本文继续介绍Java自带的性能监测工具，本文使用jmap工具来玩~

jmap (Java Memory Map) 命令可以生成Java应用程序的堆快照和对象统计信息，对生成的堆快照进行分析，可以分析堆中对象所占用内存的情况，检查大对象等~

先使用jmap -help查看一下jmap命令的基本语法以及选择项（option）说明~

[root@dev18 ~]# jmap -helpUsage:    jmap [option] 
    
             (to connect to running process)    jmap [option] 
     
      
               (to connect to a core file)    jmap [option] [server_id@]
       
                (to connect to remote debug server)where 
         is one of:    
         
           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:
          
            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=
           
             dump heap to 
            
              Example: jmap -dump:live,format=b,file=heap.bin 
             
               -F force. Use with -dump:
               
               
                 or -histo to force a heap dump or histogram when 
                
                  does not respond. The "live" suboption is not supported in this mode. -h | -help to print this help message -J
                 
                   to pass 
                  
                    directly to the runtime system[root@dev18 ~]#
                  
                 
                
               
              
             
            
           
          
         
       
      
     
    

jmap基本语法：

jmap [option] 
    

jmap需要pid，所以需要和jps配合使用，也就是先使用jps获取pid信息，然后使用jmap来处理~

[root@dev18 ~]# jps -l1656 org.elasticsearch.bootstrap.Elasticsearch12905 org.apache.zookeeper.server.quorum.QuorumPeerMain4308 /srv/activemq/apache-activemq-5.14.3//bin/activemq.jar27265 sun.tools.jps.Jps[root@dev18 ~]#

接下来，本文使用Zookeeper的服务进程为例，其pid为12905

使用 jmap -heap 12905打印java heap的情况，如

[root@dev18 ~]# jmap -heap 12905Attaching to process ID 12905, please wait...Debugger attached successfully.Server compiler detected.JVM version is 24.71-b01using thread-local object allocation.Parallel GC with 2 thread(s)Heap Configuration:   MinHeapFreeRatio = 0   MaxHeapFreeRatio = 100   MaxHeapSize      = 1004535808 (958.0MB)   NewSize          = 1310720 (1.25MB)   MaxNewSize       = 17592186044415 MB   OldSize          = 5439488 (5.1875MB)   NewRatio         = 2   SurvivorRatio    = 8   PermSize         = 21757952 (20.75MB)   MaxPermSize      = 85983232 (82.0MB)   G1HeapRegionSize = 0 (0.0MB)Heap Usage:PS Young GenerationEden Space:   capacity = 15204352 (14.5MB)   used     = 12229848 (11.663291931152344MB)   free     = 2974504 (2.8367080688476562MB)   80.43649607691272% usedFrom Space:   capacity = 524288 (0.5MB)   used     = 32768 (0.03125MB)   free     = 491520 (0.46875MB)   6.25% usedTo Space:   capacity = 524288 (0.5MB)   used     = 0 (0.0MB)   free     = 524288 (0.5MB)   0.0% usedPS Old Generation   capacity = 41418752 (39.5MB)   used     = 3584192 (3.41815185546875MB)   free     = 37834560 (36.08184814453125MB)   8.65354900118671% usedPS Perm Generation   capacity = 22020096 (21.0MB)   used     = 8938800 (8.524703979492188MB)   free     = 13081296 (12.475296020507812MB)   40.59382847377232% used2776 interned Strings occupying 230000 bytes.[root@dev18 ~]#

使用 jmap -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。 

下面的例子使用jmap生成pid为12905的Java程序的对象统计信息（直方图的形式），并将生成的统计信息输出到时/usr/local目录下的12905.histo文件中，如：

[root@dev18 ~]# jmap -histo 12905 >/usr/local/12905.histo

[root@dev18 ~]# vim /usr/local/12905.histo 

num     #instances         #bytes  class name----------------------------------------------   1:         30204        5384536  [B   2:         17521        2252160  
    
        3:         17521        2186552  
     
         4:          1515        1644272  
      
          5:         39568        1582720  java.util.HashMap$KeyIterator   6:         10809        1457680  [C   7:          1515        1040840  
       
