where <option> is one of:   

<none>               to print same info as Solaris pmap   
-heap                to print java heap summary  
-histo[:live]        to print histogram of java object heap; if the "live" suboption is specified, only count live objects      
-permstat            to print permanent generation statistics  
-finalizerinfo       to print information on objects awaiting finalization   
-dump:<dump-options> to dump java heap in hprof binary format                        
　　　　　　　　　　    dump-options:  
                     live         dump only live objects; if not specified,all objects in the heap are dumped.                             
　　　　　　　　　　    format=b     binary format  
                     file=<file>  dump heap to <file>  
                     Example: jmap -dump:live,format=b,file=heap.bin <pid>      
-F                   force. Use with -dump:<dump-options> <pid> or -histo to force a heap dump or histogram when <pid> does not respond. The "live" suboption is not supported  in this mode.  
-h | -help           to print this help message  
-J<flag>             to pass <flag> directly to the runtime system  

参数如下：  
-heap：打印jvm heap的情况  
-histo：打印jvm heap的直方图。其输出信息包括类名，对象数量，对象占用大小。  
-histo：live ：同上，但是只答应存活对象的情况  
-permstat：打印permanent generation heap情况  

[root@localhost ~]# jmap -heap 27900
Attaching to process ID 27900, please wait...
Debugger attached successfully.
Client compiler detected.
JVM version is 20.45-b01
using thread-local object allocation.
Mark Sweep Compact GC
Heap Configuration: #堆内存初始化配置
   MinHeapFreeRatio = 40     #-XX:MinHeapFreeRatio设置JVM堆最小空闲比率  
   MaxHeapFreeRatio = 70   #-XX:MaxHeapFreeRatio设置JVM堆最大空闲比率  
   MaxHeapSize = 100663296 (96.0MB)   #-XX:MaxHeapSize=设置JVM堆的最大大小
   NewSize = 1048576 (1.0MB)     #-XX:NewSize=设置JVM堆的‘新生代’的默认大小
   MaxNewSize = 4294901760 (4095.9375MB) #-XX:MaxNewSize=设置JVM堆的‘新生代’的最大大小
   OldSize = 4194304 (4.0MB)  #-XX:OldSize=设置JVM堆的‘老生代’的大小
   NewRatio = 2    #-XX:NewRatio=:‘新生代’和‘老生代’的大小比率
   SurvivorRatio = 8  #-XX:SurvivorRatio=设置年轻代中Eden区与Survivor区的大小比值
   PermSize = 12582912 (12.0MB) #-XX:PermSize=<value>:设置JVM堆的‘持久代’的初始大小  
   MaxPermSize = 67108864 (64.0MB) #-XX:MaxPermSize=<value>:设置JVM堆的‘持久代’的最大大小  
Heap Usage:
New Generation (Eden + 1 Survivor Space): #新生代区内存分布，包含伊甸园区+1个Survivor区
   capacity = 30212096 (28.8125MB)
   used = 27103784 (25.848182678222656MB)
   free = 3108312 (2.9643173217773438MB)
   89.71169693092462% used
Eden Space: #Eden区内存分布
   capacity = 26869760 (25.625MB)
   used = 26869760 (25.625MB)
   free = 0 (0.0MB)
   100.0% used
From Space: #其中一个Survivor区的内存分布
   capacity = 3342336 (3.1875MB)
   used = 234024 (0.22318267822265625MB)
   free = 3108312 (2.9643173217773438MB)
   7.001809512867647% used
To Space: #另一个Survivor区的内存分布
   capacity = 3342336 (3.1875MB)
   used = 0 (0.0MB)
   free = 3342336 (3.1875MB)
   0.0% used
tenured generation:   #当前的Old区内存分布  
   capacity = 67108864 (64.0MB)
   used = 67108816 (63.99995422363281MB)
   free = 48 (4.57763671875E-5MB)
   99.99992847442627% used
Perm Generation:     #当前的 “持久代” 内存分布
   capacity = 14417920 (13.75MB)
   used = 14339216 (13.674942016601562MB)
   free = 78704 (0.0750579833984375MB)
   99.45412375710227% used

[root@localhost ~]# jmap -heap 4951
Attaching to process ID 4951, please wait...
Debugger attached successfully.
Client compiler detected.
JVM version is 20.45-b01

using thread-local object allocation.
Parallel GC with 1 thread(s)

Heap Configuration:
   MinHeapFreeRatio = 40
   MaxHeapFreeRatio = 70
   MaxHeapSize      = 734003200 (700.0MB)
   NewSize          = 314572800 (300.0MB)
   MaxNewSize       = 314572800 (300.0MB)
   OldSize          = 4194304 (4.0MB)
   NewRatio         = 2
   SurvivorRatio    = 8
   PermSize         = 104857600 (100.0MB)
   MaxPermSize      = 104857600 (100.0MB)

Heap Usage:
PS Young Generation
Eden Space:
   capacity = 235929600 (225.0MB)
   used     = 169898184 (162.02753448486328MB)
   free     = 66031416 (62.97246551513672MB)
   72.01223754882812% used
From Space:
   capacity = 39321600 (37.5MB)
   used     = 0 (0.0MB)
   free     = 39321600 (37.5MB)
   0.0% used
To Space:
   capacity = 39321600 (37.5MB)
   used     = 0 (0.0MB)
   free     = 39321600 (37.5MB)
   0.0% used
PS Old Generation
   capacity = 419430400 (400.0MB)
   used     = 0 (0.0MB)
   free     = 419430400 (400.0MB)
   0.0% used
PS Perm Generation
   capacity = 104857600 (100.0MB)
   used     = 14140688 (13.485610961914062MB)
   free     = 90716912 (86.51438903808594MB)
   13.485610961914062% used

 JAVA_OPTS="-Xms700m -Xmx700m -Xmn300m -Xss1024K -XX:PermSize=100m -XX:MaxPermSize=100 -XX:+UseParallelGC -XX:ParallelGCThreads=1 -XX:+PrintGCTimeStamps
 -XX:+PrintGCDetails -Xloggc:/opt/logs/gc.log -verbose:gc -XX:+DisableExplicitGC"

#instance 是对象的实例个数 
#bytes 是总占用的字节数 
class name 对应的就是 Class 文件里的 class 的标识 
B 代表 byte
C 代表 char
D 代表 double
F 代表 float
I 代表 int
J 代表 long
Z 代表 boolean
前边有 [ 代表数组， [I 就相当于 int[]
对象用 [L+ 类名表示

-XX:+HeapDumpOnOutOfMemoryError -XX:HeapDumpPath=/path

jmap  -dump:format=b,file=/path/heap.bin 进程ID  

如果只dump heap中的存活对象，则加上选项-live。 

最后在eclipse中安装MAT插件（http://www.eclipse.org/mat/），然后在eclipse中，file---->open，打开这个文件heap.bin，利用现成的OOM工具进行分析。
具体操作方法：
首先输入网址http://www.eclipse.org/mat/previousReleases.php，然后查看你自己的Eclipse版本，我的是Indigo的，所以点击链接“Previous Releases”，选择Indigo版本的URLhttp://download.eclipse.org/mat/1.1.0/update-site/