前言
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在了解jmap之前需要回顾一下jvm内存模型相关的知识，了解jvm内存模型的基础上，更容易理解。
1、 jmap命令简介
jmap（Java Virtual Machine Memory Map）是JDK提供的一个可以生成Java虚拟机的堆转储快照dump文件的命令行工具。除此以外，jmap命令还可以查看finalize执行队列、Java堆和方法区的详细信息，比如空间使用率、当前使用的什么垃圾回收器、分代情况等等。 配合jstack命令使用，通过分析线程堆栈，和内存堆存储的信息，基本上可以分析90%以上线上遇到的问题。
基本语法：
jmap [options] pid
[app@dzopdev-010010195187 ~]$ jmap -h
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
2. options
2.1、none
to print same info as Solaris pmap 不带选项参数，jmap打印共享对象映，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。
2.2、-heap
打印堆的摘要信息
- 垃圾回收器, Parallel GC
- Heap Configuration 堆配置信息
- Heap Usage 堆内存空间使用信息，包括分代情况，每个代的总容量、已使用内存、可使用内存。如果某一代被继续细分(例如，年轻代，老年代， eden ， from， to 等)，则包含细分的空间的内存使用信息。
[app@dzopdev-010010195187 ~]$ jmap -heap 3994
Attaching to process ID 3994, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 25.191-b12
using thread-local object allocation.
Parallel GC with 13 thread(s)
Heap Configuration:
MinHeapFreeRatio = 0
MaxHeapFreeRatio = 100
MaxHeapSize = 1073741824 (1024.0MB)
NewSize = 357564416 (341.0MB)
MaxNewSize = 357564416 (341.0MB)
OldSize = 716177408 (683.0MB)
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 = 318242816 (303.5MB)
used = 73695144 (70.28116607666016MB)
free = 244547672 (233.21883392333984MB)
23.156891623281766% used
From Space:
capacity = 19398656 (18.5MB)
used = 11316704 (10.792449951171875MB)
free = 8081952 (7.707550048828125MB)
58.33756730363176% used
To Space:
capacity = 18874368 (18.0MB)
used = 0 (0.0MB)
free = 18874368 (18.0MB)
0.0% used
PS Old Generation
capacity = 716177408 (683.0MB)
used = 52877440 (50.4278564453125MB)
free = 663299968 (632.5721435546875MB)
7.383287912930088% used
33738 interned Strings occupying 3267696 bytes.
[app@dzopdev-010010195187 ~]$
2.3、-histo[:live]
显示Java堆中对象的统计信息，包括：对象数量、占用内存大小(单位：字节)和类的完全限定名。 这边我直接看的开发环境的进程比较多，重定向到日志的log文件里面了。
[app@dzopdev-010010195187 ~]$ jmap -histo 6770 >jmap.6770.log
[app@dzopdev-010010195187 ~]$ vi jmap.6770.log
num #instances #bytes class name
----------------------------------------------
1: 51151 250484560 [I
2: 181787 26623120 [C
3: 33083 15735520 [B
4: 146598 3518352 java.lang.String
5: 86394 3455760 ch.qos.logback.core.status.InfoStatus
6: 36608 1933944 [Ljava.lang.Object;
7: 57458 1838656 java.util.concurrent.ConcurrentHashMap$Node
8: 20534 1806992 java.lang.reflect.Method
9: 13462 1485000 java.lang.Class
10: 31080 1243200 java.util.LinkedHashMap$Entry
11: 14651 1133808 [Ljava.util.HashMap$Node;
12: 57482 919712 java.lang.Object
13: 25192 806144 java.util.HashMap$Node
14: 13312 745472 java.util.LinkedHashMap
15: 28529 684696 java.util.ArrayList
16: 2654 683760 [Ljava.util.concurrent.ConcurrentHashMap$Node;
17: 24930 550632 [Ljava.lang.Class;
18: 15830 538880 [Ljava.lang.String;
19: 4500 504000 java.net.SocksSocketImpl
20: 6606 475632 java.lang.reflect.Field
21: 79 324848 [Ljava.nio.ByteBuffer;
22: 11726 281424 java.util.concurrent.ConcurrentLinkedQueue$Node
23: 5421 260208 java.util.HashMap
24: 3204 256320 java.lang.reflect.Constructor
25: 2020 242400 org.springframework.boot.loader.jar.JarEntry
26: 7413 237216 java.util.concurrent.locks.AbstractQueuedSynchronizer$Node
27: 7060 225920 java.util.ArrayList$Itr
28: 4491 215568 java.net.SocketInputStream
29: 4491 215568 java.net.SocketOutputStream
30: 4051 194448 java.nio.HeapCharBuffer
31: 6071 194272 java.net.InetAddress$InetAddressHolder
32: 8005 192120 java.lang.StringBuilder
33: 4794 191760 java.lang.ref.SoftReference
34: 2849 182336 java.util.concurrent.ConcurrentHashMap
"jmap.6770.log" 5896L, 559623C
如果指定了live参数，则只计算活动的对象.
