombat.fruitbat.Brickbat的断言:

java -ea:com.wombat.fruitbat… -da:com.wombat.fruitbat.Brickbat MyClass

-disablesystemassertions
-dsa

禁用所有系统类中的断言。

-enableassertions[:[packagename]…|:classname]
-ea[:[packagename]…|:classname]

启用断言。默认情况下,断言是在所有的包和类禁用。

不带任何参数,-enableassertions(-ea)启用所有的包和类的断言。

如果 packagename 以…结尾,则能在指定的包和子包的使用断言。

如果参数为…,那么可以启用当前工作目录下未命名的包的断言。

以类名为参数,可以启用指定的类的断言。

在-enableassertions(-ea)选项适用于所有的class loader和system class(不具有类加载器)。

有一个例外:如果选项没有参数,那么它不适用于system class。这可以启用所有类的断言,除了system class。-enablesystemassertions选项,启用所有系统类的断言。

要明确禁用特定包或类断言,可使用-disableassertions(-da)选项。如果一个命令包含这些选项的多个实例,则它们在装载任何类之前按顺序处理。例如,要运行MyClass应用,要启动 com.wombat.fruitbat(和任何子包)的断言,但禁用类com.wombat.fruitbat.Brickbat的断言,则:

java -ea:com.wombat.fruitbat… -da:com.wombat.fruitbat.Brickbat MyClass

-enablesystemassertions
-esa

启用所有系统类的断言。

-help/-?

显示的使用信息java,而无需实际运行JVM命令。

-jar filename

执行封装在JAR文件中的程序。

filename是JAR文件名,JAR文件包含一个包含一个manifest文件,manifest 有一项为 Main-Class:classname,指定了定义了public static void main(String[] args) 方法的类,做为入口。

当使用-jar选项,指定的JAR文件是所有用户类的来源,和其他class path设置将被忽略。

有关JAR文件的详细信息,请参阅以下资源:

  • jar(1)

  • Lesson: Packaging Programs in JAR Files at

http://docs.oracle.com/javase/tutorial/deployment/jar/index.html

-javaagent:jarpath[=options]

加载指定的Java语言的代理。

有关更多信息,请参见java.lang.instrument在Java API文档中包描述:


http://docs.oracle.com/javase/8/docs/api/java/lang/instrument/package-summary.html

-jre-restrict-search

包括在版本搜索中用户专用的JRE。

-no-jre-restrict-search

不包括在版本搜索中用户专用的JRE。

-server

选择了Java HotSpot的 Server VM。JDK的64位版本仅支持 Server VM。

对于默认的JVM选择,请参阅服务器级计算机在检测:


http://docs.oracle.com/javase/8/docs/technotes/guides/vm/server-class.html

-showversion

显示版本信息,并继续该应用程序的执行。

-splash:imgname

以imgname指定的图片做为闪屏幕。例如启动应用时要显示images中的splash.gif:

-splash:images/splash.gif

-verbose:class

显示每个加载的类的信息。

-verbose:gc

显示每个GC事件信息。

-verbose:jni

显示对本地方法的使用和其他JNI活动的信息。

-version

显示版本信息然后退出。

-version:release

指定用于运行应用程序的发布版本。

如果指定的版本不存在,并且发现了一个适当的实现,则会使用这个适当的实现。

该release参数指定“任何确切的版本字符串”或“版本字符串并用空格分隔范围”的列表。

一个版本字符串格式如下:1.x.0_u(其中x是主版本号,并u是更新版本号)。

一个版本范围由一个”版本字符串”后跟”一个加号”(+),表示该版本或更高版本,或版本字符串后跟星号(*)指定匹配的版本字符串。版本字符串和范围之间,可以用一个 OR 或 & 或 AND 相结合,来表示两个范围。

如果运行class文件要求JRE 6u13(1.6.0_13),或任何从6u10中(1.6.0_10)起的JRE,指定以下内容:

-version:”1.6.0_13 1.6* & 1.6.0_10+”

如果有空格,则一定要使用引号;

对于JAR文件,则优先查看manifest文件中的版本要求。

命令行选项:非标准选项(Non-Standard Options)

下面的选项是Java HotSpot VM的通用选项。

-X

显示所有的可用的-X选项;

-Xbatch

禁用后台编译。默认情况下,在JVM将编译方法作为后台任务,直到背景编译完成,才在解释模式下运行方法。

-Xbatch禁用后台编译,所有方法的编译做为前台任务进行直到完成。

等同于 -XX:-BackgroundCompilation.

-Xbootclasspath:path

在指定的以:分割的文件夹、JAR文件、ZIP文档中,搜索 boot class 文件。用于替换包含在JDK中的 boot class 文件。

-Xbootclasspath/a:path

将指定的以:分割的文件夹、JAR文件、ZIP文档,附加到默认的bootstrap class 路径的后面。

-Xbootclasspath/p:path

将指定的以:分割的文件夹、JAR文件、ZIP文档,附加到默认的bootstrap class 路径的前面。

-Xcheck:jni

执行JNI函数时,执行附加检查。具体的说,在处理JNI请求之前,先验证传递给JNI的参数以及运行时环境数据。遇到的任何无效数据表明本机代码中的问题,在这种情况下,JVM将以一个不可恢复的错误终止。使用该选项是会使性能下降。

-Xcomp

在第一次调用方法强制编译。默认情况下,ClientVM(-client)通过执行1000次解释方法、ServerVM( -server)通过解释10000次方法调用收集的信息来决定有效率的编译。指定-Xcomp选项,禁用解释方法调用以效率为代价提高编译性能。

可以通过-XX:CompileThreshold,指定在编译之前,解释的方法调用的次数。

-Xdebug
Java8中该选项用于兼容以前版本。只是为了兼容。

-Xdiag

显示附加的诊断信息。

-Xfuture

启用严格的类文件格式检查,强制实施与规范的一致,类文件格式规范。

鼓励开发者在新的代码中使用该选项,严格的检查将成为未来版本中的默认值。

-Xint

以解释模式运行java 应用。此模式下不使用JIT。

-Xinternalversion

显示比 -version 命令更多的信息,然后退出。

-Xloggc:filename

将GC事件日志输出到filename指定的文件中。

日志格式与-verbose:gc的输出相同,但附加过去了的时间,时间是从第一个GC事件前每一记录的事件过去的时间。

如果同时使用,-Xloggc选项会覆盖-verbose:gc。

Example:

-Xloggc:garbage-collection.log

-Xmaxjitcodesize=size

指定JIT编译的代码的最大码缓存大小(以字节为单位)。

附加字母k或K表示千字节,m或M指示兆字节,g或G表示GB。

默认的最大缓存大小为 240MB; 如果禁用分层编译的选项-XX:-TieredCompilation,则默认大小为48 MB:

-Xmaxjitcodesize = 240m

此选项等同于-XX:ReservedCodeCacheSize。

-Xmixed

用解释器执行所有的字节码,而 hot mothod 编译成 native code。

-Xmnsize

设置 young generation 的初始和最大的堆大小,字节为单位。附加字母k或K表示千字节,m或M指示兆字节,g或G表示GB。

堆的 young generation 用于产生新对象。在该区域发生的GC往往比其他区域频繁。

该区域如果过大,将导致 full gc,会花费很长时间。

如果太小,会导致频繁的小型GC。

Oracle推荐的大小为:这个堆大小的1/2 ~ 1/4。

下面的例子说明如何设置young generation初始和最大值为256 MB:

-Xmn256m

-Xmn262144k

-Xmn268435456

可以使用-XX:NewSize设置初始大小和-XX:MaxNewSize设置的最大尺寸。

-Xmssize
设置堆的初始大小,以byte为单位。参数size必须是1024的倍数且大于1MB。参数size附加字母k或K表示KB;m或M指示MB;g或G表示GB。

下面的例子说明了如何分配堆大小为6MB:

-Xms6291456


-Xms6144k


-Xms6m

如果不设置这个选项,那么初始大小将被设置为老年代和新生代的大小之和。新生代堆的初始大小可以通过-Xmn-XX:NewSize选项来设置。

-Xmxsize
指定”内存分配池”的最大大小(字节为单位)。参数size必须是1024的倍数且大于2MB。参数size附加字母k或K表示千字节,m或M指示兆字节,g或G表示GB。

默认值在运行时决定,取决系统配置。对于服务器部署,通常将-Xms并且-Xmx设置为相同的值。参见《Java SE HotSpot虚拟机垃圾收集调优指南》

下面的例子说明如何分配内存的最大允许大小为80 MB:

-Xmx83886080


-Xmx81920k


-Xmx80m

该-Xmx选项等效于-XX:MaxHeapSize。

-Xnoclassgc

禁用class的GC。减少GC时间,减少停顿。

当在启动时指定-Xnoclassgc,应用程序的class objects将保留,将总是被视作存活。

这会导致更多的内存被永久占用,如果不小心使用,会抛出
内存溢出异常。

-Xprof

剖析正在运行的程序并将分析数据发送到标准输出。

程序开发使用,不建议生产使用。

-Xrs

减少由JVM使用的系统信号。

Shutdown hook通过运行用户清理代码,用于有序的关闭Java应用,即使JVM终止突然。

JVM中捕捉signal来实现shutdown hooks意外终止。

JVM使用SIGHUP,SIGINT以及SIGTERM启动shutdown hooks的运行。

JVM使用类似的机制,来实现用于调试功能的线程堆栈dump。JVM使用SIGQUIT执行线程转储。

嵌入了JVM的应用经常需要捕获信号,如SIGINT或SIGTERM,这可能会导致与JVM信号处理程序干扰。

该-Xrs选项可用来解决这个问题。当-Xrs被使用时,JVM不改变SIGINT,SIGTERM,SIGHUP和SIGQUIT信号。

指定-Xrs的两个后果:

