jcmd工具在Java 9中的重要性是什么？

“jcmd”是 JVM 诊断工具，它是一个命令行工具，用于在本地针对给定 JVM 运行诊断命令机器。该工具从Java 7版本开始就包含在JDK安装中，可以用“%java_home%\bin\jcmd.exe”程序文件表示。如果我们在环境变量“path”中包含“%java_home%\bin”目录，我们可以运行“jcmd -h”命令来查看所有目录的完整列表选项如下

C:\Users\User>jcmd -h
Usage: jcmd
   or: jcmd -l
   or: jcmd -h

   command must be a valid jcmd command for the selected jvm.
   Use the command "help" to see which commands are available.
   If the pid is 0, commands will be sent to all Java processes.
   The main class argument will be used to match (either partially
   or fully) the class used to start Java.
   If no options are given, lists Java processes (same as -l).

   PerfCounter.print display the counters exposed by this process
   -f read and execute commands from the file
   -l list JVM processes on the local machine
   -h this help

示例

public class JCmdToolTest {
   public static void main(String args[]) {
      Runtime runtime = Runtime.getRuntime();
      System.out.println("Free memory: " + runtime.freeMemory());
      System.out.println("Total memory: " + runtime.totalMemory());
      try {
         Thread.sleep(5000);
      } catch(InterruptedException e) {
      }
   }
}

输出

Free memory: 65454560
Total memory: 67108864

我们可以使用“jcmd -l”命令列出本地上所有正在运行的 JVM机器，然后使用输出中的 PID 或类 ma​​in 来识别 JVM。

C:\Users\User>jcmd -l
6108 jdk.jcmd/sun.tools.jcmd.JCmd -l

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