[app@dzopdev-010010195187 ~]$ jmap -histo:live 6770 >jmap.live.6770.log
[app@dzopdev-010010195187 ~]$ vi jmap.live.6770.log
num #instances #bytes class name
----------------------------------------------
1: 79166 9245160 [C
2: 14760 7002048 [I
3: 8982 4487632 [B
4: 78290 1878960 java.lang.String
5: 49088 1570816 java.util.concurrent.ConcurrentHashMap$Node
6: 13283 1466384 java.lang.Class
7: 23524 940960 java.util.LinkedHashMap$Entry
8: 16569 909536 [Ljava.lang.Object;
9: 9495 751088 [Ljava.util.HashMap$Node;
10: 39881 638096 java.lang.Object
11: 6384 561792 java.lang.reflect.Method
12: 9706 543536 java.util.LinkedHashMap
13: 299 473472 [Ljava.util.concurrent.ConcurrentHashMap$Node;
14: 14304 457728 java.util.HashMap$Node
15: 1968 220416 java.net.SocksSocketImpl
16: 9000 216000 java.util.ArrayList
17: 1593 191160 org.springframework.boot.loader.jar.JarEntry
18: 37 152144 [Ljava.nio.ByteBuffer;
19: 3671 144552 [Ljava.lang.String;
20: 6449 138912 [Ljava.lang.Class;
21: 3413 136520 java.lang.ref.SoftReference
22: 2649 127152 java.util.HashMap
23: 2453 117744 org.apache.tomcat.util.buf.ByteChunk
24: 2925 117000 java.lang.ref.Finalizer
25: 1618 116496 org.springframework.core.annotation.AnnotationAttributes
26: 2268 108864 org.apache.tomcat.util.buf.CharChunk
27: 2207 105936 org.apache.tomcat.util.buf.MessageBytes
28: 3270 104640 java.util.LinkedList
29: 4106 98544 java.util.LinkedList$Node
30: 1959 94032 java.net.SocketInputStream
31: 1959 94032 java.net.SocketOutputStream
32: 1100 88000 java.lang.reflect.Constructor
33: 2071 82840 java.util.TreeMap$Entry
34: 1431 80136 java.lang.invoke.MemberName
"jmap.liev.6770.log" 5489L, 525768C
2.4、 -clstats
显示Java堆中元空间的类加载器的统计信息，to print class loader statistics。
[app@dzopdev-010010195187 ~]$ jmap -clstats 6770 >jmap.clstats.6770.log
finding class loader instances ..done.
computing per loader stat ..done.
please wait.. computing liveness.liveness analysis may be inaccurate ...
[app@dzopdev-010010195187 ~]$ vim jmap.clstats.6770.log
Attaching to process ID 6770, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 25.191-b12
class_loader classes bytes parent_loader alive? type
<bootstrap> 3059 5221648 null live <internal>
0x00000000e1908890 1 1474 0x00000000e0005148 dead sun/reflect/DelegatingClassLoader@0x000000010000a028
0x00000000e1a8aaa8 1 880 0x00000000e0005148 dead sun/reflect/DelegatingClassLoader@0x000000010000a028
0x00000000e009a608 1 1471 0x00000000e0005148 dead sun/reflect/DelegatingClassLoader@0x000000010000a028
0x00000000e1524b50 1 1476 0x00000000e0005148 dead sun/reflect/DelegatingClassLoader@0x000000010000a028
0x00000000e1b217b0 1 894 0x00000000e0005148 dead sun/reflect/DelegatingClassLoader@0x000000010000a028
0x00000000e0def4d8 1 1474 0x00000000e0005148 dead sun/reflect/DelegatingClassLoader@0x000000010000a028
0x00000000e1a8b3a0 1 880 0x00000000e0005148 dead sun/reflect/DelegatingClassLoader@0x000000010000a028
0x00000000e20eca18 1 1472 null dead sun/reflect/DelegatingClassLoader@0x000000010000a028
0x00000000e05fb748 1 1473 0x00000000e0005148 dead sun/reflect/DelegatingClassLoader@0x000000010000a028
