本文详细介绍了jmap命令的各种功能，包括生成Java程序的dump文件、查看堆内对象统计信息、展示类加载器详情及F-Queue队列信息。通过多个实例展示了如何使用jmap进行Java应用程序的内存诊断。
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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