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jmap语法格式如下：
- jmap [option] pid
- jmap [option] executable core
- jmap [option] [server-id@]remote-hostname-or-ip
如果运行在64位JVM上，可能需要指定-J-d64命令选项参数。
- jmap -permstat pid
打印进程的类加载器和类加载器加载的持久代对象信息，输出：类加载器名称、对象是否存活(不可靠)、对象地址、父类加载器、已加载的类大小等信息，如下图：
使用jmap -heap pid查看进程堆内存使用情况，包括使用的GC算法、堆配置参数和各代中堆内存使用情况。比如下面的例子：
- root@ubuntu:/# jmap -heap 21711
- Attaching to process ID 21711, please wait...
- Debugger attached successfully.
- Server compiler detected.
- JVM version is 20.10-b01
-
- using thread-local object allocation.
- Parallel GC with 4 thread(s)
-
- Heap Configuration:
- MinHeapFreeRatio = 40
- MaxHeapFreeRatio = 70
- MaxHeapSize = 2067791872 (1972.0MB)
- NewSize = 1310720 (1.25MB)
- MaxNewSize = 17592186044415 MB
- OldSize = 5439488 (5.1875MB)
- NewRatio = 2
- SurvivorRatio = 8
- PermSize = 21757952 (20.75MB)
- MaxPermSize = 85983232 (82.0MB)
-
- Heap Usage:
- PS Young Generation
- Eden Space:
- capacity = 6422528 (6.125MB)
- used = 5445552 (5.1932830810546875MB)
- free = 976976 (0.9317169189453125MB)
- 84.78829520089286% used
- From Space:
- capacity = 131072 (0.125MB)
- used = 98304 (0.09375MB)
- free = 32768 (0.03125MB)
- 75.0% used
- To Space:
- capacity = 131072 (0.125MB)
- used = 0 (0.0MB)
- free = 131072 (0.125MB)
- 0.0% used
- PS Old Generation
- capacity = 35258368 (33.625MB)
- used = 4119544 (3.9287033081054688MB)
- free = 31138824 (29.69629669189453MB)
- 11.683876009235595% used
- PS Perm Generation
- capacity = 52428800 (50.0MB)
- used = 26075168 (24.867218017578125MB)
- free = 26353632 (25.132781982421875MB)
- 49.73443603515625% used
- ....
使用jmap -histo[:live] pid查看堆内存中的对象数目、大小统计直方图，如果带上live则只统计活对象，如下：
- root@ubuntu:/# jmap -histo:live 21711 | more
- num #instances #bytes class name----------------------------------------------
- 1: 38445 5597736 <constMethodKlass>
- 2: 38445 5237288 <methodKlass>
- 3: 3500 3749504 <constantPoolKlass>
- 4: 60858 3242600 <symbolKlass>
- 5: 3500 2715264 <instanceKlassKlass>
- 6: 2796 2131424 <constantPoolCacheKlass>
- 7: 5543 1317400 [I
- 8: 13714 1010768 [C
- 9: 4752 1003344 [B
- 10: 1225 639656 <methodDataKlass>
- 11: 14194 454208 java.lang.String
- 12: 3809 396136 java.lang.Class
- 13: 4979 311952 [S
- 14: 5598 287064 [[I
- 15: 3028 266464 java.lang.reflect.Method
- 16: 280 163520 <objArrayKlassKlass>
- 17: 4355 139360 java.util.HashMap$Entry
- 18: 1869 138568 [Ljava.util.HashMap$Entry;
- 19: 2443 97720 java.util.LinkedHashMap$Entry
- 20: 2072 82880 java.lang.ref.SoftReference
- 21: 1807 71528 [Ljava.lang.Object;
- 22: 2206 70592 java.lang.ref.WeakReference
- 23: 934 52304 java.util.LinkedHashMap
- 24: 871 48776 java.beans.MethodDescriptor
- 25: 1442 46144 java.util.concurrent.ConcurrentHashMap$HashEntry
- 26: 804 38592 java.util.HashMap
- 27: 948 37920 java.util.concurrent.ConcurrentHashMap$Segment
- 28: 1621 35696 [Ljava.lang.Class;
- 29: 1313 34880 [Ljava.lang.String;
- 30: 1396 33504 java.util.LinkedList$Entry
- 31: 462 33264 java.lang.reflect.Field
- 32: 1024 32768 java.util.Hashtable$Entry
- 33: 948 31440 [Ljava.util.concurrent.ConcurrentHashMap$HashEntry;
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