Java内存分析工具jmap

Content

• 1. jmap
• 1.1 概述
• 1.2 参数
• 1.3 options参数
• 2. 用法
• 2.1 jmap -dump:live,format=b,file=dump.hprof 129665
• 2.2 jmap -heap 129665
• 2.3 jmap -finalizerinfo 129665
• 2.4 jmap -histo:live 129665
• 2.5 jmap -clstats 129665
• 2.6 -F

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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。

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> jmap Usage:     jmap [option] <pid>         (to connect to running process)     jmap [option] <executable <core>         (to connect to a core file)     jmap [option] [server_id@]<remote server IP or hostname>         (to connect to remote debug server)   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     -clstats             to print class loader 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

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；

 

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2. 用法

所有测试基于如下JDK版本：

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> java -version java 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。

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> jmap -dump:live,format=b,file=dump.hprof 129665 Dumping 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。

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> jmap -heap 129665 Attaching to process ID 129665, please wait... Debugger attached successfully. Server compiler detected. JVM version is 25.121-b13   using parallel threads in the new generation. using thread-local object allocation. Concurrent Mark-Sweep GC   Heap 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% used Eden Space:    capacity = 644349952 (614.5MB)    used     = 291033744 (277.55140686035156MB)    free     = 353316208 (336.94859313964844MB)    45.167031222189024% used From Space:    capacity = 80478208 (76.75MB)    used     = 0 (0.0MB)    free     = 80478208 (76.75MB)    0.0% used To Space:    capacity = 80478208 (76.75MB)    used     = 0 (0.0MB)    free     = 80478208 (76.75MB)    0.0% used concurrent mark-sweep generation:    capacity = 1342177280 (1280.0MB)    used     = 4730600 (4.511451721191406MB)    free     = 1337446680 (1275.4885482788086MB)    0.3524571657180786% used   7119 interned Strings occupying 644232 bytes.

2.3 jmap -finalizerinfo 129665

打印等待回收的对象信息。

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> jmap -finalizerinfo 129665 Attaching to process ID 129665, please wait... Debugger attached successfully. Server compiler detected. JVM version is 25.121-b13 Number 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

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> 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。　　

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jmap -clstats 129665 Attaching to process ID 129665, please wait... Debugger attached successfully. Server compiler detected. JVM version is 25.121-b13 finding 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   <bootstrap>   1574    2848839   null      live    <internal> 0x00000000b002b6a0  1224    2203269 0x00000000b002d248  live    sun/misc/Launcher$AppClassLoader@0x000000010000f6a0 0x00000000b02f3a68  0   0   0x00000000b002b6a0  dead    java/util/ResourceBundle$RBClassLoader@0x000000010007aad8 0x00000000b0003e58  1   1471      null      dead    sun/reflect/DelegatingClassLoader@0x0000000100009df8 0x00000000b002ce78  1   1471      null      dead    sun/reflect/DelegatingClassLoader@0x0000000100009df8 0x00000000b002d248  112 240746    null      live    sun/misc/Launcher$ExtClassLoader@0x000000010000fa48   total = 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：

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jmap -F -histo 129665 | grep com.netflix Iterating over heap. This may take a while... 733:        1   40  com.netflix.hystrix.strategy.HystrixPlugins 873:        1   24  com.netflix.hystrix.strategy.properties.HystrixDynamicPropertiesSystemProperties$4 1047:       1   16  com.netflix.hystrix.strategy.properties.HystrixDynamicPropertiesSystemProperties Heap traversal took 7.434 seconds.