jvm 性能调优工具之 jmap

概述

命令jmap是一个多功能的命令。它可以生成 java 程序的 dump 文件， 也可以查看堆内对象示例的统计信息、查看 ClassLoader 的信息以及 finalizer 队列。

jmap 用法

[root@localhost ~]# jmap -help 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 [root@localhost ~]#

参数：

option： 选项参数。 pid： 需要打印配置信息的进程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。 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，用此选项参数标识服务器。 remote server IP or hostname 远程调试服务器的IP地址或主机名。

option

no option： 查看进程的内存映像信息,类似 Solaris pmap 命令。 heap： 显示Java堆详细信息 histo[:live]： 显示堆中对象的统计信息 clstats：打印类加载器信息 finalizerinfo： 显示在F-Queue队列等待Finalizer线程执行finalizer方法的对象 dump:<dump-options>：生成堆转储快照 F： 当-dump没有响应时，使用-dump或者-histo参数. 在这个模式下,live子参数无效. help：打印帮助信息 J<flag>：指定传递给运行jmap的JVM的参数

示例一：no option

命令：jmap pid 描述：查看进程的内存映像信息,类似 Solaris pmap 命令。

使用不带选项参数的jmap打印共享对象映射，将会打印目标虚拟机中加载的每个共享对象的起始地址、映射大小以及共享对象文件的路径全称。这与Solaris的pmap工具比较相似。

[root@localhost ~]# jmap 9824 Attaching to process ID 9824, please wait... Debugger attached successfully. Server compiler detected. JVM version is 25.201-b09 0x0000000000400000 8K /home/jdk/bin/java 0x0000003c2de00000 157K /lib64/ld-2.12.so 0x0000003c2e200000 1885K /lib64/libc-2.12.so 0x0000003c2e600000 143K /lib64/libpthread-2.12.so 0x0000003c2ea00000 22K /lib64/libdl-2.12.so 0x0000003c2ee00000 46K /lib64/librt-2.12.so 0x0000003c2f200000 585K /lib64/libm-2.12.so 0x0000003c39200000 91K /lib64/libgcc_s-4.4.7-20120601.so.1 0x00007fa996d92000 50K /home/jdk/jre/lib/amd64/libmanagement.so 0x00007fa99739c000 276K /home/jdk/jre/lib/amd64/libsunec.so 0x00007fa9975da000 114K /home/jdk/jre/lib/amd64/libnet.so 0x00007fa9977f2000 91K /home/jdk/jre/lib/amd64/libnio.so 0x00007fa9cb62c000 124K /home/jdk/jre/lib/amd64/libzip.so 0x00007fa9cb848000 64K /lib64/libnss_files-2.12.so 0x00007fa9cba67000 226K /home/jdk/jre/lib/amd64/libjava.so 0x00007fa9cbc96000 64K /home/jdk/jre/lib/amd64/libverify.so 0x00007fa9cbfa6000 16645K /home/jdk/jre/lib/amd64/server/libjvm.so 0x00007fa9ccf92000 106K /home/jdk/lib/amd64/jli/libjli.so [root@localhost ~]#

