jdk安装后会自带一些小工具，jmap命令(Java Memory Map)是其中之一。主要用于打印指定Java进程(或核心文件、远程调试服务器)的共享对象内存映射或堆内存细节。

例1：
命令：jmap -heap pid
描述：显示Java堆详细信息
打印一个堆的摘要信息，包括使用的GC算法、堆配置信息和各内存区域内存使用信息

jmap -heap 5611
Attaching to process ID 5611, please wait...
Debugger attached successfully.
Client compiler detected.
JVM version is 25.271-b09

using thread-local object allocation.
Mark Sweep Compact GC

Heap Configuration:
   MinHeapFreeRatio         = 40
   MaxHeapFreeRatio         = 70
   MaxHeapSize              = 268435456 (256.0MB)
   NewSize                  = 5570560 (5.3125MB)
   MaxNewSize               = 89456640 (85.3125MB)
   OldSize                  = 11206656 (10.6875MB)
   NewRatio                 = 2
   SurvivorRatio            = 8
   MetaspaceSize            = 12582912 (12.0MB)
   CompressedClassSpaceSize = 1073741824 (1024.0MB)
   MaxMetaspaceSize         = 4294963200 (4095.99609375MB)
   G1HeapRegionSize         = 0 (0.0MB)

Heap Usage:
New Generation (Eden + 1 Survivor Space):
   capacity = 5046272 (4.8125MB)
   used     = 1646832 (1.5705413818359375MB)
   free     = 3399440 (3.2419586181640625MB)
   32.63462611607143% used
Eden Space:
   capacity = 4521984 (4.3125MB)
   used     = 1614560 (1.539764404296875MB)
   free     = 2907424 (2.772735595703125MB)
   35.70468183876812% used
From Space:
   capacity = 524288 (0.5MB)
   used     = 32272 (0.0307769775390625MB)
   free     = 492016 (0.4692230224609375MB)
   6.1553955078125% used
To Space:
   capacity = 524288 (0.5MB)
   used     = 0 (0.0MB)
   free     = 524288 (0.5MB)
   0.0% used
tenured generation:
   capacity = 11206656 (10.6875MB)
   used     = 7409520 (7.0662689208984375MB)
   free     = 3797136 (3.6212310791015625MB)
   66.11713610197368% used

5109 interned Strings occupying 397496 bytes.

例2
命令：jmap -histo:live pid
描述：显示堆中对象的统计信息

其中包括每个Java类、对象数量、内存大小(单位：字节)、完全限定的类名。打印的虚拟机内部的类名称将会带有一个’*’前缀。如果指定了live子选项，则只计算活动的对象。

num     #instances         #bytes  class name
----------------------------------------------
   1:          2580        2986640  [B
   2:         27304        2316368  [C
   3:          7626        1577272  [I
   4:          8714         418272  java.util.concurrent.ConcurrentHashMap$EntryIterator
   5:         25061         400976  java.lang.String
   6:          6301         296872  [Ljava.lang.Object;
   7:          2981         284776  java.lang.Class
   8:          8714         209136  java.util.concurrent.ConcurrentHashMap$MapEntry
   9:          8679         208296  java.util.HashMap$Node
  10:          4289         171560  java.util.HashMap
  11:          3617         146984  [Ljava.util.HashMap$Node;
  12:          1425         125400  java.lang.reflect.Method
  13:          1676         120672  java.lang.reflect.Constructor
  14:          1776         113664  java.util.regex.Matcher
  15:          3480          83520  java.util.Hashtable$Entry
  16:          3046          73104  java.util.concurrent.ConcurrentHashMap$Node
  17:           687          71448  java.io.ObjectStreamClass
  18:           723          63816  [Ljava.util.Hashtable$Entry;
  19:           155          58280  java.lang.Thread
  20:          1750          56000  java.io.ObjectStreamClass$WeakClassKey