1 作用
- 生成 java 程序的 dump 文件
- 查看堆内对象示例的统计信息
- 查看 ClassLoader 的信息
- finalizer 队列
2 使用
2.1 生成 java 程序的 dump 文件
[root@kvm15 zengfiles]# jmap -dump:format=b,file=test 25274
Dumping heap to /root/zengfiles/test ...
Heap dump file created
[root@kvm15 zengfiles]# ls
jdk-6u45-linux-x64.bin jdk-7u80-linux-x64.tar.gz jdk-9.0.1_linux-x64_bin.tar.gz test
这个命令执行，JVM会将整个heap的信息dump写入到一个文件，heap如果比较大的话，就会导致这个过程比较耗时，并且执行的过程中为了保证dump的信息是可靠的，所以会暂停应用， 线上系统慎用。
然后用分析工具分析，比如jhat、eclipse Memory Analyzer、jvisualvm，这里以jhat为例：
[root@kvm15 zengfiles]# jhat -J-Xmx1024M test
Reading from test...
Dump file created Thu Apr 18 14:14:11 CST 2019
Snapshot read, resolving...
Resolving 3706025 objects...
Chasing references, expect 741 dots.....................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................
Eliminating duplicate references.....................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................
Snapshot resolved.
Started HTTP server on port 7000
Server is ready.
访问7000端口：
可以统计实例总数、占用大小、：
image.png
image.png
2.2 查看进程的内存映像信息
[root@kvm15 ~]# jmap 25274
Attaching to process ID 25274, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 25.151-b12
0x0000000000400000 7K /usr/java/jdk1.8.0_151/bin/java
0x00007f9942dec000 48K /usr/java/jdk1.8.0_151/jre/lib/amd64/libinstrument.so
0x00007f998049a000 90K /usr/java/jdk1.8.0_151/jre/lib/amd64/libnio.so
0x00007f99808ab000 49K /usr/java/jdk1.8.0_151/jre/lib/amd64/libmanagement.so
0x00007f9980eb4000 66K /usr/lib64/libbz2.so.1.0.6
0x00007f99810c4000 153K /usr/lib64/liblzma.so.5.2.2
0x00007f99812ea000 88K /usr/lib64/libz.so.1.2.7
0x00007f9981500000 97K /usr/lib64/libelf-0.168.so
0x00007f9981718000 19K /usr/lib64/libattr.so.1.1.0
0x00007f998191d000 86K /usr/lib64/libgcc_s-4.8.5-20150702.so.1
0x00007f9981b33000 290K /usr/lib64/libdw-0.168.so
0x00007f9981d7a000 19K /usr/lib64/libcap.so.2.22
0x00007f9981f7f000 108K /usr/lib64/libresolv-2.17.so
0x00007f9982199000 27K /usr/lib64/libnss_dns-2.17.so
0x00007f99882e0000 113K /usr/java/jdk1.8.0_151/jre/lib/amd64/libnet.so
0x00007f999c02b000 64K /usr/lib64/libnss_myhostname.so.2
0x00007f999de5c000 125K /usr/java/jdk1.8.0_151/jre/lib/amd64/libzip.so
0x00007f999e078000 60K /usr/lib64/libnss_files-2.17.so
0x00007f999e28b000 220K /usr/java/jdk1.8.0_151/jre/lib/amd64/libjava.so
0x00007f999e4b7000 64K /usr/java/jdk1.8.0_151/jre/lib/amd64/libverify.so
0x00007f999e6c6000 43K /usr/lib64/librt-2.17.so
0x00007f999e8ce000 1112K /usr/lib64/libm-2.17.so
0x00007f999ebd0000 16615K /usr/java/jdk1.8.0_151/jre/lib/amd64/server/libjvm.so
0x00007f999fbc7000 2077K /usr/lib64/libc-2.17.so
0x00007f999ff8a000 19K /usr/lib64/libdl-2.17.so
0x00007f99a018e000 101K /usr/java/jdk1.8.0_151/lib/amd64/jli/libjli.so
0x00007f99a03a4000 141K /usr/lib64/libpthread-2.17.so
0x00007f99a05c0000 160K /usr/lib64/ld-2.17.so
2.3 显示Java堆详细信息（配置参数以及使用情况）
[root@kvm15 ~]# jmap -heap 25274
Attaching to process ID 25274, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 25.151-b12
using thread-local object allocation.