SIGQUIT 线程转储不可用。

用户代码负责使shutdown hook运行,例如,通过调用System.exit()当JVM将被终止。

-Xshare:mode

设置类数据共享(CDS)模式。

可能的mode包括以下内容:

auto: 如果可能,请使用CDS。这是Java HotSpot 32-Bit Client VM的默认值。

on: 要求使用的CDS。打印错误消息并退出,如果CDS不能使用。

off:不要使用CDS。这是Java HotSpot 32-Bit Server VM、Java HotSpot 64-Bit Client VM, and Java HotSpot 64-Bit Server VM的默认值。

你应该随着每一个新版本的JDK重新生成CDS归档。

-XshowSettings:category

显示设置并继续。可能的category包括以下内容::

all:显示设置的所有类别。这是默认值。

locale: 显示相关区域设置。

properties: 显示相关的系统属性设置。

vm:显示JVM的设置。

-Xsssize
设置线程堆的大小(以字节为单位)。后缀k或K到指示KB,m或者M以指示MB,g或者G以指示GB。

默认值取决于平台:

Linux/ARM (32-bit): 320 KB

Linux/i386 (32-bit): 320 KB

Linux/x64 (64-bit): 1024 KB

OS X (64-bit): 1024 KB

Oracle Solaris/i386 (32-bit): 320 KB

Oracle Solaris/x64 (64-bit): 1024 KB

下面的示例设置线程的堆栈大小为1024KB:

-Xss1m


-Xss1024k


-Xss1048576

此选项等同于-XX:ThreadStackSize。

-Xusealtsigs

对JVM的内部信号使用替代信号,而不是SIGUSR1和SIGUSR2。此选项等同于-XX:+UseAltSigs。

-Xverify:mode
设置字节码校验器的模式。
字节码验证可以确保类文件正确并满足《Java虚拟机规范》中的约束 。
见4.10章节“的验证class文件”:
https://docs.oracle.com/javase/specs/jvms/se8/html/jvms-4.html#jvms-4.10

当运行动态生成或不可信的类文件时,不要关闭验证。因为这降低了由Java提供的保护,并且诊断不正常的字节码问题比较困难。
可能的mode如下:

remote:
验证所有没有被bootstrap class loader加载的字节码。这是默认的。

all:验证所有字节码。

none:禁止所有字节码的验证。

命令行选项:高级运行时选项(Advanced Runtime Options)

这些选项用于控制 HotSpot 虚拟机运行时的行为。

-XX:+CheckEndorsedAndExtDirs

该选项能防止java命令运行java程序,如果应用使用了赞同-标准(“endorsed-standards”)重写机制或者扩展机制。

能够防止该选项java命令从如果它使用了赞同-标准超越机构或扩展机制运行的Java应用程序。

该选项通过检查以下内容来确定应用是否使用了这些机制之一:

java.ext.dirs或java.endorsed.dirs系统属性设置。

lib/endorsed目录存在并且不为空。

lib/ext目录包含JDK以外的任何JAR文件。

全系统的特定平台的扩展目录中包含的任何JAR文件。

-XX:+DisableAttachMechanism

禁止工具附加到JVM。

默认情况下,允许连接机制。可以使用如jcmd, jstack, jmap, jinfo。

-XX:ErrorFile=filename

发生无法恢复错误时,错误写入的文件名。默认写入当前文件夹下的hs_err_pidPID.log,pid是导致错误的pid。下面是例子:(%p代表了PID):

-XX:ErrorFile=/var/log/java/hs_err_pid%p.log

如果文件无法创建,则写入系统临时文件夹。(/tmp)

-XX:+FailOverToOldVerifier

当新类型检查失败,自动切换到旧验证。

默认情况下,该选项被禁用,对于最近的字节码版本它会被忽略(即视为禁用)。

可以对旧版本的字节码的类启用该选项。

-XX:+FlightRecorder

应用程序的运行过程中,使用JavaFlight Recorder(JFR)的。

这是一个商业的功能,结合-XX:+UnlockCommercialFeatures选项一起用,如下所示:

java -XX:+UnlockCommercialFeatures -XX:+FlightRecorder

如果没有提供这个选项,通过提供适当的启用jcmd诊断命令,JFR仍然可以在JVM中运行。

-XX:-FlightRecorder

应用程序的运行过程中,禁用JFR。

这是一个商业的功能,结合-XX:+UnlockCommercialFeatures选项一起用:

java -XX:+UnlockCommercialFeatures -XX:-FlightRecorder

如果提供该选项,Java的飞行记录器不能在运行的JVM中使用。

-XX:FlightRecorderOptions=parameter=value

使用参数控制JFR。

这是一个商业的功能,结合-XX:+UnlockCommercialFeatures选项一起用。

只能在JFR使用时(使用了-XX:+FlightRecorder),设置该参数。

下面是可用的参数:

defaultrecording={true|false}

在后台持续记录,还是运行指定的时间。默认false。如果要持续记录,是指为true。

disk={true|false}

是否将持续的记录写到硬盘。默认false。如果要写入硬盘,设置true,并设置defaultrecording=true。

dumponexit={true|false}

当JVM以控制方式结束时,是否生成JRF数据的dump文件。默认false,即不生成。如果要生成,设置true,并且设置defaultrecording=true.

转储文件被写入位置,由dumponexitpath定义。

dumponexitpath=path

如果设置了dumponexit=true,那么当JVM已控制方式退出时,该参数指定了dump文件的写入位置。只有设置了 defaultrecording=true 时,该才书才有效。

如果路径为文件夹,则写入以日期、时间为文件名的文件。如果指定的文件已经存在,则写入文件名为“文件名+日期+时间戳”的新文件名。

globalbuffersize=size

指定主存储器用于保存数据的总量(字节)。

追加k或K到指定KB大小,m或M以MB为单位指定大小,g或G以指定GB的大小。

默认,尺寸被设置为462848字节。

loglevel={quiet|error|warning|info|debug|trace}

日志等级。默认info。

maxage=time

设置硬盘数据保存的最长时间。

附加s表示秒,m为分钟,h几小时,d几天。

缺省情况下,15m。

disk=true时,此参数才有效。

maxchunksize=size

指定的记录中的数据块的最大大小(字节)。

追加k或K到指定KB大小,m或M以MB为单位指定大小,g或G以指定GB的大小。

默认情况下,数据块的最大大小被设置为12 MB。

maxsize=size

指定硬盘的记录的最大数据大小(字节)。

追加k或K到指定KB大小,m或M以MB为单位指定大小,g或G以指定GB的大小。

缺省情况下,磁盘数据的最大大小没有限制,此参数设置为0。

disk=true时,此参数才有效。

repository=path

指定临时磁盘存储库(目录)的路径。

默认情况下,使用系统的临时目录。

samplethreads={true|false}

指定线程采样是否已启用。

只有当采样事件与此参数一起启用时,才会发生线程采样。

默认情况下,启用此参数。

settings=path

指定事件设置文件的路径和名称。

默认情况下,使用default.jfc,位于JAVA_HOME/jre/lib/jfr。

stackdepth=depth

JFR堆栈跟踪的深度。

默认情况下,深度被设定为64的方法调用。最大为2048,最小值为1。

threadbuffersize=size

指定每个线程的本地缓冲区的大小(以字节为单位)。

追加k或K到指定KB大小,m或M以MB为单位指定大小,g或G以指定GB的大小。

这个参数值越高,允许收集更多的数据,而无需将其刷新到全局存储。

它可以在一个富线程环境中增加应用足迹。

缺省情况下,本地缓冲器大小设置为5KB。

您可以通过用逗号分隔指定多个参数值。例如,指示JFR写一个连续记录到磁盘,数据块的最大大小设置为10 MB:

-XX:FlightRecorderOptions=defaultrecording=true,disk=true,maxchunksize=10M

-XX:LargePageSizeInBytes=size

在Solaris上,设置了用于Java堆的大页面的最大大小(byte)。

该尺寸参数必须是2的幂(2,4,8,16,…)。

附加字母k或K表示千字节,m或M指示兆字节,g或G表示GB。

缺省情况下,大小设置为0,这意味着在JVM自动选择大页面的大小。

下面的例子说明如何将大页面大小为4MB:
-XX:LargePageSizeInBytes=4m

-XX:MaxDirectMemorySize=size

设置NIO的直接缓冲区分配的最大总大小。

附加字母k或K表示千字节,m或M指示兆字节,g或G表示GB。

缺省情况下,为0,即在JVM自动选择为NIO直接缓冲区分配的大小。

下面的例子说明,如何将NIO大小设置为1024 KB:

-XX:MaxDirectMemorySize=1m

-XX:MaxDirectMemorySize=1024k

-XX:MaxDirectMemorySize=1048576

-XX:NativeMemoryTracking=mode

JVM native 内存使用的追踪模式:

off

默认情况下,是不追踪 java native 内存使用。

summary

只追踪,JVM子系统的使用情况,如Java heap, class, code, thread。

detail

除了追踪子系统之外,还追踪专有CallSite的内存使用,专有虚拟内存区域,和承诺区。

-XX:ObjectAlignmentInBytes=alignment
设置Java对象的内存对齐(以字节为单位)。
默认情况下,该值被设置为8byte。指定的值应该是二的幂,并且必须>=8 and <=256。
此选项使得,大Java堆使用与压缩指针成为可能。

以字节为单位的堆大小限制的计算如下:

4GB * ObjectAlignmentInBytes

注意:随着对齐值的增加,对象之间的未使用的空间也将增加。其结果是,你可能感觉不到使用压缩指针与大的Java堆大小的任何好处。

-XX:OnError=string
Sets a custom command or a series of semicolon-separated commands to run when an irrecoverable error occurs. If the string contains spaces, then it must be enclosed in quotation marks.