           8:          1368        1032128  
        
            9:         40162         919944  [Ljava.lang.Object;  10:         36461         875064  java.util.ArrayList  11:         35427         850248  java.lang.Long  12:         15953         765744  java.nio.HeapByteBuffer  13:          5384         387648  org.apache.zookeeper.server.Request  14:         11669         373408  java.util.HashMap$Entry  15:          3771         301296  [Ljava.util.HashMap$Entry;  16:           612         296376  [I  17:          5687         272976  java.util.HashMap  18:          3805         254448  [S  19:          7029         224928  java.util.concurrent.locks.AbstractQueuedSynchronizer$Node  20:           453         203136  
         
           21: 7138 171312 java.util.concurrent.LinkedBlockingQueue$Node 22: 7008 168192 java.lang.String 23: 1671 161696 java.lang.Class 24: 2507 133624 [[I 25: 5223 125352 java.util.Collections$UnmodifiableRandomAccessList 26: 3513 112416 java.io.DataInputStream 27: 3513 112416 java.io.DataOutputStream 28: 3511 112352 org.apache.zookeeper.proto.ReplyHeader 29: 1872 89856 org.apache.zookeeper.txn.TxnHeader 30: 5486 87776 java.util.HashSet 31: 3513 84312 org.apache.jute.BinaryOutputArchive 32: 3512 84288 java.io.ByteArrayOutputStream 33: 139 75616 
          
            34: 1506 72288 org.apache.zookeeper.KeeperException$NodeExistsException 35: 1871 59872 org.apache.zookeeper.server.quorum.QuorumPacket 36: 3561 56976 java.util.HashMap$KeySet 37: 3513 56208 org.apache.zookeeper.server.ByteBufferInputStream 38: 3513 56208 org.apache.jute.BinaryInputArchive 39: 3512 56192 org.apache.zookeeper.server.SessionTrackerImpl$SessionSet 40: 1873 44952 java.util.LinkedList$Node 41: 1871 44904 org.apache.zookeeper.server.quorum.Leader$Proposal 42: 516 41280 java.lang.reflect.Method 43: 1045 33440 java.util.concurrent.ConcurrentHashMap$HashEntry 44: 1694 27104 java.lang.Object 45: 1506 24096 org.apache.zookeeper.txn.ErrorTxn 46: 106 16960 org.apache.zookeeper.server.NIOServerCnxn 47: 426 13632 java.util.concurrent.locks.ReentrantLock$NonfairSync 48: 230 12704 [Ljava.util.concurrent.ConcurrentHashMap$HashEntry;
          
         
        
       
      
     
    

#instances, #bytes以及class name列说明：

instances列：表示当前类有多少个实例。bytes列：说明当前类的实例总共占用了多少个字节class name列：表示的就是当前类的名称，class name 解读：   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

也可以不生成文件，直接使用jmap -histo 12905查看，因为内容比较多，可以采用添加| more来限定~

[root@dev18 ~]# jmap -histo 12905 | more num     #instances         #bytes  class name----------------------------------------------   1:         30330        5407864  [B   2:         17521        2252160  
    
        3:         17521        2186552  
     
         4:          1515        1644272  
      
          5:         39770        1590800  java.util.HashMap$KeyIterator   6:         10827        1460848  [C   7:          1515        1040840  
       
           8:          1368        1032128  
        
            9:         40346         923032  [Ljava.lang.Object;  10:         36645         879480  java.util.ArrayList  11:         35608         854592  java.lang.Long  12:         16025         769200  java.nio.HeapByteBuffer  13:          5402         388944  org.apache.zookeeper.server.Request  14:         11723         375136  java.util.HashMap$Entry  15:           613         357464  [I  16:          3789         302736  [Ljava.util.HashMap$Entry;  17:          5705         273840  java.util.HashMap  18:          3805         254448  [S  19:          7065         226080  java.util.concurrent.locks.AbstractQueuedSynchronizer$Node  20:           453         203136  
         
           21: 7174 172176 java.util.concurrent.LinkedBlockingQueue$Node 22: 7008 168192 java.lang.String 23: 1671 161696 java.lang.Class 24: 2507 133624 [[I 25: 5241 125784 java.util.Collections$UnmodifiableRandomAccessList 26: 3531 112992 java.io.DataInputStream 27: 3531 112992 java.io.DataOutputStream 28: 3529 112928 org.apache.zookeeper.proto.ReplyHeader 29: 1872 89856 org.apache.zookeeper.txn.TxnHeader 30: 5504 88064 java.util.HashSet 31: 3531 84744 org.apache.jute.BinaryOutputArchive 32: 3530 84720 java.io.ByteArrayOutputStream 33: 139 75616 
          