Parallel GC with 2 thread(s)
Heap Configuration:
MinHeapFreeRatio = 0
MaxHeapFreeRatio = 100
MaxHeapSize = 918552576 (876.0MB)
NewSize = 19398656 (18.5MB)
MaxNewSize = 306184192 (292.0MB)
OldSize = 39321600 (37.5MB)
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 = 262668288 (250.5MB)
used = 247940704 (236.45468139648438MB)
free = 14727584 (14.045318603515625MB)
94.3930863858221% used
From Space:
capacity = 4194304 (4.0MB)
used = 0 (0.0MB)
free = 4194304 (4.0MB)
0.0% used
To Space:
capacity = 18350080 (17.5MB)
used = 0 (0.0MB)
free = 18350080 (17.5MB)
0.0% used
PS Old Generation
capacity = 103284736 (98.5MB)
used = 42724416 (40.74517822265625MB)
free = 60560320 (57.75482177734375MB)
41.36566317020939% used
30627 interned Strings occupying 4009112 bytes.
2.4 显示堆中存活的对象的统计信息
[root@kvm15 zengfiles]# jmap -histo:live 25274 | more
num #instances #bytes class name
----------------------------------------------
1: 108533 10911296 [C
2: 141720 4535040 java.util.HashMap$Node
3: 107257 2574168 java.lang.String
4: 41996 2548736 [Ljava.lang.Object;
5: 9388 1630688 [Ljava.util.HashMap$Node;
6: 45205 1446560 java.util.concurrent.ConcurrentHashMap$Node
7: 11532 1275600 java.lang.Class
8: 13645 1200760 java.lang.reflect.Method
9: 35722 857328 java.util.ArrayList
10: 2831 782424 [B
11: 19338 773520 java.util.LinkedHashMap$Entry
12: 31471 755304 io.termd.core.term.Feature
13: 13489 647472 org.aspectj.weaver.reflect.ShadowMatchImpl
14: 30539 488624 io.termd.core.term.OpCode$Literal
15: 4488 480392 [I
16: 8216 460096 java.util.LinkedHashMap
17: 241 437232 [Ljava.util.concurrent.ConcurrentHashMap$Node;
18: 13489 431648 org.aspectj.weaver.patterns.ExposedState
19: 26476 423616 io.termd.core.term.Sequence
20: 22915 366640 java.lang.Object
21: 10080 217328 [Ljava.lang.Class;
22: 3931 188688 java.util.HashMap
23: 2708 134904 [Ljava.lang.String;
24: 3076 123040 java.lang.ref.SoftReference
25: 1347 107760 java.lang.reflect.Constructor
26: 1218 87696 org.springframework.core.annotation.AnnotationAttributes
27: 2700 86400 com.taobao.text.Style$Composite
28: 2647 84704 io.termd.core.term.OpCode$Printf
29: 4643 74288 io.termd.core.term.OpCode$PushParam
30: 2281 72992 java.lang.ref.WeakReference
2.5 打印类加载信息
-clstats是-permstat的替代方案，在JDK8之前，-permstat用来打印类加载器的数据
打印Java堆内存的永久保存区域的类加载器的智能统计信息。对于每个类加载器而言，它的名称、活跃度、地址、父类加载器、它所加载的类的数量和大小都会被打印。此外，包含的字符串数量和大小也会被打印。
[root@kvm15 zengfiles]# jmap -clstats 25274 | more
Attaching to process ID 25274, please wait...
finding class loader instances ..Debugger attached successfully.
Server compiler detected.