0x00000000e0705268 1 880 0x00000000e0005148 dead sun/reflect/DelegatingClassLoader@0x000000010000a028
0x00000000e1c1e3d8 1 880 0x00000000e0005148 dead sun/reflect/DelegatingClassLoader@0x000000010000a028
0x00000000e1524448 1 1473 0x00000000e0005148 dead sun/reflect/DelegatingClassLoader@0x000000010000a028
0x00000000e1528d48 1 880 0x00000000e0005148 dead sun/reflect/DelegatingClassLoader@0x000000010000a028
0x00000000e19089b0 1 1474 0x00000000e0005148 dead sun/reflect/DelegatingClassLoader@0x000000010000a028
0x00000000e1a8b788 1 880 0x00000000e0005148 dead sun/reflect/DelegatingClassLoader@0x000000010000a028
0x00000000e2301310 1 1474 0x00000000e0005148 dead sun/reflect/DelegatingClassLoader@0x000000010000a028
0x00000000e152ae70 1 1495 null dead sun/reflect/DelegatingClassLoader@0x000000010000a028
0x00000000e1a8b080 1 1482 0x00000000e0005148 dead sun/reflect/DelegatingClassLoader@0x000000010000a028
0x00000000e22ada00 1 880 0x00000000e0005148 dead sun/reflect/DelegatingClassLoader@0x000000010000a028
0x00000000e22b2400 1 1475 null dead sun/reflect/DelegatingClassLoader@0x000000010000a028
0x00000000e0dda8f0 1 880 0x00000000e0005148 dead sun/reflect/DelegatingClassLoader@0x000000010000a028
0x00000000e0def0f0 1 880 0x00000000e0005148 dead sun/reflect/DelegatingClassLoader@0x000000010000a028
0x00000000e0796948 1 880 0x00000000e0005148 dead sun/reflect/DelegatingClassLoader@0x000000010000a028
0x00000000e04a6c78 1 1473 0x00000000e0005148 dead sun/reflect/DelegatingClassLoader@0x000000010000a028
0x00000000e05fb268 1 1473 0x00000000e0005148 dead sun/reflect/DelegatingClassLoader@0x000000010000a028
0x00000000e1524768 1 1473 0x00000000e0005148 dead sun/reflect/DelegatingClassLoader@0x000000010000a028
0x00000000e1526668 1 1474 0x00000000e0005148 dead sun/reflect/DelegatingClassLoader@0x000000010000a028
0x00000000e1b21a88 1 1471 0x00000000e0005148 dead sun/reflect/DelegatingClassLoader@0x000000010000a028
0x00000000e0dda4e0 1 1474 0x00000000e0005148 dead sun/reflect/DelegatingClassLoader@0x000000010000a028
0x00000000e000a840 48 81649 0x00000000e000a8a0 dead sun/misc/Launcher$AppClassLoader@0x000000010000f8d8
"jmap.clstats.6770.log" 144L, 14061C
2.5、-finalizerinfo
to print information on objects awaiting finalization
[app@dzopdev-010010195187 ~]$ jmap -finalizerinfo 6770
Attaching to process ID 6770, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 25.191-b12
Number of objects pending for finalization: 0
[app@dzopdev-010010195187 ~]$
2.6、-dump:
Example: jmap -dump:live,format=b,file=heap.bin 生成Java虚拟机的堆转储快照dump文件。一般上线出现问题了，通过日志无法分析问题的原因，会联系运维拉去堆快照dump文件，方便问题的定位，或者jvm参数指定内存溢出的转储dump文件。
[app@dzopdev-010010195187 ~]$ jmap -dump:live,format=b,file=/home/app/heap.6770.bin 6770
Dumping heap to /home/app/heap.6770.bin ...
Heap dump file created
[app@dzopdev-010010195187 ~]$
dump文件如何分析，当然我们主要是借助一些分析工具去做的分析，比如jprofile。
注意：
一般发生致命问题后，Java进程有时可以继续运行，但有时会挂掉。为了能够保留Java应用发生致命错误前的运行状态，JVM在死掉前产生两个文件，分别为JavaCore及HeapDump文件，便于后面问题分析和定位。
JavaCore和Heapdump区别后面会详细介绍一下，JavaCode主要是分析CPU的，比如程序在某个点卡死的现象。JavaCode文件主要保存的是Java应用各线程在某一时刻的运行的位置。
2.7、-F
强制模式。如果指定的pid没有响应，可以配合-dump或-histo一起使用。此模式下，不支持live参数。
3、HeapDump分析工具JProfiler
将上面Heapdump文件导入来用jprofile工具做堆内存分析。