The following example shows how the -XX:OnError option can be used to run the gcore command to create the core image, and the debugger is started to attach to the process in case of an irrecoverable error (the %p designates the current process):

-XX:OnError=”gcore %p;dbx – %p”
-XX:OnOutOfMemoryError=string
Sets a custom command or a series of semicolon-separated commands to run when an OutOfMemoryError exception is first thrown. If the string contains spaces, then it must be enclosed in quotation marks. For an example of a command string, see the description of the -XX:OnError option.

-XX:+PerfDataSaveToFile
If enabled, saves jstat(1) binary data when the Java application exits. This binary data is saved in a file named hsperfdata_<pid>, where <pid> is the process identifier of the Java application you ran. Use jstat to display the performance data contained in this file as follows:

jstat -class file:///<path>/hsperfdata_<pid>
jstat -gc file:///<path>/hsperfdata_<pid>
-XX:+PrintCommandLineFlags
Enables printing of ergonomically selected JVM flags that appeared on the command line. It can be useful to know the ergonomic values set by the JVM, such as the heap space size and the selected garbage collector. By default, this option is disabled and flags are not printed.

-XX:+PrintNMTStatistics
Enables printing of collected native memory tracking data at JVM exit when native memory tracking is enabled (see -XX:NativeMemoryTracking). By default, this option is disabled and native memory tracking data is not printed.

-XX:+RelaxAccessControlCheck
Decreases the amount of access control checks in the verifier. By default, this option is disabled, and it is ignored (that is, treated as disabled) for classes with a recent bytecode version. You can enable it for classes with older versions of the bytecode.

-XX:+ResourceManagement
Enables the use of Resource Management during the runtime of the application.

This is a commercial feature that requires you to also specify the -XX:+UnlockCommercialFeatures option as follows:

java -XX:+UnlockCommercialFeatures -XX:+ResourceManagement

-XX:ResourceManagementSampleInterval=value (milliseconds)
Sets the parameter that controls the sampling interval for Resource Management measurements, in milliseconds.

This option can be used only when Resource Management is enabled (that is, the -XX:+ResourceManagement option is specified).

-XX:SharedArchiveFile=path
Specifies the path and name of the class data sharing (CDS) archive file

-XX:SharedClassListFile=file_name
Specifies the text file that contains the names of the class files to store in the class data sharing (CDS) archive. This file contains the full name of one class file per line, except slashes (/) replace dots (.). For example, to specify the classes java.lang.Object and hello.Main, create a text file that contains the following two lines:

java/lang/Object
hello/Main
The class files that you specify in this text file should include the classes that are commonly used by the application. They may include any classes from the application, extension, or bootstrap class paths.

-XX:+ShowMessageBoxOnError
Enables displaying of a dialog box when the JVM experiences an irrecoverable error. This prevents the JVM from exiting and keeps the process active so that you can attach a debugger to it to investigate the cause of the error. By default, this option is disabled.

-XX:StartFlightRecording=parameter=value
Starts a JFR recording for the Java application. This is a commercial feature that works in conjunction with the -XX:+UnlockCommercialFeatures option. This option is equivalent to the JFR.start diagnostic command that starts a recording during runtime. You can set the following parameters when starting a JFR recording:

compress={true|false}
Specifies whether to compress the JFR recording log file (of type JFR) on the disk using the gzip file compression utility. This parameter is valid only if the filename parameter is specified. By default it is set to false (recording is not compressed). To enable compression, set the parameter to true.

defaultrecording={true|false}
Specifies whether the recording is a continuous background recording or if it runs for a limited time. By default, this parameter is set to false (recording runs for a limited time). To make the recording run continuously, set the parameter to true.

delay=time
Specifies the delay between the Java application launch time and the start of the recording. Append s to specify the time in seconds, m for minutes, h for hours, or d for days (for example, specifying 10m means 10 minutes). By default, there is no delay, and this parameter is set to 0.

dumponexit={true|false}
Specifies whether a dump file of JFR data should be generated when the JVM terminates in a controlled manner. By default, this parameter is set to false (dump file on exit is not generated). To enable it, set the parameter to true.

The dump file is written to the location defined by the filename parameter.

Example:

-XX:StartFlightRecording=name=test,filename=D:\test.jfr,dumponexit=true
duration=time
Specifies the duration of the recording. Append s to specify the time in seconds, m for minutes, h for hours, or d for days (for example, specifying 5h means 5 hours). By default, the duration is not limited, and this parameter is set to 0.

filename=path
Specifies the path and name of the JFR recording log file.

name=identifier
Specifies the identifier for the JFR recording. By default, it is set to Recording x.

maxage=time
Specifies the maximum age of disk data to keep for the default recording. Append s to specify the time in seconds, m for minutes, h for hours, or d for days (for example, specifying 30s means 30 seconds). By default, the maximum age is set to 15 minutes (15m).

maxsize=size
Specifies the maximum size (in bytes) of disk data to keep for the default recording. Append k or K, to specify the size in KB, m or M to specify the size in MB, g or G to specify the size in GB. By default, the maximum size of disk data is not limited, and this parameter is set to 0.

settings=path
Specifies the path and name of the event settings file (of type JFC). By default, the default.jfc file is used, which is located in JAVA_HOME/jre/lib/jfr.

You can specify values for multiple parameters by separating them with a comma. For example, to save the recording to test.jfr in the current working directory, and instruct JFR to compress the log file, specify the following:

-XX:StartFlightRecording=filename=test.jfr,compress=true
-XX:ThreadStackSize=size
Sets the thread stack size (in bytes). Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, g or G to indicate gigabytes. The default value depends on the platform:

Linux/ARM (32-bit): 320 KB

Linux/i386 (32-bit): 320 KB

Linux/x64 (64-bit): 1024 KB

OS X (64-bit): 1024 KB

Oracle Solaris/i386 (32-bit): 320 KB

Oracle Solaris/x64 (64-bit): 1024 KB

The following examples show how to set the thread stack size to 1024 KB in different units:

-XX:ThreadStackSize=1m
-XX:ThreadStackSize=1024k
-XX:ThreadStackSize=1048576
This option is equivalent to -Xss.

-XX:+TraceClassLoading
Enables tracing of classes as they are loaded. By default, this option is disabled and classes are not traced.

-XX:+TraceClassLoadingPreorder
Enables tracing of all loaded classes in the order in which they are referenced. By default, this option is disabled and classes are not traced.

-XX:+TraceClassResolution
Enables tracing of constant pool resolutions. By default, this option is disabled and constant pool resolutions are not traced.

-XX:+TraceClassUnloading
Enables tracing of classes as they are unloaded. By default, this option is disabled and classes are not traced.

-XX:+TraceLoaderConstraints
Enables tracing of the loader constraints recording. By default, this option is disabled and loader constraints recording is not traced.

-XX:+UnlockCommercialFeatures
Enables the use of commercial features. Commercial features are included with Oracle Java SE Advanced or Oracle Java SE Suite packages, as defined on the Java SE Products page at http://www.oracle.com/technetwork/java/javase/terms/products/index.html

By default, this option is disabled and the JVM runs without the commercial features. Once they were enabled for a JVM process, it is not possible to disable their use for that process.

If this option is not provided, commercial features can still be unlocked in a running JVM by using the appropriate jcmd diagnostic commands.

-XX:+UseAltSigs
Enables the use of alternative signals instead of SIGUSR1 and SIGUSR2 for JVM internal signals. By default, this option is disabled and alternative signals are not used. This option is equivalent to -Xusealtsigs.

-XX:+UseAppCDS
Enables application class data sharing (AppCDS). To use AppCDS, you must also specify values for the options -XX:SharedClassListFile and -XX:SharedArchiveFile during both CDS dump time (see the option -Xshare:dump) and application run time.

This is a commercial feature that requires you to also specify the -XX:+UnlockCommercialFeatures option. This is also an experimental feature; it may change in future releases.

See “Application Class Data Sharing”.

-XX:-UseBiasedLocking
Disables the use of biased locking. Some applications with significant amounts of uncontended synchronization may attain significant speedups with this flag enabled, whereas applications with certain patterns of locking may see slowdowns. For more information about the biased locking technique, see the example in Java Tuning White Paper at http://www.oracle.com/technetwork/java/tuning-139912.html#section4.2.5

By default, this option is enabled.

-XX:-UseCompressedOops
Disables the use of compressed pointers. By default, this option is enabled, and compressed pointers are used when Java heap sizes are less than 32 GB. When this option is enabled, object references are represented as 32-bit offsets instead of 64-bit pointers, which typically increases performance when running the application with Java heap sizes less than 32 GB. This option works only for 64-bit JVMs.

It is also possible to use compressed pointers when Java heap sizes are greater than 32GB. See the -XX:ObjectAlignmentInBytes option.

-XX:+UseHugeTLBFS
This option for Linux is the equivalent of specifying -XX:+UseLargePages. This option is disabled by default. This option pre-allocates all large pages up-front, when memory is reserved; consequently the JVM cannot dynamically grow or shrink large pages memory areas; see -XX:UseTransparentHugePages if you want this behavior.

For more information, see “Large Pages”.

-XX:+UseLargePages
Enables the use of large page memory. By default, this option is disabled and large page memory is not used.

For more information, see “Large Pages”.

-XX:+UseMembar
Enables issuing of membars on thread state transitions. This option is disabled by default on all platforms except ARM servers, where it is enabled. (It is recommended that you do not disable this option on ARM servers.)

-XX:+UsePerfData
Enables the perfdata feature. This option is enabled by default to allow JVM monitoring and performance testing. Disabling it suppresses the creation of the hsperfdata_userid directories. To disable the perfdata feature, specify -XX:-UsePerfData.

-XX:+UseTransparentHugePages
On Linux, enables the use of large pages that can dynamically grow or shrink. This option is disabled by default. You may encounter performance problems with transparent huge pages as the OS moves other pages around to create huge pages; this option is made available for experimentation.

For more information, see “Large Pages”.

