linux JVM内存分析（一）

下图中22455是tomcat的进程号，这里不重复

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查看tomcat 占用内存:ps aux | grep tomcat | grep -v grep | awk -F ' ' '{print $5,$6}'

第一个数字是占用的虚拟内存大小，第二数字是占用物理内存大小，单位byte

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查看具体占用状况: jmap -histo 进程号 | more

其中排在最上面的几个缩写说明

[C is a char[]
[S is a short[]
[I is a int[]
[B is a byte[]
[[I is a int[][]

上面的输出中[C对象占用Heap这么多，每每跟String有关，String其内部使用final char[]数组来保存数据的

 

constMethodKlass/ methodKlass/ constantPoolKlass/ constantPoolCacheKlass/ instanceKlassKlass/ methodDataKlass与Classloader相关，常驻于Perm区。

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查看堆转储快照(重要，比较直接)

jmap命令用于生成堆转储快照；jhat命令用于启动堆转储快照的web server，以便于经过界面查看。

1. 运行命令（22455是tomcat进程号）: jmap -dump:format=b,file=dumpfile20190528 22455

[root@XXXXXX temp]# jmap -dump:format=b,file=dumpfile20190528 22455
Dumping heap to /opt/apache-tomcat-8.5.8/temp/dumpfile20190528 ...
Heap dump file created
 则当前目录下会生成一个dumpfile20190528文件，即堆转储快照文件。
 

     2. 运行命令:   jhat dumpfile20190528

[root@XXXXXXXXX temp]# jhat dumpfile20190528 
Reading from dumpfile20190528...
Dump file created Tue May 28 15:59:18 CST 2019
Snapshot read, resolving...
Resolving 22266807 objects...
Chasing references, expect 4453 dots......................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................
Snapshot resolved.
Started HTTP server on port 7000
Server is ready.

 浏览器访问 http://localhost:7000，展现的是堆转储快照信息界面：

1. 第一项package是项目中的包路径及实例列表
2. 第二项是实例的统计信息
3. 比较实用的是Show heap histogram

    Show heap histogram

   

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jmap输出jvm参数（java8）

使用命令：../usr/java/jdk1.8.0_111/bin/jmap -heap 22455