示例二：heap

命令：jmap -heap pid 描述：显示Java堆详细信息

打印一个堆的摘要信息，包括使用的GC算法、堆配置信息和各内存区域内存使用信息

[root@localhost ~]# jmap -heap 9824 Attaching to process ID 9824, please wait... Debugger attached successfully. Server compiler detected. JVM version is 25.201-b09 using thread-local object allocation. Parallel GC with 8 thread(s) Heap Configuration: MinHeapFreeRatio = 0 MaxHeapFreeRatio = 100 MaxHeapSize = 2048917504 (1954.0MB) NewSize = 42991616 (41.0MB) MaxNewSize = 682622976 (651.0MB) OldSize = 87031808 (83.0MB) NewRatio = 2 SurvivorRatio = 8 MetaspaceSize = 21807104 (20.796875MB) CompressedClassSpaceSize = 1073741824 (1024.0MB) MaxMetaspaceSize = 17592186044415 MB G1HeapRegionSize = 0 (0.0MB) Heap Usage: PS Young Generation Eden Space: capacity = 57671680 (55.0MB) used = 15361488 (14.649856567382812MB) free = 42310192 (40.35014343261719MB) 26.636102849786933% used From Space: capacity = 2097152 (2.0MB) used = 1417568 (1.351898193359375MB) free = 679584 (0.648101806640625MB) 67.59490966796875% used To Space: capacity = 2097152 (2.0MB) used = 0 (0.0MB) free = 2097152 (2.0MB) 0.0% used PS Old Generation capacity = 196083712 (187.0MB) used = 57017984 (54.3765869140625MB) free = 139065728 (132.6234130859375MB) 29.078388724097593% used 19040 interned Strings occupying 2101824 bytes. [root@localhost ~]#

示例三：histo[:live]

命令：jmap -histo:live pid 描述：显示堆中对象的统计信息

其中包括每个Java类、对象数量、内存大小(单位：字节)、完全限定的类名。打印的虚拟机内部的类名称将会带有一个’*’前缀。如果指定了live子选项，则只计算活动的对象。

[root@localhost ~]# jmap -histo:live 10027 num #instances #bytes class name ---------------------------------------------- 1: 12160 857048 [C 2: 1164 395200 [B 3: 2687 307408 java.lang.Class 4: 12145 291480 java.lang.String 5: 2315 146336 [Ljava.lang.Object; 6: 1303 114664 java.lang.reflect.Method 7: 1588 80424 [I 8: 1980 63360 java.util.HashMap$Node 9: 1689 54048 java.util.concurrent.ConcurrentHashMap$Node 10: 1309 52360 java.util.LinkedHashMap$Entry 11: 264 38080 [Ljava.util.HashMap$Node; 12: 1098 35136 java.util.Hashtable$Entry 13: 1971 31536 java.lang.Object 14: 1233 29096 [Ljava.lang.Class; 15: 571 22840 java.lang.ref.SoftReference 16: 527 17776 [Ljava.lang.String; 17: 424 16960 java.math.BigInteger 18: 625 15000 java.util.concurrent.atomic.AtomicLong 19: 301 14448 java.util.HashMap 20: 50 13712 [Ljava.util.concurrent.ConcurrentHashMap$Node; 21: 413 13216 java.lang.ref.WeakReference

示例四：clstats

命令：jmap -clstats pid 描述：打印类加载器信息

-clstats是-permstat的替代方案，在JDK8之前，-permstat用来打印类加载器的数据 打印Java堆内存的永久保存区域的类加载器的智能统计信息。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。