-XX:+AllowUserSignalHandlers
Enables installation of signal handlers by the application. By default, this option is disabled and the application is not allowed to install signal handlers.

命令行选项:高级JIT编译器选项(Advanced JIT Compiler Options)

These options control the dynamic just-in-time (JIT) compilation performed by the Java HotSpot VM.

-XX:+AggressiveOpts
Enables the use of aggressive performance optimization features, which are expected to become default in upcoming releases. By default, this option is disabled and experimental performance features are not used.

-XX:AllocateInstancePrefetchLines=lines
Sets the number of lines to prefetch ahead of the instance allocation pointer. By default, the number of lines to prefetch is set to 1:

-XX:AllocateInstancePrefetchLines=1
Only the Java HotSpot Server VM supports this option.

-XX:AllocatePrefetchDistance=size
Sets the size (in bytes) of the prefetch distance for object allocation. Memory about to be written with the value of new objects is prefetched up to this distance starting from the address of the last allocated object. Each Java thread has its own allocation point.

Negative values denote that prefetch distance is chosen based on the platform. Positive values are bytes to prefetch. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, g or G to indicate gigabytes. The default value is set to -1.

The following example shows how to set the prefetch distance to 1024 bytes:

-XX:AllocatePrefetchDistance=1024
Only the Java HotSpot Server VM supports this option.

-XX:AllocatePrefetchInstr=instruction
Sets the prefetch instruction to prefetch ahead of the allocation pointer. Only the Java HotSpot Server VM supports this option. Possible values are from 0 to 3. The actual instructions behind the values depend on the platform. By default, the prefetch instruction is set to 0:

-XX:AllocatePrefetchInstr=0
Only the Java HotSpot Server VM supports this option.

-XX:AllocatePrefetchLines=lines
Sets the number of cache lines to load after the last object allocation by using the prefetch instructions generated in compiled code. The default value is 1 if the last allocated object was an instance, and 3 if it was an array.

The following example shows how to set the number of loaded cache lines to 5:

-XX:AllocatePrefetchLines=5
Only the Java HotSpot Server VM supports this option.

-XX:AllocatePrefetchStepSize=size
Sets the step size (in bytes) for sequential prefetch instructions. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, g or G to indicate gigabytes. By default, the step size is set to 16 bytes:

-XX:AllocatePrefetchStepSize=16
Only the Java HotSpot Server VM supports this option.

-XX:AllocatePrefetchStyle=style
Sets the generated code style for prefetch instructions. The style argument is an integer from 0 to 3:

0
Do not generate prefetch instructions.

1
Execute prefetch instructions after each allocation. This is the default parameter.

2
Use the thread-local allocation block (TLAB) watermark pointer to determine when prefetch instructions are executed.

3
Use BIS instruction on SPARC for allocation prefetch.

Only the Java HotSpot Server VM supports this option.

-XX:+BackgroundCompilation
Enables background compilation. This option is enabled by default. To disable background compilation, specify -XX:-BackgroundCompilation (this is equivalent to specifying -Xbatch).

-XX:CICompilerCount=threads
Sets the number of compiler threads to use for compilation. By default, the number of threads is set to 2 for the server JVM, to 1 for the client JVM, and it scales to the number of cores if tiered compilation is used. The following example shows how to set the number of threads to 2:

-XX:CICompilerCount=2
-XX:CodeCacheMinimumFreeSpace=size
Sets the minimum free space (in bytes) required for compilation. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, g or G to indicate gigabytes. When less than the minimum free space remains, compiling stops. By default, this option is set to 500 KB. The following example shows how to set the minimum free space to 1024 MB:

-XX:CodeCacheMinimumFreeSpace=1024m
-XX:CompileCommand=command,method[,option]
Specifies a command to perform on a method. For example, to exclude the indexOf() method of the String class from being compiled, use the following:

-XX:CompileCommand=exclude,java/lang/String.indexOf
Note that the full class name is specified, including all packages and subpackages separated by a slash (/). For easier cut and paste operations, it is also possible to use the method name format produced by the -XX:+PrintCompilation and -XX:+LogCompilation options:

-XX:CompileCommand=exclude,java.lang.String::indexOf
If the method is specified without the signature, the command will be applied to all methods with the specified name. However, you can also specify the signature of the method in the class file format. In this case, you should enclose the arguments in quotation marks, because otherwise the shell treats the semicolon as command end. For example, if you want to exclude only the indexOf(String) method of the String class from being compiled, use the following:

-XX:CompileCommand=”exclude,java/lang/String.indexOf,(Ljava/lang/String;)I”
You can also use the asterisk (*) as a wildcard for class and method names. For example, to exclude all indexOf() methods in all classes from being compiled, use the following:

-XX:CompileCommand=exclude,*.indexOf
The commas and periods are aliases for spaces, making it easier to pass compiler commands through a shell. You can pass arguments to -XX:CompileCommand using spaces as separators by enclosing the argument in quotation marks:

-XX:CompileCommand=”exclude java/lang/String indexOf”
Note that after parsing the commands passed on the command line using the -XX:CompileCommand options, the JIT compiler then reads commands from the .hotspot_compiler file. You can add commands to this file or specify a different file using the -XX:CompileCommandFile option.

To add several commands, either specify the -XX:CompileCommand option multiple times, or separate each argument with the newline separator (\n). The following commands are available:

break
Set a breakpoint when debugging the JVM to stop at the beginning of compilation of the specified method.

compileonly
Exclude all methods from compilation except for the specified method. As an alternative, you can use the -XX:CompileOnly option, which allows to specify several methods.

dontinline
Prevent inlining of the specified method.

exclude
Exclude the specified method from compilation.

help
Print a help message for the -XX:CompileCommand option.

inline
Attempt to inline the specified method.

log
Exclude compilation logging (with the -XX:+LogCompilation option) for all methods except for the specified method. By default, logging is performed for all compiled methods.

option
This command can be used to pass a JIT compilation option to the specified method in place of the last argument (option). The compilation option is set at the end, after the method name. For example, to enable the BlockLayoutByFrequency option for the append() method of the StringBuffer class, use the following:

-XX:CompileCommand=option,java/lang/StringBuffer.append,BlockLayoutByFrequency
You can specify multiple compilation options, separated by commas or spaces.

print
Print generated assembler code after compilation of the specified method.

quiet
Do not print the compile commands. By default, the commands that you specify with the -XX:CompileCommand option are printed; for example, if you exclude from compilation the indexOf() method of the String class, then the following will be printed to standard output:

CompilerOracle: exclude java/lang/String.indexOf
You can suppress this by specifying the -XX:CompileCommand=quiet option before other -XX:CompileCommand options.

-XX:CompileCommandFile=filename
Sets the file from which JIT compiler commands are read. By default, the .hotspot_compiler file is used to store commands performed by the JIT compiler.

Each line in the command file represents a command, a class name, and a method name for which the command is used. For example, this line prints assembly code for the toString() method of the String class:

print java/lang/String toString
For more information about specifying the commands for the JIT compiler to perform on methods, see the -XX:CompileCommand option.

-XX:CompileOnly=methods
Sets the list of methods (separated by commas) to which compilation should be restricted. Only the specified methods will be compiled. Specify each method with the full class name (including the packages and subpackages). For example, to compile only the length() method of the String class and the size() method of the List class, use the following:

-XX:CompileOnly=java/lang/String.length,java/util/List.size
Note that the full class name is specified, including all packages and subpackages separated by a slash (/). For easier cut and paste operations, it is also possible to use the method name format produced by the -XX:+PrintCompilation and -XX:+LogCompilation options:

-XX:CompileOnly=java.lang.String::length,java.util.List::size
Although wildcards are not supported, you can specify only the class or package name to compile all methods in that class or package, as well as specify just the method to compile methods with this name in any class:

-XX:CompileOnly=java/lang/String
-XX:CompileOnly=java/lang
-XX:CompileOnly=.length
-XX:CompileThreshold=invocations
Sets the number of interpreted method invocations before compilation. By default, in the server JVM, the JIT compiler performs 10,000 interpreted method invocations to gather information for efficient compilation. For the client JVM, the default setting is 1,500 invocations. This option is ignored when tiered compilation is enabled; see the option -XX:+TieredCompilation. The following example shows how to set the number of interpreted method invocations to 5,000:

-XX:CompileThreshold=5000
You can completely disable interpretation of Java methods before compilation by specifying the -Xcomp option.

-XX:+DoEscapeAnalysis
Enables the use of escape analysis. This option is enabled by default. To disable the use of escape analysis, specify -XX:-DoEscapeAnalysis. Only the Java HotSpot Server VM supports this option.

-XX:InitialCodeCacheSize=size
Sets the initial code cache size (in bytes). Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, g or G to indicate gigabytes. The default value is set to 500 KB. The initial code cache size should be not less than the system’s minimal memory page size. The following example shows how to set the initial code cache size to 32 KB:

-XX:InitialCodeCacheSize=32k
-XX:+Inline
Enables method inlining. This option is enabled by default to increase performance. To disable method inlining, specify -XX:-Inline.

-XX:InlineSmallCode=size
Sets the maximum code size (in bytes) for compiled methods that should be inlined. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, g or G to indicate gigabytes. Only compiled methods with the size smaller than the specified size will be inlined. By default, the maximum code size is set to 1000 bytes:

-XX:InlineSmallCode=1000
-XX:+LogCompilation
Enables logging of compilation activity to a file named hotspot.log in the current working directory. You can specify a different log file path and name using the -XX:LogFile option.

By default, this option is disabled and compilation activity is not logged. The -XX:+LogCompilation option has to be used together with the -XX:UnlockDiagnosticVMOptions option that unlocks diagnostic JVM options.

You can enable verbose diagnostic output with a message printed to the console every time a method is compiled by using the -XX:+PrintCompilation option.

