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【转载】jmap详解

2013-05-07 17:33:37| 分类：默认分类 | 标签： |举报 |字号大中小订阅

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本文转载自hbluojiahui《jmap详解》

1. jmap -heap pid

查看java 堆（heap）使用情况

        using thread-local object allocation.
        Parallel GC with 4 thread(s)          //GC 方式          Heap Configuration:       //堆内存初始化配置
         MinHeapFreeRatio=40     //对应jvm启动参数-XX:MinHeapFreeRatio设置JVM堆最小空闲比率(default 40)
   MaxHeapFreeRatio=70 //对应jvm启动参数 -XX:MaxHeapFreeRatio设置JVM堆最大空闲比率(default 70)
   MaxHeapSize=512.0MB //对应jvm启动参数-XX:MaxHeapSize=设置JVM堆的最大大小
         NewSize = 1.0MB          //对应jvm启动参数-XX:NewSize=设置JVM堆的‘新生代’的默认大小
   MaxNewSize =4095MB   //对应jvm启动参数-XX:MaxNewSize=设置JVM堆的‘新生代’的最大大小
   OldSize = 4.0MB            //对应jvm启动参数-XX:OldSize=<value>:设置JVM堆的‘老生代’的大小
   NewRatio = 8         //对应jvm启动参数-XX:NewRatio=:‘新生代’和‘老生代’的大小比率
         SurvivorRatio = 8    //对应jvm启动参数-XX:SurvivorRatio=设置年轻代中Eden区与Survivor区的大小比值
PermSize= 16.0MB       //对应jvm启动参数-XX:PermSize=<value>:设置JVM堆的‘永生代’的初始大小
          MaxPermSize=64.0MB //对应jvm启动参数-XX:MaxPermSize=<value>:设置JVM堆的‘永生代’的最大大小

          Heap Usage:               //堆内存分步
          PS Young Generation
          Eden Space:          //Eden区内存分布
    capacity = 20381696 (19.4375MB) //Eden区总容量
          used     = 20370032 (19.426376342773438MB) //Eden区已使用
    free     = 11664 (0.0111236572265625MB) //Eden区剩余容量
            99.94277218147106% used //Eden区使用比率
         From Space:        //其中一个Survivor区的内存分布
     capacity = 8519680 (8.125MB)
   used     = 32768 (0.03125MB)
   free     = 8486912 (8.09375MB)
             0.38461538461538464% used
        To Space:            //另一个Survivor区的内存分布
            capacity = 9306112 (8.875MB)
            used     = 0 (0.0MB)
free     = 9306112 (8.875MB)
0.0% used
        PS Old Generation //当前的Old区内存分布
capacity = 366280704 (349.3125MB)
used     = 322179848 (307.25464630126953MB)
free     = 44100856 (42.05785369873047MB)
87.95982001825573% used
        PS Perm Generation //当前的 “永生代” 内存分布
            capacity = 32243712 (30.75MB)
            used     = 28918584 (27.57891082763672MB)
free     = 3325128 (3.1710891723632812MB)
            89.68751488662348% used

2. jmap -histo pid

查看堆内存(histogram)中的对象数量，大小

num #instances #bytes class name

                序号         实例个数            字节数       类名
        ----------------------------------------------
   1:       3174877      107858256 [C
   2:       3171499       76115976 java.lang.String
   3:       1397884       38122240 [B
   4:        214690       37785440 com.tongli.book.form.Book
   5:        107345       18892720 com.tongli.book.form.Book
   6:         65645       13953440 [Ljava.lang.Object;
   7:         59627        7648416 <constMethodKlass>
   8:        291852        7004448 java.util.HashMap$Entry
   9:        107349        6871176 [[B

..........

total 9150732 353969416

3. jmap - dump pid

将内存使用的详细情况输出到文件

jmap -dump:format=b,file=m.datpid

用jhat命令可以参看 jhat -port 5000 m.dat
在浏览器中访问：http://localhost:5000/ 查看详细信息

sun 官方文档 http://download.oracle.com/javase/1.5.0/docs/tooldocs/share/jmap.html

class name对应的就是Class文件里的class的标识
B代表byte
C代表char
D代表double
F代表float
I代表int
J代表long
Z代表boolean
前边有[代表数组，[I 就相当于int[]

对象用[L+类名表示

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