            34: 1506 72288 org.apache.zookeeper.KeeperException$NodeExistsException 35: 1871 59872 org.apache.zookeeper.server.quorum.QuorumPacket 36: 3579 57264 java.util.HashMap$KeySet 37: 3531 56496 org.apache.zookeeper.server.ByteBufferInputStream 38: 3531 56496 org.apache.jute.BinaryInputArchive 39: 3530 56480 org.apache.zookeeper.server.SessionTrackerImpl$SessionSet 40: 1873 44952 java.util.LinkedList$Node 41: 1871 44904 org.apache.zookeeper.server.quorum.Leader$Proposal 42: 516 41280 java.lang.reflect.Method 43: 1045 33440 java.util.concurrent.ConcurrentHashMap$HashEntry 44: 1694 27104 java.lang.Object 45: 1506 24096 org.apache.zookeeper.txn.ErrorTxn 46: 106 16960 org.apache.zookeeper.server.NIOServerCnxn 47: 426 13632 java.util.concurrent.locks.ReentrantLock$NonfairSync 48: 230 12704 [Ljava.util.concurrent.ConcurrentHashMap$HashEntry; 49: 308 12320 java.lang.ref.SoftReference
          
         
        
       
      
     
    

使用jmap -permstat 12905打印permanent generation heap情况 

[root@dev18 ~]# jmap -permstat 12905Attaching to process ID 12905, please wait...Debugger attached successfully.Server compiler detected.JVM version is 24.71-b01finding class loader instances ..done.computing per loader stat ..done.please wait.. computing liveness.liveness analysis may be inaccurate ...class_loader	classes	bytes	parent_loader	alive?	type
    
     	1267	7343336	  null  	live	
     
      0x00000000c4292130	1	3048	  null  	dead	sun/reflect/DelegatingClassLoader@0x00000000bf04ffc80x00000000c420b210	0	0	  null  	dead	sun/misc/Launcher$ExtClassLoader@0x00000000bf1c01200x00000000c4292170	1	1888	  null  	dead	sun/reflect/DelegatingClassLoader@0x00000000bf04ffc80x00000000c42921b0	1	1888	  null  	dead	sun/reflect/DelegatingClassLoader@0x00000000bf04ffc80x00000000c4292358	2	32144	  null  	dead	javax/management/remote/rmi/NoCallStackClassLoader@0x00000000bf546b680x00000000c42921f0	1	1888	  null  	dead	sun/reflect/DelegatingClassLoader@0x00000000bf04ffc80x00000000c420b1c0	401	2657256	0x00000000c420b210	dead	sun/misc/Launcher$AppClassLoader@0x00000000bf21a9c80x00000000c42920e0	4	13776	  null  	dead	javax/management/remote/rmi/NoCallStackClassLoader@0x00000000bf546b680x00000000c4292318	1	3048	  null  	dead	sun/reflect/DelegatingClassLoader@0x00000000bf04ffc80x00000000c42922d8	1	1888	  null  	dead	sun/reflect/DelegatingClassLoader@0x00000000bf04ffc80x00000000c4292298	1	3048	  null  	dead	sun/reflect/DelegatingClassLoader@0x00000000bf04ffc80x00000000c4312bc8	0	0	0x00000000c420b1c0	dead	java/util/ResourceBundle$RBClassLoader@0x00000000bf6dce700x00000000c4292258	1	3056	  null  	dead	sun/reflect/DelegatingClassLoader@0x00000000bf04ffc8total = 14	1682	10066264	    N/A    	alive=1, dead=13	    N/A    [root@dev18 ~]#
     
    

jmap还有一个功能是可以手工生成指定程序的堆快照文件~ 使用-dump选项~

如：

jmap -dump:format=b,file=/usr/local/12905.hprof 12905

该示例为pid为12905的程序生成一个二进制的dump文件，文件放置在/usr/local目录下，

名字为12905.hprof~

[root@dev18 ~]# jmap -dump:format=b,file=/usr/local/12905.hprof 12905Dumping heap to /usr/local/12905.hprof ...Heap dump file created[root@dev18 ~]#

针对这种产生的dump文件，可以使用JDK自带jhat(Java Heap Analyse Tool)小工具来查看，可以将对中的对象以html的形式展示，包括对象的数量、大小等信息，并支持对象查询语言 （OQL），jhat将会在下一篇文章中介绍~