-XX:MaxInlineSize=size
Sets the maximum bytecode size (in bytes) of a method to be inlined. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, g or G to indicate gigabytes. By default, the maximum bytecode size is set to 35 bytes:

-XX:MaxInlineSize=35
-XX:MaxNodeLimit=nodes
Sets the maximum number of nodes to be used during single method compilation. By default, the maximum number of nodes is set to 65,000:

-XX:MaxNodeLimit=65000
-XX:MaxTrivialSize=size
Sets the maximum bytecode size (in bytes) of a trivial method to be inlined. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, g or G to indicate gigabytes. By default, the maximum bytecode size of a trivial method is set to 6 bytes:

-XX:MaxTrivialSize=6
-XX:+OptimizeStringConcat
Enables the optimization of String concatenation operations. This option is enabled by default. To disable the optimization of String concatenation operations, specify -XX:-OptimizeStringConcat. Only the Java HotSpot Server VM supports this option.

-XX:+PrintAssembly
Enables printing of assembly code for bytecoded and native methods by using the external disassembler.so library. This enables you to see the generated code, which may help you to diagnose performance issues.

By default, this option is disabled and assembly code is not printed. The -XX:+PrintAssembly option has to be used together with the -XX:UnlockDiagnosticVMOptions option that unlocks diagnostic JVM options.

-XX:+PrintCompilation
Enables verbose diagnostic output from the JVM by printing a message to the console every time a method is compiled. This enables you to see which methods actually get compiled. By default, this option is disabled and diagnostic output is not printed.

You can also log compilation activity to a file by using the -XX:+LogCompilation option.

-XX:+PrintInlining
Enables printing of inlining decisions. This enables you to see which methods are getting inlined.

By default, this option is disabled and inlining information is not printed. The -XX:+PrintInlining option has to be used together with the -XX:+UnlockDiagnosticVMOptions option that unlocks diagnostic JVM options.

-XX:ReservedCodeCacheSize=size
Sets the maximum code cache size (in bytes) for JIT-compiled code. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, g or G to indicate gigabytes. The default maximum code cache size is 240 MB; if you disable tiered compilation with the option -XX:-TieredCompilation, then the default size is 48 MB. This option has a limit of 2 GB; otherwise, an error is generated. The maximum code cache size should not be less than the initial code cache size; see the option -XX:InitialCodeCacheSize. This option is equivalent to -Xmaxjitcodesize.

-XX:RTMAbortRatio=abort_ratio
The RTM abort ratio is specified as a percentage (%) of all executed RTM transactions. If a number of aborted transactions becomes greater than this ratio, then the compiled code will be deoptimized. This ratio is used when the -XX:+UseRTMDeopt option is enabled. The default value of this option is 50. This means that the compiled code will be deoptimized if 50% of all transactions are aborted.

-XX:RTMRetryCount=number_of_retries
RTM locking code will be retried, when it is aborted or busy, the number of times specified by this option before falling back to the normal locking mechanism. The default value for this option is 5. The -XX:UseRTMLocking option must be enabled.

-XX:-TieredCompilation
Disables the use of tiered compilation. By default, this option is enabled. Only the Java HotSpot Server VM supports this option.

-XX:+UseAES
Enables hardware-based AES intrinsics for Intel, AMD, and SPARC hardware. Intel Westmere (2010 and newer), AMD Bulldozer (2011 and newer), and SPARC (T4 and newer) are the supported hardware. UseAES is used in conjunction with UseAESIntrinsics.

-XX:+UseAESIntrinsics
UseAES and UseAESIntrinsics flags are enabled by default and are supported only for Java HotSpot Server VM 32-bit and 64-bit. To disable hardware-based AES intrinsics, specify -XX:-UseAES -XX:-UseAESIntrinsics. For example, to enable hardware AES, use the following flags:

-XX:+UseAES -XX:+UseAESIntrinsics
To support UseAES and UseAESIntrinsics flags for 32-bit and 64-bit use -server option to choose Java HotSpot Server VM. These flags are not supported on Client VM.

-XX:+UseCodeCacheFlushing
Enables flushing of the code cache before shutting down the compiler. This option is enabled by default. To disable flushing of the code cache before shutting down the compiler, specify -XX:-UseCodeCacheFlushing.

-XX:+UseCondCardMark
Enables checking of whether the card is already marked before updating the card table. This option is disabled by default and should only be used on machines with multiple sockets, where it will increase performance of Java applications that rely heavily on concurrent operations. Only the Java HotSpot Server VM supports this option.

-XX:+UseRTMDeopt
Auto-tunes RTM locking depending on the abort ratio. This ratio is specified by -XX:RTMAbortRatio option. If the number of aborted transactions exceeds the abort ratio, then the method containing the lock will be deoptimized and recompiled with all locks as normal locks. This option is disabled by default. The -XX:+UseRTMLocking option must be enabled.

-XX:+UseRTMLocking
Generate Restricted Transactional Memory (RTM) locking code for all inflated locks, with the normal locking mechanism as the fallback handler. This option is disabled by default. Options related to RTM are only available for the Java HotSpot Server VM on x86 CPUs that support Transactional Synchronization Extensions (TSX).

RTM is part of Intel’s TSX, which is an x86 instruction set extension and facilitates the creation of multithreaded applications. RTM introduces the new instructions XBEGIN, XABORT, XEND, and XTEST. The XBEGIN and XEND instructions enclose a set of instructions to run as a transaction. If no conflict is found when running the transaction, the memory and register modifications are committed together at the XEND instruction. The XABORT instruction can be used to explicitly abort a transaction and the XEND instruction to check if a set of instructions are being run in a transaction.

A lock on a transaction is inflated when another thread tries to access the same transaction, thereby blocking the thread that did not originally request access to the transaction. RTM requires that a fallback set of operations be specified in case a transaction aborts or fails. An RTM lock is a lock that has been delegated to the TSX’s system.

RTM improves performance for highly contended locks with low conflict in a critical region (which is code that must not be accessed by more than one thread concurrently). RTM also improves the performance of coarse-grain locking, which typically does not perform well in multithreaded applications. (Coarse-grain locking is the strategy of holding locks for long periods to minimize the overhead of taking and releasing locks, while fine-grained locking is the strategy of trying to achieve maximum parallelism by locking only when necessary and unlocking as soon as possible.) Also, for lightly contended locks that are used by different threads, RTM can reduce false cache line sharing, also known as cache line ping-pong. This occurs when multiple threads from different processors are accessing different resources, but the resources share the same cache line. As a result, the processors repeatedly invalidate the cache lines of other processors, which forces them to read from main memory instead of their cache.

-XX:+UseSHA
Enables hardware-based intrinsics for SHA crypto hash functions for SPARC hardware. UseSHA is used in conjunction with the UseSHA1Intrinsics, UseSHA256Intrinsics, and UseSHA512Intrinsics options.

The UseSHA and UseSHA*Intrinsics flags are enabled by default, and are supported only for Java HotSpot Server VM 64-bit on SPARC T4 and newer.

This feature is only applicable when using the sun.security.provider.Sun provider for SHA operations.

To disable all hardware-based SHA intrinsics, specify -XX:-UseSHA. To disable only a particular SHA intrinsic, use the appropriate corresponding option. For example: -XX:-UseSHA256Intrinsics.

-XX:+UseSHA1Intrinsics
Enables intrinsics for SHA-1 crypto hash function.

-XX:+UseSHA256Intrinsics
Enables intrinsics for SHA-224 and SHA-256 crypto hash functions.

-XX:+UseSHA512Intrinsics
Enables intrinsics for SHA-384 and SHA-512 crypto hash functions.

-XX:+UseSuperWord
Enables the transformation of scalar operations into superword operations. This option is enabled by default. To disable the transformation of scalar operations into superword operations, specify -XX:-UseSuperWord. Only the Java HotSpot Server VM supports this option.

命令行选项:高级可维护性选项(Advanced Serviceability Options)

These options provide the ability to gather system information and perform extensive debugging.

-XX:+ExtendedDTraceProbes
Enables additional dtrace tool probes that impact the performance. By default, this option is disabled and dtrace performs only standard probes.

-XX:+HeapDumpOnOutOfMemory
Enables the dumping of the Java heap to a file in the current directory by using the heap profiler (HPROF) when a java.lang.OutOfMemoryError exception is thrown. You can explicitly set the heap dump file path and name using the -XX:HeapDumpPath option. By default, this option is disabled and the heap is not dumped when an OutOfMemoryError exception is thrown.

-XX:HeapDumpPath=path
Sets the path and file name for writing the heap dump provided by the heap profiler (HPROF) when the -XX:+HeapDumpOnOutOfMemoryError option is set. By default, the file is created in the current working directory, and it is named java_pidpid.hprof where pid is the identifier of the process that caused the error. The following example shows how to set the default file explicitly (%p represents the current process identificator):

-XX:HeapDumpPath=./java_pid%p.hprof
The following example shows how to set the heap dump file to /var/log/java/java_heapdump.hprof:

-XX:HeapDumpPath=/var/log/java/java_heapdump.hprof
-XX:LogFile=path
Sets the path and file name where log data is written. By default, the file is created in the current working directory, and it is named hotspot.log.

The following example shows how to set the log file to /var/log/java/hotspot.log:

-XX:LogFile=/var/log/java/hotspot.log
-XX:+PrintClassHistogram
Enables printing of a class instance histogram after a Control+C event (SIGTERM). By default, this option is disabled.

Setting this option is equivalent to running the jmap -histo command, or the jcmd pid GC.class_histogram command, where pid is the current Java process identifier.

-XX:+PrintConcurrentLocks
Enables printing of java.util.concurrent locks after a Control+C event (SIGTERM). By default, this option is disabled.

Setting this option is equivalent to running the jstack -l command or the jcmd pid Thread.print -l command, where pid is the current Java process identifier.

