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jmap 命令如下：
[root@admin ~]# 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常用的 options 参数：
- heap 显示 Java 堆详细信息
- histo 显示堆中对象的统计信息
- permstat 显示堆永久区的类加载器的统计信息
- finalizerinfo 显示在 F-Queue 队列等待 Finalizer 线程执行 finalize 方法的对象
- dump 生成堆转储快照
例如，要生成堆快照文件：
[root@admin ~]# jmap -dump:live,format=b,file=/home/dump.hprof 43616
Dumping heap to /home/dump.hprof ...
Heap dump file created如果要打印 heap 的概要信息，GC 算法等，可以使用 -heap ：
[root@admin ~]# jmap -heap 43616
Attaching to process ID 43616, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 24.45-b08
using thread-local object allocation.
Parallel GC with 4 thread(s)
Heap Configuration:
MinHeapFreeRatio = 40
MaxHeapFreeRatio = 70
MaxHeapSize = 3191865344 (3044.0MB)
NewSize = 1310720 (1.25MB)
MaxNewSize = 17592186044415 MB
OldSize = 5439488 (5.1875MB)
NewRatio = 2
SurvivorRatio = 8
PermSize = 21757952 (20.75MB)
MaxPermSize = 85983232 (82.0MB)
G1HeapRegionSize = 0 (0.0MB)
Heap Usage:
PS Young Generation
Eden Space:
capacity = 50855936 (48.5MB)
used = 2034448 (1.9402008056640625MB)
free = 48821488 (46.55979919433594MB)
4.000414032297036% used
From Space:
capacity = 7864320 (7.5MB)
used = 0 (0.0MB)
free = 7864320 (7.5MB)
0.0% used
To Space:
capacity = 7864320 (7.5MB)
used = 0 (0.0MB)
free = 7864320 (7.5MB)
0.0% used
PS Old Generation
capacity = 51380224 (49.0MB)
used = 473272 (0.45134735107421875MB)
free = 50906952 (48.54865264892578MB)
0.9211170430086097% used
PS Perm Generation
capacity = 22020096 (21.0MB)
used = 2571712 (2.45257568359375MB)
free = 19448384 (18.54742431640625MB)
11.678931826636905% used
1453 interned Strings occupying 135688 bytes.-finalizerinfo:
[root@admin ~]# jmap -finalizerinfo 43616
Attaching to process ID 43616, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 24.45-b08
Number of objects pending for finalization: 0看到当前并没有等待 finalize 的对象。
-histo：
显示堆中对象的统计信息。
-histo:live 这个命令执行，JVM会先触发gc，然后再统计信息。
[root@admin ~]# jmap -histo:live 43616 | more
num #instances #bytes class name
----------------------------------------------
1: 5734 739488 <methodKlass>
2: 5734 662424 <constMethodKlass>
3: 380 453824 <constantPoolKlass>
4: 349 279520 <constantPoolCacheKlass>
5: 1834 264656 [C
6: 380 257248 <instanceKlassKlass>
7: 528 89504 [B
8: 443 54200 java.lang.Class
9: 627 41648 [[I
10: 1683 40392 java.lang.String
11: 580 35536 [S
12: 785 31400 java.util.TreeMap$Entry
13: 46 24656 <objArrayKlassKlass>
14: 314 13184 [Ljava.lang.Object;
15: 186 7584 [Ljava.lang.String;
16: 13 6152 <methodDataKlass>
17: 8 4288 <typeArrayKlassKlass>
18: 112 3584 java.util.Hashtable$Entry
19: 11 2288 <klassKlass>
20: 61 1952 java.util.concurrent.ConcurrentHashMap$HashEntry
21: 38 1824 sun.util.locale.LocaleObjectCache$CacheEntry-permstat
[root@admin ~]# jmap -permstat 43616
Attaching to process ID 43616, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 24.45-b08
finding class loader instances ..done.
computing per loader stat ..done.
please wait.. computing liveness...............................................................................done.
class_loader classes bytes parent_loader alive? type
<bootstrap> 377 2522712 null live <internal>
0x0000000741c00d98 0 0 null live sun/misc/Launcher$ExtClassLoader@0x000000073cbc1478
0x0000000741c00738 11 123552 0x0000000741c00d98 live sun/misc/Launcher$AppClassLoader@0x000000073cc24c50
total = 3 388 2646264 N/A alive=3, dead=0 N/A