jmap
jdk安装后会自带一些小工具，jmap命令(Memory Map for Java)是其中之一。主要用于打印指定Java进程(或核心文件、远程调试服务器)的共享对象内存映射或堆内存细节。
jmap命令可以获得运行中的jvm的堆的快照，从而可以离线分析堆，以检查内存泄漏，检查一些严重影响性能的大对象的创建，检查系统中什么对象最多，各种对象所占内存的大小等等。可以使用jmap生成Heap Dump。
如果不想使用jmap命令，要想获取Java堆转储快照还有一些比较“暴力”的手段：譬如在前面用过的 -XX:+HeapDumpOnOutOfMemoryError参数，可以让虚拟机在OOM异常出现之后自动生成dump文件，通过-XX:+HeapDumpOnCtrlBreak参数可以使用[ctrl]+[Break]键让虚拟机生成dump文件，又或者在Linux系统下通过Kill -3 命令发送进程退出信息“恐吓”一下虚拟机，也能拿到dump文件。
jmap的作用并不仅仅是为了获取dump文件，他还可以查询finalize执行队列，java堆和永久代的详细信息，如空间使用率、当前用的是哪种收集器等。
jmap命令格式
[root@ady01 ~]# 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
主要选项：
| 选项 | 作用 |
|---|---|
| -dump | 生成java堆转储快照，格式为：-dump:[live,]format=b,file=<filename>,其中live子参数说明是否只dump出存活对象 |
| -finalizerinfo | 显示在F-Queue中等待Finalizer线程执行finalize方法的对象，只在linux/solaris平台下有效 |
| -heap | 显示堆详细信息，如使用哪种回收期、参数配置、分带状况等，只在linux/solaris平台下有效 |
| -histo | 显示堆中对象统计信息，包括类、实例数量和合计容量 |
| -permstat | 以ClassLoader为统计口径显示永久代内存状况，只在linux/solaris平台下有效 |
| -F | 当虚拟机进程对-dump选项没有响应时，可以使用这个选项强制生成dump快照，只在linux/solaris平台下有效 |
jmap -dump：生成java堆转储快照
生成java对转存快照，格式：jmap -dump:[live,]format=b,file=文件名
C:\Users\Think>jmap -dump:live,format=b,file=D:/1.hprof 24956
Dumping heap to D:\1.hprof ...
Heap dump file created
可以使用jdk提供的jvisualvm.exe查看hprof文件
jmap -heap：显示堆详细信息
显示堆详细信息
命令格式 ：jmap -heap
[root@ady01 ~]# jmap -heap 25867
Attaching to process ID 25867, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 25.181-b13
using thread-local object allocation.
Parallel GC with 2 thread(s)
Heap Configuration:
MinHeapFreeRatio = 0
MaxHeapFreeRatio = 100
MaxHeapSize = 4164943872 (3972.0MB)
NewSize = 87031808 (83.0MB)
MaxNewSize = 1388314624 (1324.0MB)
OldSize = 175112192 (167.0MB)
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 = 47710208 (45.5MB)
used = 2632072 (2.5101394653320312MB)
free = 45078136 (42.98986053466797MB)
5.516790033696772% used
From Space:
capacity = 1048576 (1.0MB)
used = 770128 (0.7344512939453125MB)
free = 278448 (0.2655487060546875MB)
73.44512939453125% used
To Space:
capacity = 524288 (0.5MB)
used = 0 (0.0MB)
free = 524288 (0.5MB)
0.0% used
PS Old Generation
capacity = 220200960 (210.0MB)
used = 98595728 (94.02821350097656MB)
free = 121605232 (115.97178649902344MB)
44.77533976236979% used
38803 interned Strings occupying 4463232 bytes.
jmap -histo：显示堆中对象统计信息
显示堆中对象统计信息，包括类、实例数量和合计容量
命令格式 ：jmap -histo[:live]
C:\Users\Think>jmap -histo 28252
num #instances #bytes class name
----------------------------------------------
1: 309006 16963968 [C
2: 1081 7275840 [I
3: 41164 3156952 [B
4: 90125 2163000 java.lang.String
5: 21000 672000 java.util.UUID
6: 21000 336000 com.jvm.test8.Test8$User
7: 21000 336000 com.jvm.test8.Test8$User$UserBuilder
8: 799 300072 [Ljava.lang.Object;
9: 7557 181368 java.lang.StringBuilder
10: 772 87704 java.lang.Class
11: 1026 65664 sun.nio.fs.WindowsFileAttributes
12: 1026 49248 sun.nio.fs.WindowsPath$WindowsPathWithAttributes
13: 837 33480 java.util.TreeMap$Entry
14: 1032 33024 java.lang.ref.WeakReference
15: 775 31000 sun.nio.fs.WindowsPath
16: 1028 24672 sun.nio.fs.WindowsPathParser$Result
17: 424 13568 java.io.File
18: 168 12096 java.lang.reflect.Field
19: 299 11960 java.util.LinkedHashMap$Entry
20: 323 11200 [Ljava.lang.String;
21: 173 11072 java.net.URL
22: 341 10912 sun.misc.FDBigInteger
23: 312 9984 java.util.Hashtable$Entry
24: 66 8992 [Ljava.util.HashMap$Node;
25: 267 8544 java.util.HashMap$Node
26: 195 7800 java.lang.ref.Finalizer
27: 264 6336 java.lang.StringBuffer
28: 121 4840 java.lang.ref.SoftReference
29: 29 4816 [Ljava.util.Hashtable$Entry;
30: 50 4800 java.util.jar.JarFile$JarFileEntry
31: 105 4552 [[C
32: 53 4240 [Ljava.util.WeakHashMap$Entry;
33: 74 4144 sun.misc.URLClassPath$JarLoader
34: 258 4128 java.lang.Integer
35: 50 4000 java.util.zip.ZipEntry
36: 79 3792 java.net.NetworkInterface
37: 150 3600 java.net.Parts
38: 111 3552 java.util.concurrent.ConcurrentHashMap$Node
39: 134 3216 java.security.Provider$ServiceKey
40: 50 3200 java.util.jar.JarFile
41: 55 3080 sun.nio.cs.UTF_8$Encoder
42: 8 3008 java.lang.Thread
43: 62 2976 java.util.HashMap
44: 51 2856 java.util.zip.ZipFile$ZipFileInputStream
45: 114 2736 java.io.ExpiringCache$Entry
46: 53 2544 java.util.WeakHashMap
47: 30 2400 java.lang.reflect.Constructor
48: 56 2240 java.util.WeakHashMap$Entry
49: 39 2184 java.util.zip.ZipFile$ZipFileInflaterInputStream