-XX:+UnlockDiagnosticVMOptions
Unlocks the options intended for diagnosing the JVM. By default, this option is disabled and diagnostic options are not available.

命令行选项:高级GC选项(Advanced Garbage Collection Options)

These options control how garbage collection (GC) is performed by the Java HotSpot VM.

-XX:+AggressiveHeap
Enables Java heap optimization. This sets various parameters to be optimal for long-running jobs with intensive memory allocation, based on the configuration of the computer (RAM and CPU). By default, the option is disabled and the heap is not optimized.

-XX:+AlwaysPreTouch
Enables touching of every page on the Java heap during JVM initialization. This gets all pages into the memory before entering the main() method. The option can be used in testing to simulate a long-running system with all virtual memory mapped to physical memory. By default, this option is disabled and all pages are committed as JVM heap space fills.

-XX:+CMSClassUnloadingEnabled
Enables class unloading when using the concurrent mark-sweep (CMS) garbage collector. This option is enabled by default. To disable class unloading for the CMS garbage collector, specify -XX:-CMSClassUnloadingEnabled.

-XX:CMSExpAvgFactor=percent
Sets the percentage of time (0 to 100) used to weight the current sample when computing exponential averages for the concurrent collection statistics. By default, the exponential averages factor is set to 25%. The following example shows how to set the factor to 15%:

-XX:CMSExpAvgFactor=15
-XX:CMSInitiatingOccupancyFraction=percent
Sets the percentage of the old generation occupancy (0 to 100) at which to start a CMS collection cycle. The default value is set to -1. Any negative value (including the default) implies that -XX:CMSTriggerRatio is used to define the value of the initiating occupancy fraction.

The following example shows how to set the occupancy fraction to 20%:

-XX:CMSInitiatingOccupancyFraction=20
-XX:+CMSScavengeBeforeRemark
Enables scavenging attempts before the CMS remark step. By default, this option is disabled.

-XX:CMSTriggerRatio=percent
Sets the percentage (0 to 100) of the value specified by -XX:MinHeapFreeRatio that is allocated before a CMS collection cycle commences. The default value is set to 80%.

The following example shows how to set the occupancy fraction to 75%:

-XX:CMSTriggerRatio=75
-XX:ConcGCThreads=threads
Sets the number of threads used for concurrent GC. The default value depends on the number of CPUs available to the JVM.

For example, to set the number of threads for concurrent GC to 2, specify the following option:

-XX:ConcGCThreads=2
-XX:+DisableExplicitGC
Enables the option that disables processing of calls to System.gc(). This option is disabled by default, meaning that calls to System.gc() are processed. If processing of calls to System.gc() is disabled, the JVM still performs GC when necessary.

-XX:+ExplicitGCInvokesConcurrent
Enables invoking of concurrent GC by using the System.gc() request. This option is disabled by default and can be enabled only together with the -XX:+UseConcMarkSweepGC option.

-XX:+ExplicitGCInvokesConcurrentAndUnloadsClasses
Enables invoking of concurrent GC by using the System.gc() request and unloading of classes during the concurrent GC cycle. This option is disabled by default and can be enabled only together with the -XX:+UseConcMarkSweepGC option.

-XX:G1HeapRegionSize=size
Sets the size of the regions into which the Java heap is subdivided when using the garbage-first (G1) collector. The value can be between 1 MB and 32 MB. The default region size is determined ergonomically based on the heap size.

The following example shows how to set the size of the subdivisions to 16 MB:

-XX:G1HeapRegionSize=16m
-XX:+G1PrintHeapRegions
Enables the printing of information about which regions are allocated and which are reclaimed by the G1 collector. By default, this option is disabled.

-XX:G1ReservePercent=percent
Sets the percentage of the heap (0 to 50) that is reserved as a false ceiling to reduce the possibility of promotion failure for the G1 collector. By default, this option is set to 10%.

The following example shows how to set the reserved heap to 20%:

-XX:G1ReservePercent=20
-XX:InitialHeapSize=size
Sets the initial size (in bytes) of the memory allocation pool. This value must be either 0, or a multiple of 1024 and greater than 1 MB. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, g or G to indicate gigabytes. The default value is chosen at runtime based on system configuration. See the section “Ergonomics” in Java SE HotSpot Virtual Machine Garbage Collection Tuning Guide at http://docs.oracle.com/javase/8/docs/technotes/guides/vm/gctuning/index.html.

The following examples show how to set the size of allocated memory to 6 MB using various units:

-XX:InitialHeapSize=6291456
-XX:InitialHeapSize=6144k
-XX:InitialHeapSize=6m
If you set this option to 0, then the initial size will be set as the sum of the sizes allocated for the old generation and the young generation. The size of the heap for the young generation can be set using the -XX:NewSize option.

-XX:InitialSurvivorRatio=ratio
Sets the initial survivor space ratio used by the throughput garbage collector (which is enabled by the -XX:+UseParallelGC and/or -XX:+UseParallelOldGC options). Adaptive sizing is enabled by default with the throughput garbage collector by using the -XX:+UseParallelGC and -XX:+UseParallelOldGC options, and survivor space is resized according to the application behavior, starting with the initial value. If adaptive sizing is disabled (using the -XX:-UseAdaptiveSizePolicy option), then the -XX:SurvivorRatio option should be used to set the size of the survivor space for the entire execution of the application.

The following formula can be used to calculate the initial size of survivor space (S) based on the size of the young generation (Y), and the initial survivor space ratio (R):

S=Y/(R+2)
The 2 in the equation denotes two survivor spaces. The larger the value specified as the initial survivor space ratio, the smaller the initial survivor space size.

By default, the initial survivor space ratio is set to 8. If the default value for the young generation space size is used (2 MB), the initial size of the survivor space will be 0.2 MB.

The following example shows how to set the initial survivor space ratio to 4:

-XX:InitialSurvivorRatio=4
-XX:InitiatingHeapOccupancyPercent=percent
Sets the percentage of the heap occupancy (0 to 100) at which to start a concurrent GC cycle. It is used by garbage collectors that trigger a concurrent GC cycle based on the occupancy of the entire heap, not just one of the generations (for example, the G1 garbage collector).

By default, the initiating value is set to 45%. A value of 0 implies nonstop GC cycles. The following example shows how to set the initiating heap occupancy to 75%:

-XX:InitiatingHeapOccupancyPercent=75
-XX:MaxGCPauseMillis=time
Sets a target for the maximum GC pause time (in milliseconds). This is a soft goal, and the JVM will make its best effort to achieve it. By default, there is no maximum pause time value.

The following example shows how to set the maximum target pause time to 500 ms:

-XX:MaxGCPauseMillis=500
-XX:MaxHeapSize=size
Sets the maximum size (in byes) of the memory allocation pool. This value must be a multiple of 1024 and greater than 2 MB. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, g or G to indicate gigabytes. The default value is chosen at runtime based on system configuration. For server deployments, -XX:InitialHeapSize and -XX:MaxHeapSize are often set to the same value. See the section “Ergonomics” in Java SE HotSpot Virtual Machine Garbage Collection Tuning Guide at http://docs.oracle.com/javase/8/docs/technotes/guides/vm/gctuning/index.html.

The following examples show how to set the maximum allowed size of allocated memory to 80 MB using various units:

-XX:MaxHeapSize=83886080
-XX:MaxHeapSize=81920k
-XX:MaxHeapSize=80m
On Oracle Solaris 7 and Oracle Solaris 8 SPARC platforms, the upper limit for this value is approximately 4,000 MB minus overhead amounts. On Oracle Solaris 2.6 and x86 platforms, the upper limit is approximately 2,000 MB minus overhead amounts. On Linux platforms, the upper limit is approximately 2,000 MB minus overhead amounts.

The -XX:MaxHeapSize option is equivalent to -Xmx.

-XX:MaxHeapFreeRatio=percent
Sets the maximum allowed percentage of free heap space (0 to 100) after a GC event. If free heap space expands above this value, then the heap will be shrunk. By default, this value is set to 70%.

The following example shows how to set the maximum free heap ratio to 75%:

-XX:MaxHeapFreeRatio=75
-XX:MaxMetaspaceSize=size
Sets the maximum amount of native memory that can be allocated for class metadata. By default, the size is not limited. The amount of metadata for an application depends on the application itself, other running applications, and the amount of memory available on the system.

The following example shows how to set the maximum class metadata size to 256 MB:

-XX:MaxMetaspaceSize=256m
-XX:MaxNewSize=size
Sets the maximum size (in bytes) of the heap for the young generation (nursery). The default value is set ergonomically.

-XX:MaxTenuringThreshold=threshold
Sets the maximum tenuring threshold for use in adaptive GC sizing. The largest value is 15. The default value is 15 for the parallel (throughput) collector, and 6 for the CMS collector.

The following example shows how to set the maximum tenuring threshold to 10:

-XX:MaxTenuringThreshold=10
-XX:MetaspaceSize=size
Sets the size of the allocated class metadata space that will trigger a garbage collection the first time it is exceeded. This threshold for a garbage collection is increased or decreased depending on the amount of metadata used. The default size depends on the platform.

-XX:MinHeapFreeRatio=percent
Sets the minimum allowed percentage of free heap space (0 to 100) after a GC event. If free heap space falls below this value, then the heap will be expanded. By default, this value is set to 40%.

The following example shows how to set the minimum free heap ratio to 25%:

-XX:MinHeapFreeRatio=25
-XX:NewRatio=ratio
Sets the ratio between young and old generation sizes. By default, this option is set to 2. The following example shows how to set the young/old ratio to 1:

-XX:NewRatio=1
-XX:NewSize=size
Sets the initial size (in bytes) of the heap for the young generation (nursery). Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, g or G to indicate gigabytes.

The young generation region of the heap is used for new objects. GC is performed in this region more often than in other regions. If the size for the young generation is too low, then a large number of minor GCs will be performed. If the size is too high, then only full GCs will be performed, which can take a long time to complete. Oracle recommends that you keep the size for the young generation between a half and a quarter of the overall heap size.

