JVM基础篇

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1.JVM的组成 

  • 类加载器(ClassLoader) 
  • 运行时数据区(Runtime Data Area) 
  • (堆、栈) 执行引擎(Execution Engine)
  •  c++/C 本地库接口(Native Interface) 

2. jvm的运行参数 

2.1 三种参数类型 

jvm的参数类型分为三类,分别是: 

 标准参数   -help   -version 

 -X参数 (非标准参数)   -Xint    -Xcomp 

 -XX参数(非stable参数) -XX:newSize   -XX:+UseSerialGC  

2.2 标准参数 ​ 

jvm的标准参数,一般都是很稳定的,在未来的JVM版本中不会改变,可以使用java -help 检索出所有的标准参数。

[liuyh@VMcentos jdk1.8.0_231]$ java -help
Usage: java [-options] class [args...]
           (to execute a class)
   or  java [-options] -jar jarfile [args...]
           (to execute a jar file)
where options include:
    -d32          use a 32-bit data model if available
    -d64          use a 64-bit data model if available
    -server       to select the "server" VM
                  The default VM is server.

    -cp <class search path of directories and zip/jar files>
    -classpath <class search path of directories and zip/jar files>
                  A : separated list of directories, JAR archives,
                  and ZIP archives to search for class files.
    -D<name>=<value>
                  set a system property
    -verbose:[class|gc|jni]
                  enable verbose output
    -version      print product version and exit
    -version:<value>
                  Warning: this feature is deprecated and will be removed
                  in a future release.
                  require the specified version to run
    -showversion  print product version and continue
    -jre-restrict-search | -no-jre-restrict-search
                  Warning: this feature is deprecated and will be removed
                  in a future release.
                  include/exclude user private JREs in the version search
    -? -help      print this help message
    -X            print help on non-standard options
    -ea[:<packagename>...|:<classname>]
    -enableassertions[:<packagename>...|:<classname>]
                  enable assertions with specified granularity
    -da[:<packagename>...|:<classname>]
    -disableassertions[:<packagename>...|:<classname>]
                  disable assertions with specified granularity
    -esa | -enablesystemassertions
                  enable system assertions
    -dsa | -disablesystemassertions
                  disable system assertions
    -agentlib:<libname>[=<options>]
                  load native agent library <libname>, e.g. -agentlib:hprof
                  see also, -agentlib:jdwp=help and -agentlib:hprof=help
    -agentpath:<pathname>[=<options>]
                  load native agent library by full pathname
    -javaagent:<jarpath>[=<options>]
                  load Java programming language agent, see java.lang.instrument
    -splash:<imagepath>
                  show splash screen with specified image
See http://www.oracle.com/technetwork/java/javase/documentation/index.html for more details.

2.2.1 -d32和-d64

​ 指定程序是运行在32位还是64位环境。java -version命令可以查看到默认的运行环境(64-Bit):

[liuyh@VMcentos jdk1.8.0_231]$ java -version
java version "1.8.0_231"
Java(TM) SE Runtime Environment (build 1.8.0_231-b11)
Java HotSpot(TM) 64-Bit Server VM (build 25.231-b11, mixed mode)

2.2.2 -client和-server

​ 可以通过-server或-client设置jvm的运行参数。java -version命令可以查看到默认的运行环境(Server VM):

[liuyh@VMcentos jdk1.8.0_231]$ java -version
java version "1.8.0_231"
Java(TM) SE Runtime Environment (build 1.8.0_231-b11)
Java HotSpot(TM) 64-Bit Server VM (build 25.231-b11, mixed mode)

  • 它们的区别是Server VM的初始堆空间会大一些,默认使用的是并行垃圾回收器,启 动慢运行快。  
  • Client VM相对来讲会保守一些,初始堆空间会小一些,使用串行的垃圾回收器,它 的目标是为了让JVM的启动速度更快,但运行速度会比Serverm模式慢些。
  •  JVM在启动的时候会根据硬件和操作系统自动选择使用Server还是Client类型的 JVM。 

 2.2.3 -D设置系统属性参数 

public class JVMTest {

    public static void main(String[] args) {
        String test = System.getProperty("test");
        if (test == null) {
            System.out.println("系统属性test为空");
        } else {
            System.out.println("系统属性test= "+test);
        }
    }
}

