本文已参与「新人创作礼」活动，一起开启掘金创作之路。
1. jamp
root@node131:~/bes_seeyon_logs# jmap -heap 32051
Attaching to process ID 32051, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 25.71-b00
using thread-local object allocation.
Garbage-First (G1) GC with 8 thread(s)
Heap Configuration:
MinHeapFreeRatio = 40
MaxHeapFreeRatio = 70
MaxHeapSize = 16777216000 (16000.0MB)
NewSize = 1363144 (1.2999954223632812MB)
MaxNewSize = 5591007232 (5332.0MB)
OldSize = 5452592 (5.1999969482421875MB)
NewRatio = 2
SurvivorRatio = 8
MetaspaceSize = 4194304000 (4000.0MB)
CompressedClassSpaceSize = 1073741824 (1024.0MB)
MaxMetaspaceSize = 4194304000 (4000.0MB)
G1HeapRegionSize = 4194304 (4.0MB)
Heap Usage:
Exception in thread "main" java.lang.reflect.InvocationTargetException
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at sun.tools.jmap.JMap.runTool(JMap.java:201)
at sun.tools.jmap.JMap.main(JMap.java:130)
Caused by: java.lang.RuntimeException: unknown CollectedHeap type : class sun.jvm.hotspot.gc_interface.CollectedHeap
at sun.jvm.hotspot.tools.HeapSummary.run(HeapSummary.java:144)
at sun.jvm.hotspot.tools.Tool.startInternal(Tool.java:260)
at sun.jvm.hotspot.tools.Tool.start(Tool.java:223)
at sun.jvm.hotspot.tools.Tool.execute(Tool.java:118)
at sun.jvm.hotspot.tools.HeapSummary.main(HeapSummary.java:49)
... 6 more
它可以打印出 Eden区总容量 、已使用的容量、剩余的空间容量、两个Survivor区的总容量、已使用的容量和剩余的空间容量、老年代的总容量、已使用的容量和剩余的容量。但是 一般不用jmap命令，原因有2个 ：1 没有jstat 显示的全 2 没有GC相关统计。
# 该命令了解当前jvm中对象占用情况，它会按照对象占用空间大小降序排列，这样就可以快速了解到那个对象占用了大量内存
root@node131:~/bes_seeyon_logs# jmap -histo 32051
num #instances #bytes class name
----------------------------------------------
1: 24122377 2122532560 [C
2: 5169030 782332368 [B
3: 467896 338246792 [I
4: 13109748 314633952 java.lang.String
5: 5465828 131179872 java.lang.StringBuilder
6: 2668662 106746480 java.io.ObjectStreamField
7: 2201879 97641048 [Ljava.lang.Object;
8: 938397 82578936 java.lang.reflect.Method
9: 607426 63172304 java.io.ObjectStreamClass
10: 1703416 54509312 java.util.concurrent.locks.AbstractQueuedSynchronizer$Node
11: 1272133 40708256 java.util.HashMap$Node
12: 1288689 30928536 java.lang.Long
13: 1059702 25432848 java.util.ArrayList
14: 458851 24076192 [Ljava.util.HashMap$Node;
15: 475831 22839888 java.util.HashMap
16: 434167 17366680 java.util.TreeMap$Entry
17: 511656 16372992 java.util.concurrent.ConcurrentHashMap$Node
18: 507610 16243520 java.io.File
19: 500892 16028544 java.lang.ref.WeakReference
20: 249458 15965312 java.io.ObjectInputStream
21: 270501 15800704 [Ljava.io.ObjectInputStream$HandleTable$HandleList;
22: 282202 15316888 [Ljava.io.ObjectStreamField;
23: 354535 14601512 [Ljava.lang.String;
24: 260206 14571536 java.io.ObjectInputStream$BlockDataInputStream
....
2. jamp 生成当前堆内存快照
# 使用jmap 命令生成个堆内存快照到一个文件当中，因dump.hprof是二进制文件，不可直接打开
root@node131:~/bes_seeyon_logs# jmap -dump:live,format=b,file=dump.hprof 32051
Dumping heap to /data/BES952/Node/10.XX.XX.XX/instances/ins13/logs/dump.hprof ...
Heap dump file created
root@node131:~/bes_seeyon_logs# sz dump.hprof
3. 利用jhat 在浏览器中分析堆转出快照
C:\Users\test\Desktop>jhat -port 7000 dump.hprof
Reading from dump.hprof...
Dump file created Fri Sep 24 17:17:07 CST 2021
Snapshot read, resolving...
Resolving 18466626 objects...
Chasing references, expect 3693 dots.............................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................
Eliminating duplicate references.............................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................
Snapshot resolved.
Started HTTP server on port 7000
Server is ready.
由于我的 dump.hprof 文件比较大，所以分析的时间会长一些
4. jhat的用法
C:\Users\test>jhat -h
Usage: jhat [-stack <bool>] [-refs <bool>] [-port <port>] [-baseline <file>] [-debug <int>] [-version] [-h|-help] <file>
-J<flag> Pass <flag> directly to the runtime system. For
example, -J-mx512m to use a maximum heap size of 512MB
-stack false: Turn off tracking object allocation call stack.
-refs false: Turn off tracking of references to objects
-port <port>: Set the port for the HTTP server. Defaults to 7000
-exclude <file>: Specify a file that lists data members that should
be excluded from the reachableFrom query.
-baseline <file>: Specify a baseline object dump. Objects in
both heap dumps with the same ID and same class will
be marked as not being "new".
-debug <int>: Set debug level.
0: No debug output
1: Debug hprof file parsing
2: Debug hprof file parsing, no server
-version Report version number
-h|-help Print this help and exit
<file> The file to read
For a dump file that contains multiple heap dumps,
you may specify which dump in the file
by appending "#<number>" to the file name, i.e. "foo.hprof#3".
All boolean options default to "true"
从上边的默认参数可以看到jhat的默认端口是7000