The following examples show how to set the initial size of young generation to 256 MB using various units:

-XX:NewSize=256m
-XX:NewSize=262144k
-XX:NewSize=268435456
The -XX:NewSize option is equivalent to -Xmn.

-XX:ParallelGCThreads=threads
Sets the number of threads used for parallel garbage collection in the young and old generations. The default value depends on the number of CPUs available to the JVM.

For example, to set the number of threads for parallel GC to 2, specify the following option:

-XX:ParallelGCThreads=2
-XX:+ParallelRefProcEnabled
Enables parallel reference processing. By default, this option is disabled.

-XX:+PrintAdaptiveSizePolicy
Enables printing of information about adaptive generation sizing. By default, this option is disabled.

-XX:+PrintGC
Enables printing of messages at every GC. By default, this option is disabled.

-XX:+PrintGCApplicationConcurrentTime
Enables printing of how much time elapsed since the last pause (for example, a GC pause). By default, this option is disabled.

-XX:+PrintGCApplicationStoppedTime
Enables printing of how much time the pause (for example, a GC pause) lasted. By default, this option is disabled.

-XX:+PrintGCDateStamps
Enables printing of a date stamp at every GC. By default, this option is disabled.

-XX:+PrintGCDetails
Enables printing of detailed messages at every GC. By default, this option is disabled.

-XX:+PrintGCTaskTimeStamps
Enables printing of time stamps for every individual GC worker thread task. By default, this option is disabled.

-XX:+PrintGCTimeStamps
Enables printing of time stamps at every GC. By default, this option is disabled.

-XX:+PrintStringDeduplicationStatistics
Prints detailed deduplication statistics. By default, this option is disabled. See the -XX:+UseStringDeduplication option.

-XX:+PrintTenuringDistribution
Enables printing of tenuring age information. The following is an example of the output:

Desired survivor size 48286924 bytes, new threshold 10 (max 10)
– age 1: 28992024 bytes, 28992024 total
– age 2: 1366864 bytes, 30358888 total
– age 3: 1425912 bytes, 31784800 total

Age 1 objects are the youngest survivors (they were created after the previous scavenge, survived the latest scavenge, and moved from eden to survivor space). Age 2 objects have survived two scavenges (during the second scavenge they were copied from one survivor space to the next). And so on.

In the preceding example, 28 992 024 bytes survived one scavenge and were copied from eden to survivor space, 1 366 864 bytes are occupied by age 2 objects, etc. The third value in each row is the cumulative size of objects of age n or less.

By default, this option is disabled.

-XX:+ScavengeBeforeFullGC
Enables GC of the young generation before each full GC. This option is enabled by default. Oracle recommends that you do not disable it, because scavenging the young generation before a full GC can reduce the number of objects reachable from the old generation space into the young generation space. To disable GC of the young generation before each full GC, specify -XX:-ScavengeBeforeFullGC.

-XX:SoftRefLRUPolicyMSPerMB=time
Sets the amount of time (in milliseconds) a softly reachable object is kept active on the heap after the last time it was referenced. The default value is one second of lifetime per free megabyte in the heap. The -XX:SoftRefLRUPolicyMSPerMB option accepts integer values representing milliseconds per one megabyte of the current heap size (for Java HotSpot Client VM) or the maximum possible heap size (for Java HotSpot Server VM). This difference means that the Client VM tends to flush soft references rather than grow the heap, whereas the Server VM tends to grow the heap rather than flush soft references. In the latter case, the value of the -Xmx option has a significant effect on how quickly soft references are garbage collected.

The following example shows how to set the value to 2.5 seconds:

-XX:SoftRefLRUPolicyMSPerMB=2500
-XX:StringDeduplicationAgeThreshold=threshold
String objects reaching the specified age are considered candidates for deduplication. An object’s age is a measure of how many times it has survived garbage collection. This is sometimes referred to as tenuring; see the -XX:+PrintTenuringDistribution option. Note that String objects that are promoted to an old heap region before this age has been reached are always considered candidates for deduplication. The default value for this option is 3. See the -XX:+UseStringDeduplication option.

-XX:SurvivorRatio=ratio
Sets the ratio between eden space size and survivor space size. By default, this option is set to 8. The following example shows how to set the eden/survivor space ratio to 4:

-XX:SurvivorRatio=4
-XX:TargetSurvivorRatio=percent
Sets the desired percentage of survivor space (0 to 100) used after young garbage collection. By default, this option is set to 50%.

The following example shows how to set the target survivor space ratio to 30%:

-XX:TargetSurvivorRatio=30
-XX:TLABSize=size
Sets the initial size (in bytes) of a thread-local allocation buffer (TLAB). Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, g or G to indicate gigabytes. If this option is set to 0, then the JVM chooses the initial size automatically.

The following example shows how to set the initial TLAB size to 512 KB:

-XX:TLABSize=512k
-XX:+UseAdaptiveSizePolicy
Enables the use of adaptive generation sizing. This option is enabled by default. To disable adaptive generation sizing, specify -XX:-UseAdaptiveSizePolicy and set the size of the memory allocation pool explicitly (see the -XX:SurvivorRatio option).

-XX:+UseCMSInitiatingOccupancyOnly
Enables the use of the occupancy value as the only criterion for initiating the CMS collector. By default, this option is disabled and other criteria may be used.

-XX:+UseConcMarkSweepGC
Enables the use of the CMS garbage collector for the old generation. Oracle recommends that you use the CMS garbage collector when application latency requirements cannot be met by the throughput (-XX:+UseParallelGC) garbage collector. The G1 garbage collector (-XX:+UseG1GC) is another alternative.

By default, this option is disabled and the collector is chosen automatically based on the configuration of the machine and type of the JVM. When this option is enabled, the -XX:+UseParNewGC option is automatically set and you should not disable it, because the following combination of options has been deprecated in JDK 8: -XX:+UseConcMarkSweepGC -XX:-UseParNewGC.

-XX:+UseG1GC
Enables the use of the garbage-first (G1) garbage collector. It is a server-style garbage collector, targeted for multiprocessor machines with a large amount of RAM. It meets GC pause time goals with high probability, while maintaining good throughput. The G1 collector is recommended for applications requiring large heaps (sizes of around 6 GB or larger) with limited GC latency requirements (stable and predictable pause time below 0.5 seconds).

By default, this option is disabled and the collector is chosen automatically based on the configuration of the machine and type of the JVM.

-XX:+UseGCOverheadLimit
Enables the use of a policy that limits the proportion of time spent by the JVM on GC before an OutOfMemoryError exception is thrown. This option is enabled, by default and the parallel GC will throw an OutOfMemoryError if more than 98% of the total time is spent on garbage collection and less than 2% of the heap is recovered. When the heap is small, this feature can be used to prevent applications from running for long periods of time with little or no progress. To disable this option, specify -XX:-UseGCOverheadLimit.

-XX:+UseNUMA
Enables performance optimization of an application on a machine with nonuniform memory architecture (NUMA) by increasing the application’s use of lower latency memory. By default, this option is disabled and no optimization for NUMA is made. The option is only available when the parallel garbage collector is used (-XX:+UseParallelGC).

-XX:+UseParallelGC
Enables the use of the parallel scavenge garbage collector (also known as the throughput collector) to improve the performance of your application by leveraging multiple processors.

By default, this option is disabled and the collector is chosen automatically based on the configuration of the machine and type of the JVM. If it is enabled, then the -XX:+UseParallelOldGC option is automatically enabled, unless you explicitly disable it.

-XX:+UseParallelOldGC
Enables the use of the parallel garbage collector for full GCs. By default, this option is disabled. Enabling it automatically enables the -XX:+UseParallelGC option.

-XX:+UseParNewGC
Enables the use of parallel threads for collection in the young generation. By default, this option is disabled. It is automatically enabled when you set the -XX:+UseConcMarkSweepGC option. Using the -XX:+UseParNewGC option without the -XX:+UseConcMarkSweepGC option was deprecated in JDK 8.

-XX:+UseSerialGC
Enables the use of the serial garbage collector. This is generally the best choice for small and simple applications that do not require any special functionality from garbage collection. By default, this option is disabled and the collector is chosen automatically based on the configuration of the machine and type of the JVM.

-XX:+UseSHM
On Linux, enables the JVM to use shared memory to setup large pages.

For more information, see “Large Pages”.

-XX:+UseStringDeduplication
Enables string deduplication. By default, this option is disabled. To use this option, you must enable the garbage-first (G1) garbage collector. See the -XX:+UseG1GC option.

String deduplication reduces the memory footprint of String objects on the Java heap by taking advantage of the fact that many String objects are identical. Instead of each String object pointing to its own character array, identical String objects can point to and share the same character array.

-XX:+UseTLAB
Enables the use of thread-local allocation blocks (TLABs) in the young generation space. This option is enabled by default. To disable the use of TLABs, specify -XX:-UseTLAB.

不推荐使用的选项和已经移除的选项

These options were included in the previous release, but have since been considered unnecessary.

-Xincgc
Enables incremental garbage collection. This option was deprecated in JDK 8 with no replacement.

-Xrunlibname
Loads the specified debugging/profiling library. This option was superseded by the -agentlib option.

-XX:CMSIncrementalDutyCycle=percent
Sets the percentage of time (0 to 100) between minor collections that the concurrent collector is allowed to run. This option was deprecated in JDK 8 with no replacement, following the deprecation of the -XX:+CMSIncrementalMode option.

-XX:CMSIncrementalDutyCycleMin=percent
Sets the percentage of time (0 to 100) between minor collections that is the lower bound for the duty cycle when -XX:+CMSIncrementalPacing is enabled. This option was deprecated in JDK 8 with no replacement, following the deprecation of the -XX:+CMSIncrementalMode option.