JVM version is 25.151-b12
done.
computing per loader stat ..done.
please wait.. computing liveness.liveness analysis may be inaccurate ...
class_loader classes bytes parent_loader alive? type
<bootstrap> 2474 4165685 null live <internal>
0x00000000ca596758 1 880 null dead sun/reflect/DelegatingClassLoader@0x0000000100009df8
0x00000000ca596b58 1 880 0x00000000c9547130 dead sun/reflect/DelegatingClassLoader@0x0000000100009df8
0x00000000c9767270 1 1476 0x00000000c9547130 dead sun/reflect/DelegatingClassLoader@0x0000000100009df8
0x00000000ca593d50 1 880 0x00000000c9547130 dead sun/reflect/DelegatingClassLoader@0x0000000100009df8
0x00000000ca596050 1 880 0x00000000c9547130 dead sun/reflect/DelegatingClassLoader@0x0000000100009df8
0x00000000ca598950 1 1474 0x00000000c9547130 dead sun/reflect/DelegatingClassLoader@0x0000000100009df8
0x00000000ca59af50 1 1474 0x00000000c9547130 dead sun/reflect/DelegatingClassLoader@0x0000000100009df8
0x00000000ca2c4f20 1 880 0x00000000c9547130 dead sun/reflect/DelegatingClassLoader@0x0000000100009df8
0x00000000c98e9480 1 880 0x00000000c9547130 dead sun/reflect/DelegatingClassLoader@0x0000000100009df8
0x00000000ca597648 1 880 0x00000000c9547130 dead sun/reflect/DelegatingClassLoader@0x0000000100009df8
0x00000000ca598148 1 880 0x00000000c9547130 dead sun/reflect/DelegatingClassLoader@0x0000000100009df8
0x00000000ca2c4b38 1 880 0x00000000c9547130 dead sun/reflect/DelegatingClassLoader@0x0000000100009df8
0x00000000c9401b58 0 0 0x00000000c94fd660 dead java/util/ResourceBundle$RBClassLoader@0x00000001000b5ca0
0x00000000ca596f40 1 1473 0x00000000c9547130 dead sun/reflect/DelegatingClassLoader@0x0000000100009df8
0x00000000ca597a40 1 880 0x00000000c9547130 dead sun/reflect/DelegatingClassLoader@0x0000000100009df8
0x00000000c98e8b90 1 880 0x00000000c9547130 dead sun/reflect/DelegatingClassLoader@0x0000000100009df8
0x00000000ca596e78 1 1473 0x00000000c9547130 dead sun/reflect/DelegatingClassLoader@0x0000000100009df8
0x00000000c9d25ef0 1 1505 null dead sun/reflect/DelegatingClassLoader@0x0000000100009df8
0x00000000c9766f50 1 1473 0x00000000c9547130 dead sun/reflect/DelegatingClassLoader@0x0000000100009df8
0x00000000ca594070 1 880 0x00000000c9547130 dead sun/reflect/DelegatingClassLoader@0x0000000100009df8
0x00000000ca596370 1 880 0x00000000c9547130 dead sun/reflect/DelegatingClassLoader@0x0000000100009df8
0x00000000ca598c70 1 880 0x00000000c9547130 dead sun/reflect/DelegatingClassLoader@0x0000000100009df8
0x00000000ca59a870 1 880 0x00000000c9547130 dead sun/reflect/DelegatingClassLoader@0x0000000100009df8
0x00000000ca2c4100 1 880 0x00000000c9547130 dead sun/reflect/DelegatingClassLoader@0x0000000100009df8
0x00000000ca2c4c00 1 880 0x00000000c9547130 dead sun/reflect/DelegatingClassLoader@0x0000000100009df8
0x00000000ca052828 0 0 0x00000000c9547130 dead org/springframework/boot/web/embedded/tomcat/TomcatEmbeddedWebappClassLoader@0x00000001004c20b8
2.6 打印等待终结的对象信息
[root@kvm15 zengfiles]# jmap -finalizerinfo 25274 |more
Attaching to process ID 25274, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 25.151-b12
Number of objects pending for finalization: 0
我这里没有
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