linux JVM內存分析（一）

下圖中22455是tomcat的進程號，這裏不重複

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查看tomcat 佔用內存:ps aux | grep tomcat | grep -v grep | awk -F ' ' '{print $5,$6}'

第一個數字是佔用的虛擬內存大小，第二數字是佔用物理內存大小，單位byte

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查看具體佔用狀況: jmap -histo 進程號 | more

其中排在最上面的幾個縮寫說明

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

上面的輸出中[C對象佔用Heap這麼多，每每跟String有關，String其內部使用final char[]數組來保存數據的

 

constMethodKlass/ methodKlass/ constantPoolKlass/ constantPoolCacheKlass/ instanceKlassKlass/ methodDataKlass與Classloader相關，常駐於Perm區。

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查看堆轉儲快照(重要，比較直接)

jmap命令用於生成堆轉儲快照；jhat命令用於啓動堆轉儲快照的web server，以便於經過界面查看。

1. 運行命令（22455是tomcat進程號）: 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
 則當前目錄下會生成一個dumpfile20190528文件，即堆轉儲快照文件。
 

     2. 運行命令:   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.

 瀏覽器訪問 http://localhost:7000，展現的是堆轉儲快照信息界面：

1. 第一項package是項目中的包路徑及實例列表
2. 第二項是實例的統計信息
3. 比較實用的是Show heap histogram

    Show heap histogram

   

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jmap輸出jvm參數（java8）

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