JVM调试工具-jmap

jmap

jmap命令是一个可以输出所有内存中对象的工具，甚至可以将VM 中的heap，以二进制输出成文本。

打印出某个java进程（使用pid）内存内的，所有‘对象’的情况（如：产生那些对象，及其数量）。

命令用法：jmap [option]
(to connect to running process) 连接到正在运行的进程

jmap [option] <executable <core>
    (to connect to a core file)     连接到核心文件

jmap [option] [[email protected]]<remote server IP or hostname>
    (to connect to remote debug server) 连接到远程调试服务
    
pid:    目标进程的PID，进程编号，可以采用ps -ef | grep java 查看java进程的PID;

executable:     产生core dump的java可执行程序;

core:     将被打印信息的core dump文件;

remote-hostname-or-IP:     远程debug服务的主机名或ip;

server-id:     唯一id,假如一台主机上多个远程debug服务;

常用命令

打印Java堆概要信息，包括使用的GC算法、堆配置参数和各代中堆内存使用情况

> jmap -heap 23766
Attaching to process ID 31846, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 24.71-b01
 
using thread-local object allocation.
Parallel GC with 4 thread(s)//GC 方式
 
Heap Configuration: //堆内存初始化配置
   MinHeapFreeRatio = 0 //对应jvm启动参数-XX:MinHeapFreeRatio设置JVM堆最小空闲比率(default 40)
   MaxHeapFreeRatio = 100 //对应jvm启动参数 -XX:MaxHeapFreeRatio设置JVM堆最大空闲比率(default 70)
   MaxHeapSize      = 2082471936 (1986.0MB) //对应jvm启动参数-XX:MaxHeapSize=设置JVM堆的最大大小
   NewSize          = 1310720 (1.25MB)//对应jvm启动参数-XX:NewSize=设置JVM堆的‘新生代’的默认大小
   MaxNewSize       = 17592186044415 MB//对应jvm启动参数-XX:MaxNewSize=设置JVM堆的‘新生代’的最大大小
   OldSize          = 5439488 (5.1875MB)//对应jvm启动参数-XX:OldSize=<value>:设置JVM堆的‘老生代’的大小
   NewRatio         = 2 //对应jvm启动参数-XX:NewRatio=:‘新生代’和‘老生代’的大小比率
   SurvivorRatio    = 8 //对应jvm启动参数-XX:SurvivorRatio=设置年轻代中Eden区与Survivor区的大小比值 
   PermSize         = 21757952 (20.75MB)  //对应jvm启动参数-XX:PermSize=<value>:设置JVM堆的‘永生代’的初始大小
   MaxPermSize      = 85983232 (82.0MB)//对应jvm启动参数-XX:MaxPermSize=<value>:设置JVM堆的‘永生代’的最大大小
   G1HeapRegionSize = 0 (0.0MB)
 
Heap Usage://堆内存使用情况
PS Young Generation
Eden Space://Eden区内存分布
   capacity = 33030144 (31.5MB)//Eden区总容量
   used     = 1524040 (1.4534378051757812MB)  //Eden区已使用
   free     = 31506104 (30.04656219482422MB)  //Eden区剩余容量
   4.614088270399305% used //Eden区使用比率
From Space:  //其中一个Survivor区的内存分布
   capacity = 5242880 (5.0MB)
   used     = 0 (0.0MB)
   free     = 5242880 (5.0MB)
   0.0% used
To Space:  //另一个Survivor区的内存分布
   capacity = 5242880 (5.0MB)
   used     = 0 (0.0MB)
   free     = 5242880 (5.0MB)
   0.0% used
PS Old Generation //当前的Old区内存分布
   capacity = 86507520 (82.5MB)
   used     = 0 (0.0MB)
   free     = 86507520 (82.5MB)
   0.0% used
PS Perm Generation//当前的 “永生代” 内存分布
   capacity = 22020096 (21.0MB)
   used     = 2496528 (2.3808746337890625MB)
   free     = 19523568 (18.619125366210938MB)
   11.337498256138392% used
 
670 interned Strings occupying 43720 bytes.

打印Java堆中对象直方图，通过该图可以获取每个class的对象数目，占用内存大小和类全名信息，带上:live，则只统计活着的对象

> jmap -histo 23766

23766: Unable to open socket file: target process not responding or HotSpot VM not loaded
The -F option can be used when the target process is not responding

> jmap -F -histo 23766
Attaching to process ID 23766, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 25.191-b12
Iterating over heap. This may take a while...

 num     #instances         #bytes  class name
----------------------------------------------
   1:        245237       26378800  [C
   2:         18355       19262448  [B
   3:         45062       14682856  [I
   4:        245389        5889336  java.util.HashMap$Node
   5:        274529        4392464  java.lang.String
   6:         44989        4087960  [Ljava.util.HashMap$Node;
   7:         68684        3150440  [Ljava.lang.Object;
   8:         18342        1841288  java.lang.Class
   9:         28573        1828672  java.lang.reflect.Field
  10:         42818        1712720  java.util.HashMap
  11:         30488        1556088  [Ljava.lang.String;
  12:         32420        1037440  java.util.LinkedHashMap$Entry

打印等待回收的对象信息

>  jmap -finalizerinfo 23766

Attaching to process ID 23766, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 25.191-b12
Number of objects pending for finalization: 0

# 说明当前F-QUEUE队列中并没有等待Fializer线程执行finalizer方法的对象。

以hprof二进制格式将Java堆信息输出到文件内，该文件可以用MAT、VisualVM或jhat等工具查看

# 输出到文件，注意此时会暂停应用
> jmap -dump:live,format=b,file=heapDump 23766

Dumping heap to /home/oidd/heapDump ...
Heap dump file created

# 通过浏览器访问
> jhat -port 5000 heapDump

Reading from heapDump...
Dump file created Thu Sep 17 08:43:21 CST 2020
Snapshot read, resolving...
Resolving 314429 objects...
Chasing references, expect 62 dots..............................................................
Eliminating duplicate references..............................................................
Snapshot resolved.
Started HTTP server on port 5000
Server is ready.


访问：http://localhost:5000/