当java应用程序出现OOM(内存溢出)异常时，jmap是我们常用的排错指令，此指令能让我们看到jvm heap内存信息，java进程中每个对象有多少个实例，占用多少内存等信息

例一：查看jvm heap(堆内存)信息

[www@iZbp1j3zm4hugfnv44puytZ ~]$ jmap -heap 2493
Attaching to process ID 2493, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 25.73-b02

using parallel threads in the new generation.
using thread-local object allocation.
Concurrent Mark-Sweep GC

Heap Configuration:
   MinHeapFreeRatio         = 40
   MaxHeapFreeRatio         = 70
   MaxHeapSize              = 1258291200 (1200.0MB)
   NewSize                  = 1073741824 (1024.0MB)
   MaxNewSize               = 1073741824 (1024.0MB)
   OldSize                  = 184549376 (176.0MB)
   NewRatio                 = 2
   SurvivorRatio            = 8
   MetaspaceSize            = 21807104 (20.796875MB)
   CompressedClassSpaceSize = 1073741824 (1024.0MB)
   MaxMetaspaceSize         = 17592186044415 MB
   G1HeapRegionSize         = 0 (0.0MB)

Heap Usage:
New Generation (Eden + 1 Survivor Space):
   capacity = 966393856 (921.625MB)
   used     = 966393848 (921.6249923706055MB)
   free     = 8 (7.62939453125E-6MB)
   99.99999917218017% used
Eden Space:
   capacity = 859045888 (819.25MB)
   used     = 859045880 (819.2499923706055MB)
   free     = 8 (7.62939453125E-6MB)
   99.99999906873427% used
From Space:
   capacity = 107347968 (102.375MB)
   used     = 107347968 (102.375MB)
   free     = 0 (0.0MB)
   100.0% used
To Space:
   capacity = 107347968 (102.375MB)
   used     = 0 (0.0MB)
   free     = 107347968 (102.375MB)
   0.0% used
concurrent mark-sweep generation:
   capacity = 184549376 (176.0MB)
   used     = 184549344 (175.99996948242188MB)
   free     = 32 (3.0517578125E-5MB)
   99.99998266046697% used

20905 interned Strings occupying 2220488 bytes.
[www@iZbp1j3zm4hugfnv44puytZ ~]$ 

上面2493为进程号：
jvm heap(堆)内存主要分为：
　　新生代（Young Generation）又分为：Eden区和Survivor区，Survivor区有分为From Space和To Space。Eden区是对象最初分配到的地方；默认情况下，From Space和To Space的区域大小相等。
　　老年代（Old Generation）
　　永久代（Permanent Generation）

例二：java进程中每个对象有多少个实例，占用多少内存信息

[root@localhost ~]# jmap -histo 25453 | head -n 20

 num     #instances         #bytes  class name
----------------------------------------------
   1:        220855       27984664  [C
   2:         19802       18777128  [B
   3:          8735       11026312  [I
   4:        211785        5082840  java.lang.String
   5:         52714        5060544  java.util.jar.JarFile$JarFileEntry
   6:         94705        3030560  java.util.HashMap$Node
   7:         11058        1554912  [Ljava.util.HashMap$Node;
   8:         10760        1207792  java.lang.Class
   9:         21710        1136096  [Ljava.lang.Object;
  10:         32501        1040032  java.util.concurrent.ConcurrentHashMap$Node
  11:         11019         969672  java.lang.reflect.Method
  12:         15147         969408  java.net.URL
  13:         17583         703320  java.util.LinkedHashMap$Entry
  14:          9742         545552  java.util.LinkedHashMap
  15:         11037         529776  java.util.HashMap
  16:          9381         525336  java.util.concurrent.ConcurrentHashMap$ValueIterator
  17:          9147         439056  org.apache.catalina.loader.ResourceEntry
Total       1089836       92194448

上面25453为进程号,因为输出太多只取前20行：
第一列，序号，无实际意义
第二列，对象实例数量
第三列，对象实例占用总内存数，单位：字节
第四列，对象实例名称
最后一行，总实例数量与总内存占用数

另，部分示例说明（还没弄明白，后续补充）：
[C，
[B，
[I，
[J，
[L，

参考：
http://www.importnew.com/15671.html
https://blog.csdn.net/zlzlei/article/details/46472729