jcmd是一款命令行工具，可以监控jvm虚拟机性能和诊断问题。

GC.class_stats

如果报错：
GC.class_stats command requires -XX:+UnlockDiagnosticVMOptions

在启动脚本中添加参数：
-XX:+UnlockDiagnosticVMOptions

内容太多了，只选前10换行吧
jcmd 25274 GC.class_stats | head

25274:
Index Super InstBytes KlassBytes annotations    CpAll MethodCount Bytecodes MethodAll    ROAll    RWAll    Total ClassName
    1    -1  12764600        480           0        0           0         0         0       24      584      608 [C
    2 10302   5398800       1112           0     5600          40       847     20904     4224    24072    28296 java.lang.reflect.Method
    3    18   3961152        568           0     1376           8       223      1744     1032     2976     4008 java.util.concurrent.ConcurrentHashMap$Node
    4    18   3175992        624           0     8784          94      4623     51968    12144    50312    62456 java.lang.String
    5    18   2488704        632           0     2280          10       248      4760     1640     6400     8040 org.aspectj.weaver.reflect.ShadowMatchImpl

jcmd 25274 Thread.print

也来一段输出吧，根本看不懂。

"Finalizer" #3 daemon prio=8 os_prio=0 tid=0x00007f4b2814b800 nid=0x62cc in Object.wait() [0x00007f4b08513000]
   java.lang.Thread.State: WAITING (on object monitor)
        at java.lang.Object.wait(Native Method)
        at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:143)
        - locked <0x00000000c03a3948> (a java.lang.ref.ReferenceQueue$Lock)
        at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:164)
        at java.lang.ref.Finalizer$FinalizerThread.run(Finalizer.java:212)

"Reference Handler" #2 daemon prio=10 os_prio=0 tid=0x00007f4b28147000 nid=0x62cb in Object.wait() [0x00007f4b08614000]
   java.lang.Thread.State: WAITING (on object monitor)
        at java.lang.Object.wait(Native Method)
        at java.lang.Object.wait(Object.java:502)
        at java.lang.ref.Reference.tryHandlePending(Reference.java:191)
        - locked <0x00000000c03a2e88> (a java.lang.ref.Reference$Lock)
        at java.lang.ref.Reference$ReferenceHandler.run(Reference.java:153)

"VM Thread" os_prio=0 tid=0x00007f4b2813f800 nid=0x62ca runnable

"GC task thread#0 (ParallelGC)" os_prio=0 tid=0x00007f4b2801f800 nid=0x62c4 runnable

"GC task thread#1 (ParallelGC)" os_prio=0 tid=0x00007f4b28021000 nid=0x62c6 runnable

"GC task thread#2 (ParallelGC)" os_prio=0 tid=0x00007f4b28023000 nid=0x62c8 runnable

"GC task thread#3 (ParallelGC)" os_prio=0 tid=0x00007f4b28025000 nid=0x62c9 runnable

jcmd 25274 GC.run

对25274进程手动进行垃圾回收。

这个倒是简单，一般都是返回成功。

jcmd 25274 GC.run

返回内容：
25274:
Command executed successfully

怎么感觉只有这个命令能用的上，其他的根本看不懂。

其他

文档(命令行)

这文档可够简洁的。

 jcmd -h
 
Usage: jcmd <pid | main class> <command ...|PerfCounter.print|-f file>
   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 -p).

  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