-XX:+CMSIncrementalMode
Enables the incremental mode for the CMS collector. This option was deprecated in JDK 8 with no replacement, along with other options that start with CMSIncremental.

-XX:CMSIncrementalOffset=percent
Sets the percentage of time (0 to 100) by which the incremental mode duty cycle is shifted to the right within the period between minor collections. This option was deprecated in JDK 8 with no replacement, following the deprecation of the -XX:+CMSIncrementalMode option.

-XX:+CMSIncrementalPacing
Enables automatic adjustment of the incremental mode duty cycle based on statistics collected while the JVM is running. This option was deprecated in JDK 8 with no replacement, following the deprecation of the -XX:+CMSIncrementalMode option.

-XX:CMSIncrementalSafetyFactor=percent
Sets the percentage of time (0 to 100) used to add conservatism when computing the duty cycle. This option was deprecated in JDK 8 with no replacement, following the deprecation of the -XX:+CMSIncrementalMode option.

-XX:CMSInitiatingPermOccupancyFraction=percent
Sets the percentage of the permanent generation occupancy (0 to 100) at which to start a GC. This option was deprecated in JDK 8 with no replacement.

-XX:MaxPermSize=size
Sets the maximum permanent generation space size (in bytes). This option was deprecated in JDK 8, and superseded by the -XX:MaxMetaspaceSize option.

-XX:PermSize=size
Sets the space (in bytes) allocated to the permanent generation that triggers a garbage collection if it is exceeded. This option was deprecated un JDK 8, and superseded by the -XX:MetaspaceSize option.

-XX:+UseSplitVerifier
Enables splitting of the verification process. By default, this option was enabled in the previous releases, and verification was split into two phases: type referencing (performed by the compiler) and type checking (performed by the JVM runtime). This option was deprecated in JDK 8, and verification is now split by default without a way to disable it.

-XX:+UseStringCache
Enables caching of commonly allocated strings. This option was removed from JDK 8 with no replacement.

Performance Tuning Examples

The following examples show how to use experimental tuning flags to either optimize throughput or to provide lower response time.

Example 1 – Tuning for Higher Throughput
java -d64 -server -XX:+AggressiveOpts -XX:+UseLargePages -Xmn10g -Xms26g -Xmx26g
Example 2 – Tuning for Lower Response Time
java -d64 -XX:+UseG1GC -Xms26g Xmx26g -XX:MaxGCPauseMillis=500 -XX:+PrintGCTimeStamp

Large Pages

Also known as huge pages, large pages are memory pages that are significantly larger than the standard memory page size (which varies depending on the processor and operating system). Large pages optimize processor Translation-Lookaside Buffers.

A Translation-Lookaside Buffer (TLB) is a page translation cache that holds the most-recently used virtual-to-physical address translations. TLB is a scarce system resource. A TLB miss can be costly as the processor must then read from the hierarchical page table, which may require multiple memory accesses. By using a larger memory page size, a single TLB entry can represent a larger memory range. There will be less pressure on TLB, and memory-intensive applications may have better performance.

However, large pages page memory can negatively affect system performance. For example, when a large mount of memory is pinned by an application, it may create a shortage of regular memory and cause excessive paging in other applications and slow down the entire system. Also, a system that has been up for a long time could produce excessive fragmentation, which could make it impossible to reserve enough large page memory. When this happens, either the OS or JVM reverts to using regular pages.

Large Pages Support

Solaris and Linux support large pages.

Solaris

Solaris 9 and later include Multiple Page Size Support (MPSS); no additional configuration is necessary. See http://www.oracle.com/technetwork/server-storage/solaris10/overview/solaris9-features-scalability-135663.html.

Linux

The 2.6 kernel supports large pages. Some vendors have backported the code to their 2.4-based releases. To check if your system can support large page memory, try the following:

# cat /proc/meminfo | grep Huge
HugePages_Total: 0
HugePages_Free: 0
Hugepagesize: 2048 kB
If the output shows the three “Huge” variables, then your system can support large page memory but it needs to be configured. If the command prints nothing, then your system does not support large pages. To configure the system to use large page memory, login as root, and then follow these steps:

If you are using the option -XX:+UseSHM (instead of -XX:+UseHugeTLBFS), then increase the SHMMAX value. It must be larger than the Java heap size. On a system with 4 GB of physical RAM (or less), the following will make all the memory sharable:

# echo 4294967295 > /proc/sys/kernel/shmmax
If you are using the option -XX:+UseSHM or -XX:+UseHugeTLBFS, then specify the number of large pages. In the following example, 3 GB of a 4 GB system are reserved for large pages (assuming a large page size of 2048kB, then 3 GB = 3 * 1024 MB = 3072 MB = 3072 * 1024 kB = 3145728 kB and 3145728 kB / 2048 kB = 1536):

# echo 1536 > /proc/sys/vm/nr_hugepages

Note:

Note that the values contained in /proc will reset after you reboot your system, so may want to set them in an initialization script (for example, rc.local or sysctl.conf).
If you configure (or resize) the OS kernel parameters /proc/sys/kernel/shmmax or /proc/sys/vm/nr_hugepages, Java processes may allocate large pages for areas in addition to the Java heap. These steps can allocate large pages for the following areas:

Java heap

Code cache

The marking bitmap data structure for the parallel GC

Consequently, if you configure the nr_hugepages parameter to the size of the Java heap, then the JVM can fail in allocating the code cache areas on large pages because these areas are quite large in size.

Application Class Data Sharing

Application Class Data Sharing (AppCDS) extends CDS (see https://docs.oracle.com/javase/8/docs/technotes/guides/vm/class-data-sharing.html) to enable classes from the standard extensions directories (specified by the system property java.ext.dirs; see https://docs.oracle.com/javase/8/docs/technotes/guides/extensions/spec.html) and the application class path (see “Setting the Class Path”) to be placed in the shared archive. AppCDS reduces the footprint and decreases start-up time of your applications provided that a substantial number of classes are loaded from the application class path.

This is a commercial feature that requires you to also specify the -XX:+UnlockCommercialFeatures option. This is also an experimental feature; it may change in future releases.

Creating a Shared Archive File, and Running an Application with It

The following steps create a shared archive file that contains all the classes used by the test.Hello application. The last step runs the application with the shared archive file.

Create a list of all classes used by the test.Hello application. The following command creates a file named hello.classlist that contains a list of all classes used by this application:

java -Xshare:off -XX:+UnlockCommercialFeatures -XX:DumpLoadedClassList=hello.classlist -XX:+UseAppCDS -cp hello.jar test.Hello

Note that the -cp parameter must contain only JAR files; the -XX:+UseAppCDS option does not support class paths that contain directory names.

Create a shared archive, named hello.jsa, that contains all the classes in hello.classlist:

java -XX:+UnlockCommercialFeatures -Xshare:dump -XX:+UseAppCDS -XX:SharedArchiveFile=hello.jsa -XX:SharedClassListFile=hello.classlist -cp hello.jar

Note that the -cp parameter used at archive creation time must be the same as (or a prefix of) the -cp used at run time.

Run the application test.Hello with the shared archive hello.jsa:

java -XX:+UnlockCommercialFeatures -Xshare:on -XX:+UseAppCDS -XX:SharedArchiveFile=hello.jsa -cp hello.jar test.Hello

Ensure that you have specified the option -Xshare:on or -Xshare:auto.

Verify that the test.Hello application is using the class contained in the hello.jsa shared archive:

java -XX:+UnlockCommercialFeatures -Xshare:on -XX:+UseAppCDS -XX:SharedArchiveFile=hello.jsa -cp hello.jar -verbose:class test.Hello

The output of this command should contain the following text:

Loaded test.Hello from shared objects file by sun/misc/Launcher$AppClassLoader

Sharing a Shared Archive across Multiple Application Processes

You can share the same archive file across multiple applications processes that have the exact same class path or share a common class path prefix. This reduces memory usage as the archive is memory-mapped into the address space of the processes. The operating system automatically shares the read-only pages across these processes.

The following steps create a shared archive that both applications Hello and Hi can use.

Create a list of all classes used by the Hello application and another list for the Hi application:

java -XX:+UnlockCommercialFeatures -XX:DumpLoadedClassList=hello.classlist -XX:+UseAppCDS -cp common.jar:hello.jar Hello

java -XX:+UnlockCommercialFeatures -XX:DumpLoadedClassList=hi.classlist -XX:+UseAppCDS -cp common.jar:hi.jar Hi

Note that because the Hello and Hi applications share a common class path prefix (both of their class paths start with common.jar), these two applications can share a shared archive file.

Create a single list of classes used by all the applications that will share the shared archive file.

The following commands combine the files hello.classlist and hi.classlist to one file, common.classlist:

cat hello.classlist hi.classlist > common.classlist

Create a shared archive, named common.jsa, that contains all the classes in common.classlist:

java -XX:+UnlockCommercialFeatures -Xshare:dump -XX:SharedArchiveFile=common.jsa -XX:+UseAppCDS -XX:SharedClassListFile=common.classlist -cp common.jar

The value of the -cp parameter is the common class path prefix shared by the Hello and Hi applications.

Run the Hello and Hi applications with the same shared archive:

java -XX:+UnlockCommercialFeatures -Xshare:on -XX:SharedArchiveFile=common.jsa -XX:+UseAppCDS -cp common.jar:hello.jar Hello

java -XX:+UnlockCommercialFeatures -Xshare:on -XX:SharedArchiveFile=common.jsa -XX:+UseAppCDS -cp common.jar:hi.jar Hi

退出状态

当launcher被调用了错误的参数,严重的错误,或JVM抛出了异常,如下的状态码是通常由launcher返回的。然而,应用也可以通过 System.exit(exitValue) API 返回状态码。
可能的值为:

0: 执行成功。

>0: 发错误。

参考文献