启动参数VM option增加-Dtest=20200406,运行后结果为

系统属性test= 20200406

2.3 非标准参数

​ jvm的-X参数是非标准参数,在不同版本的jvm中,参数可能会有所不同,可以通过java -
X查看非标准参数。

[liuyh@VMcentos jdk1.8.0_231]$ java -X
    -Xmixed           mixed mode execution (default)
    -Xint             interpreted mode execution only
    -Xbootclasspath:<directories and zip/jar files separated by :>
                      set search path for bootstrap classes and resources
    -Xbootclasspath/a:<directories and zip/jar files separated by :>
                      append to end of bootstrap class path
    -Xbootclasspath/p:<directories and zip/jar files separated by :>
                      prepend in front of bootstrap class path
    -Xdiag            show additional diagnostic messages
    -Xnoclassgc       disable class garbage collection
    -Xincgc           enable incremental garbage collection
    -Xloggc:<file>    log GC status to a file with time stamps
    -Xbatch           disable background compilation
    -Xms<size>        set initial Java heap size
    -Xmx<size>        set maximum Java heap size
    -Xss<size>        set java thread stack size
    -Xprof            output cpu profiling data
    -Xfuture          enable strictest checks, anticipating future default
    -Xrs              reduce use of OS signals by Java/VM (see documentation)
    -Xcheck:jni       perform additional checks for JNI functions
    -Xshare:off       do not attempt to use shared class data
    -Xshare:auto      use shared class data if possible (default)
    -Xshare:on        require using shared class data, otherwise fail.
    -XshowSettings    show all settings and continue
    -XshowSettings:all
                      show all settings and continue
    -XshowSettings:vm show all vm related settings and continue
    -XshowSettings:properties
                      show all property settings and continue
    -XshowSettings:locale
                      show all locale related settings and continue

The -X options are non-standard and subject to change without notice.

2.3.1 -Xint、-Xcomp、-Xmixed 

  • 在解释模式(interpreted mode)下,-Xint标记会强制JVM执行所有的字节码,当然这 会降低运行速度,通常低10倍或更多。  
  • -Xcomp纯编译模式(如果方法无法编译,则回退到解释模式执行无法编译的方法) 
  • -Xmixed是混合模式,将解释模式与编译模式进行混合使用,由jvm自己决定,这是 jvm默认的模式,也是推荐使用的模式。  

2.3.2 -Xms与-Xmx参数 

-Xms与-Xmx分别是设置jvm的堆内存的初始大小和最大大小。 

-Xmx2048m:等价于-XX:MaxHeapSize,设置JVM最大堆内存为2048M。 

-Xms512m:等价于-XX:InitialHeapSize,设置JVM初始堆内存为1024M。 

[liuyh@VMcentos jvm]$ java -Xms1024m -Xmx2048m JVMTest
系统属性test为空

2.4 非Stable参数(-XX) 

这些参数可以被松散的聚合成三类:  

  • 行为参数(Behavioral Options):用于改变jvm的一些基础行为; 
  • 性能调优(Performance Tuning):用于jvm的性能调优; 
  • 调试参数(Debugging Options):一般用于打开跟踪、打印、输出等jvm参数,用于显示jvm更加详细的信息。
 -XX参数的使用有2种方式,一种是boolean类型,一种是非boolean类型: 

 boolean类型 

  •  格式:-XX:[±] 
  • 如:-XX:+DisableExplicitGC 表示禁用手动调用gc操作,也就是说调用System.gc()无效  

非boolean类型 

  •  格式:-XX: 
  •  如:-XX:NewRatio=1 表示新生代和老年代的比值 

2.5 查看jvm的运行参数 

2.5.1 运行java命令时打印参数 

使用java -XX:+PrintFlagsFinal -version打印运行参数 结果中参数有boolean类型和数字类型,值的操作符是=或:=,分别代 表默认值和被修改的值。  

2.5.2查看正在运行的jvm参数 

  • 使用jps -l查看java进程 
  • 然后查看某一参数的值,用法:jinfo ‐flag <参数名> <进程id> 

[liuyh@VMcentos jvm]$ jps -l
6882 sun.tools.jps.Jps
1612 /home/liuyh/apache-activemq-5.15.11//bin/activemq.jar
[liuyh@VMcentos jvm]$ jinfo -flag MaxHeapSize 1612
-XX:MaxHeapSize=1073741824