[root@localhost ~]# jmap -clstats 10027 Attaching to process ID 10027, please wait... Debugger attached successfully. Server compiler detected. JVM version is 25.201-b09 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> 1946 3496011 null live <internal> 0x00000000d7535050 1 1471 0x0000000085e13ac8 dead sun/reflect/DelegatingClassLoader@0x000000010000a028 0x00000000860c1688 1 1471 null dead sun/reflect/DelegatingClassLoader@0x000000010000a028 0x00000000d7542d90 1 1471 null dead sun/reflect/DelegatingClassLoader@0x000000010000a028 0x000000008605a340 1 880 null dead sun/reflect/DelegatingClassLoader@0x000000010000a028 0x00000000d75fc4d8 1 1473 null dead sun/reflect/DelegatingClassLoader@0x000000010000a028 0x0000000085e13b28 28 46144 null dead sun/misc/Launcher$ExtClassLoader@0x000000010000fc80 0x00000000860c15c0 1 1471 null dead sun/reflect/DelegatingClassLoader@0x000000010000a028 0x000000008605a408 1 880 null dead sun/reflect/DelegatingClassLoader@0x000000010000a028 0x00000000860c1818 1 1471 null dead sun/reflect/DelegatingClassLoader@0x000000010000a028 0x00000000d7569ec0 1 1471 0x0000000085e13ac8 dead sun/reflect/DelegatingClassLoader@0x000000010000a028 0x00000000d7571e40 1 1471 null dead sun/reflect/DelegatingClassLoader@0x000000010000a028 0x00000000d75047c8 1 1471 null dead sun/reflect/DelegatingClassLoader@0x000000010000a028 0x00000000d7589640 1 1471 null dead sun/reflect/DelegatingClassLoader@0x000000010000a028 0x00000000d7510388 1 1471 null dead sun/reflect/DelegatingClassLoader@0x000000010000a028 0x00000000d753e3c8 1 1471 null dead sun/reflect/DelegatingClassLoader@0x000000010000a028 0x00000000860c1750 1 1471 null dead sun/reflect/DelegatingClassLoader@0x000000010000a028 0x00000000d75603c8 1 1471 null dead sun/reflect/DelegatingClassLoader@0x000000010000a028 0x00000000d757c248 1 1471 0x0000000085e13ac8 dead sun/reflect/DelegatingClassLoader@0x000000010000a028 0x000000008605a020 1 1471 null dead sun/reflect/DelegatingClassLoader@0x000000010000a028 0x00000000d7557530 1 1471 null dead sun/reflect/DelegatingClassLoader@0x000000010000a028 0x00000000d75d0078 1 1471 null dead sun/reflect/DelegatingClassLoader@0x000000010000a028 0x0000000085f82050 0 0 0x0000000085e13ac8 dead java/util/ResourceBundle$RBClassLoader@0x00000001000735e0 0x00000000d750aab8 1 1471 null dead sun/reflect/DelegatingClassLoader@0x000000010000a028 0x0000000085e13ac8 391 676812 0x0000000085e13b28 dead sun/misc/Launcher$AppClassLoader@0x000000010000f8d8 0x00000000d752bd38 1 1471 null dead sun/reflect/DelegatingClassLoader@0x000000010000a028 0x00000000860c18e0 1 1471 null dead sun/reflect/DelegatingClassLoader@0x000000010000a028 0x00000000d75498f8 1 1471 null dead sun/reflect/DelegatingClassLoader@0x000000010000a028 0x000000008605a0e8 1 1474 null dead sun/reflect/DelegatingClassLoader@0x000000010000a028 0x000000008605a1b0 1 880 null dead sun/reflect/DelegatingClassLoader@0x000000010000a028 0x00000000d758ec60 1 1471 null dead sun/reflect/DelegatingClassLoader@0x000000010000a028 0x000000008605a278 1 880 null dead sun/reflect/DelegatingClassLoader@0x000000010000a028 total = 32 2393 4257796 N/A alive=1, dead=31 N/A [root@localhost ~]#

示例五：finalizerinfo

命令：jmap -finalizerinfo pid 描述：打印等待终结的对象信息

[root@localhost ~]# jmap -finalizerinfo 10027 Attaching to process ID 10027, please wait... Debugger attached successfully. Server compiler detected. JVM version is 25.201-b09 Number of objects pending for finalization: 0 [root@localhost ~]#

Number of objects pending for finalization: 0 说明当前F-QUEUE队列中并没有等待Fializer线程执行final

示例六：dump:<dump-options>

命令：jmap -dump:format=b,file=heapdump.phrof pid 描述：生成堆转储快照dump文件。

以hprof二进制格式转储Java堆到指定filename的文件中。live子选项是可选的。如果指定了live子选项，堆中只有活动的对象会被转储。想要浏览heap dump，你可以使用jhat(Java堆分析工具)读取生成的文件。

这个命令执行，JVM会将整个heap的信息dump写入到一个文件，heap如果比较大的话，就会导致这个过程比较耗时，并且执行的过程中为了保证dump的信息是可靠的，所以会暂停应用， 线上系统慎用。

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