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：

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：

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：

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]

-client

-Dproperty=value
设置系统属性值。

-d32

-d64

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

-disablesystemassertions
-dsa

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

-enablesystemassertions
-esa

-help/-?

-jar filename

• jar(1)

• The Java Archive (JAR) Files guide at http://docs.oracle.com/javase/8/docs/technotes/guides/jar/index.html

• Lesson: Packaging Programs in JAR Files at

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

-javaagent:jarpath[=options]

-jre-restrict-search

-no-jre-restrict-search

-server

-showversion

-splash:imgname

-verbose:class

-verbose:gc

-verbose:jni

-version

-version:release

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

-Xbatch

-Xbootclasspath:path

-Xbootclasspath/a:path

-Xbootclasspath/p:path

-Xcheck:jni

-Xcomp

-Xdebug
Java8中该选项用于兼容以前版本。只是为了兼容。

-Xdiag

-Xfuture

-Xint

-Xinternalversion

-Xloggc:filename

-Xmaxjitcodesize=size

-Xmixed

-Xmnsize

-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：

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或-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：

该-Xmx选项等效于-XX:MaxHeapSize。

-Xnoclassgc

-Xprof

-Xrs

-Xshare:mode

-XshowSettings:category

-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：

下面的示例设置线程的堆栈大小为1024KB：

此选项等同于-XX:ThreadStackSize。

-Xusealtsigs

-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

-XX:+DisableAttachMechanism

-XX:ErrorFile=filename

-XX:+FailOverToOldVerifier

-XX:+FlightRecorder

-XX:-FlightRecorder

-XX:FlightRecorderOptions=parameter=value

-XX:LargePageSizeInBytes=size

-XX:MaxDirectMemorySize=size

-XX:NativeMemoryTracking=mode

-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：

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

• http://docs.oracle.com/javase/8/docs/technotes/tools/unix/java.html#BGBCIEFC