1. jmap

1.1 概述

JVM Memory Map命令用于生成heap dump文件，如果不使用这个命令，还可以使用-XX:+HeapDumpOnOutOfMemoryError参数来让虚拟机出现OOM的时候自动生成dump文件。 

jmap不仅能生成dump文件，还可以查询finalize执行队列、Java堆和老年代的详细信息，如当前使用率、当前使用的是哪种收集器等。

> jmapUsage:    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 -clstats to print class loader 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
                   
                  
                 
                
               
              
             
            
           
          
        
       
      
     

1.2 参数

option：选项参数，不可同时使用多个选项参数。

pid：Java进程id。

executable：产生核心dump的Java可执行文件。

core：需要打印配置信息的核心文件。

remote-hostname-or-ip：远程调试的主机名或ip。

server-id：可选的唯一id，如果相同的远程主机上运行了多台调试服务器，用此选项参数标示服务器。

1.3 options参数

heap：显示Java堆详细信息；

histo：线下堆中对象的统计信息；

clstats：Java堆中内存的类加载器的统计信息；

finalizerinfo：显示在F-Queue队列等待Finlizer线程执行finalizer方法的对象；

dump：生成堆转储快照；

F：当-dump没有响应时，强制生成dump快照；

 

2. 用法

所有测试基于如下JDK版本：

> java -versionjava version "1.8.0_121"Java(TM) SE Runtime Environment (build 1.8.0_121-b13)Java HotSpot(TM) 64-Bit Server VM (build 25.121-b13, mixed mode)

　　

2.1 jmap -dump:live,format=b,file=dump.hprof 129665

dump堆到文件，format指定输出格式，live指明是活着的对象，file指定文件名。

> jmap -dump:live,format=b,file=dump.hprof 129665Dumping heap to /opt/huawei/inputMethod/flight/flight/dump.hprof ...Heap dump file created

　　

dump.hprof这个文件可以通过eclipse的打开：

2.2 jmap -heap 129665

打印heap的概要信息，GC使用的算法，heap的配置和使用情况，可以用此来判断内存目前的使用情况以及垃圾回收情况。

> jmap -heap 129665Attaching to process ID 129665, please wait...Debugger attached successfully.Server compiler detected.JVM version is 25.121-b13using parallel threads in the new generation.using thread-local object allocation.Concurrent Mark-Sweep GCHeap Configuration:   MinHeapFreeRatio         = 40   MaxHeapFreeRatio         = 70   MaxHeapSize              = 2147483648 (2048.0MB)   NewSize                  = 805306368 (768.0MB)   MaxNewSize               = 805306368 (768.0MB)   OldSize                  = 1342177280 (1280.0MB)   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 = 724828160 (691.25MB)   used     = 291033744 (277.55140686035156MB)   free     = 433794416 (413.69859313964844MB)   40.15210225827871% usedEden Space:   capacity = 644349952 (614.5MB)   used     = 291033744 (277.55140686035156MB)   free     = 353316208 (336.94859313964844MB)   45.167031222189024% usedFrom Space:   capacity = 80478208 (76.75MB)   used     = 0 (0.0MB)   free     = 80478208 (76.75MB)   0.0% usedTo Space:   capacity = 80478208 (76.75MB)   used     = 0 (0.0MB)   free     = 80478208 (76.75MB)   0.0% usedconcurrent mark-sweep generation:   capacity = 1342177280 (1280.0MB)   used     = 4730600 (4.511451721191406MB)   free     = 1337446680 (1275.4885482788086MB)   0.3524571657180786% used7119 interned Strings occupying 644232 bytes.

2.3 jmap -finalizerinfo 129665

打印等待回收的对象信息。

> jmap -finalizerinfo 129665Attaching to process ID 129665, please wait...Debugger attached successfully.Server compiler detected.JVM version is 25.121-b13Number of objects pending for finalization: 0

Number of objects pending for finalization:0 说明当前F-Queue队列中并没有等待Finalizer线程执行finalizer方法的对象。

2.4 jmap -histo:live 129665

打印堆的对象统计，包括对象数、内存大小等。jmap -histo:live这个命令执行，JVM会先触发gc，然后再统计信息。

第一列：编号id

第二列：实例个数

第三列：所有实例大小

第四列：类名

> jmap -histo:live 129665 | grep com.netflix 658:             1             40  com.netflix.hystrix.strategy.HystrixPlugins 790:             1             24  com.netflix.hystrix.strategy.properties.HystrixDynamicPropertiesSystemProperties$4 934:             1             16  com.netflix.hystrix.strategy.properties.HystrixDynamicPropertiesSystemProperties

2.5 jmap -clstats 129665

打印Java类加载器的智能统计信息，对于每个类加载器而言，对于每个类加载器而言，它的名称，活跃度，地址，父类加载器，它所加载的类的数量和大小都会被打印。此外，包含的字符串数量和大小也会被打印。　　

jmap -clstats 129665Attaching to process ID 129665, please wait...Debugger attached successfully.Server compiler detected.JVM version is 25.121-b13finding 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
     
      	1574	2848839	  null  	live	
      
       0x00000000b002b6a0	1224	2203269	0x00000000b002d248	live	sun/misc/Launcher$AppClassLoader@0x000000010000f6a00x00000000b02f3a68	0	0	0x00000000b002b6a0	dead	java/util/ResourceBundle$RBClassLoader@0x000000010007aad80x00000000b0003e58	1	1471	  null  	dead	sun/reflect/DelegatingClassLoader@0x0000000100009df80x00000000b002ce78	1	1471	  null  	dead	sun/reflect/DelegatingClassLoader@0x0000000100009df80x00000000b002d248	112	240746	  null  	live	sun/misc/Launcher$ExtClassLoader@0x000000010000fa48total = 6	2912	5295796	    N/A    	alive=3, dead=3	    N/A
      
     

2.6 -F

强制模式。如果指定的pid没有响应，请使用jmap -dump或jmap -histo选项。此模式下，不支持live子选项。

比如：

jmap -F -histo 129665 | grep com.netflixIterating over heap. This may take a while...733:		1	40	com.netflix.hystrix.strategy.HystrixPlugins873:		1	24	com.netflix.hystrix.strategy.properties.HystrixDynamicPropertiesSystemProperties$41047:		1	16	com.netflix.hystrix.strategy.properties.HystrixDynamicPropertiesSystemPropertiesHeap traversal took 7.434 seconds.