jvm 监控和故障处理工具：jmap

àì夳堔傛蜴生んèń 2022-03-19 01:16 230阅读 0赞

**jmap**

JVM Memory Map命令用于生成heap dump文件，如果不使用这个命令，还可以使用-XX:+HeapDumpOnOutOfMemoryError参数来让虚拟机出现OOM的时候自动生成dump文件。 jmap不仅能生成dump文件，还可以查询finalize执行队列、Java堆和永久代的详细信息，如当前使用率、当前使用的是哪种收集器等。【内存分析】

[root@localhost jdk1.7.0_79]# jmap --help
    Usage:
        jmap [option] <pid>
            (to connect to running process)
        jmap [option] <executable <core>
            (to connect to a core file)
        jmap [option] [server_id@]<remote server IP or hostname>
            (to connect to remote debug server)

**参数**

option：选项参数，不可同时使用多个选项参数

pid：java进程id，命令ps -ef | grep java获取

executable：产生核心dump的java可执行文件

core：需要打印配置信息的核心文件

remote-hostname-or-ip：远程调试的主机名或ip

server-id：可选的唯一id，如果相同的远程主机上运行了多台调试服务器，用此选项参数标识服务器

**options参数**

heap : 显示Java堆详细信息

histo : 显示堆中对象的统计信息

permstat :Java堆内存的永久保存区域的类加载器的统计信息

finalizerinfo : 显示在F-Queue队列等待Finalizer线程执行finalizer方法的对象  
dump : 生成堆转储快照  
F : 当-dump没有响应时，强制生成dump快照

**示例**

**-dump**

dump堆到文件,format指定输出格式，live指明是活着的对象,file指定文件名

[root@localhost jdk1.7.0_79]# jmap -dump:live,format=b,file=dump.hprof 24971
    Dumping heap to /usr/local/java/jdk1.7.0_79/dump.hprof ...
    Heap dump file created

**-heap**

打印heap的概要信息，GC使用的算法，heap的配置及使用情况，可以用此来判断内存目前的使用情况以及垃圾回收情况

[root@localhost jdk1.7.0_79]# jmap -heap 24971
    Attaching to process ID 24971, please wait...
    Debugger attached successfully.
    Server compiler detected.
    JVM version is 24.79-b02
     
    using thread-local object allocation.
    Parallel GC with 4 thread(s)
     
    Heap Configuration:
       MinHeapFreeRatio = 0
       MaxHeapFreeRatio = 100
       MaxHeapSize      = 4146069504 (3954.0MB)
       NewSize          = 1310720 (1.25MB)
       MaxNewSize       = 17592186044415 MB
       OldSize          = 5439488 (5.1875MB)
       NewRatio         = 2
       SurvivorRatio    = 8
       PermSize         = 21757952 (20.75MB)
       MaxPermSize      = 85983232 (82.0MB)
       G1HeapRegionSize = 0 (0.0MB)
     
    Heap Usage:
    PS Young Generation
    Eden Space:
       capacity = 517996544 (494.0MB)
       used     = 151567520 (144.54605102539062MB)
       free     = 366429024 (349.4539489746094MB)
       29.26033421566612% used
    From Space:
       capacity = 41943040 (40.0MB)
       used     = 0 (0.0MB)
       free     = 41943040 (40.0MB)
       0.0% used
    To Space:
       capacity = 40370176 (38.5MB)
       used     = 0 (0.0MB)
       free     = 40370176 (38.5MB)
       0.0% used
    PS Old Generation
       capacity = 115343360 (110.0MB)
       used     = 32927184 (31.401809692382812MB)
       free     = 82416176 (78.59819030761719MB)
       28.54709972034801% used
    PS Perm Generation
       capacity = 85983232 (82.0MB)
       used     = 54701200 (52.16712951660156MB)
       free     = 31282032 (29.832870483398438MB)
       63.6184506300019% used
     
    20822 interned Strings occupying 2441752 bytes.

**-finalizerinfo**

打印等待回收的对象信息，

[root@localhost jdk1.7.0_79]# jmap -finalizerinfo 24971
    Attaching to process ID 24971, please wait...
    Debugger attached successfully.
    Server compiler detected.
    JVM version is 24.79-b02
    Number of objects pending for finalization: 0

**`Number of objects pending ``for` `finalization: 0 `****`说明当前F-QUEUE队列中并没有等待Fializer线程执行final`**

**`izer方法的对象。`**

**-histo**

打印堆的对象统计，包括对象数、内存大小等等。jmap -histo:live 这个命令执行，JVM会先触发gc，然后再统计信息

[root@localhost jdk1.7.0_79]# jmap -histo:live 24971 | more
     
     num     #instances         #bytes  class name
    ----------------------------------------------
       1:        100134       14622728  <constMethodKlass>
       2:        100134       12830128  <methodKlass>
       3:         88438       12708392  [C
       4:          8271       10163584  <constantPoolKlass>
       5:         27806        9115784  [B
       6:          8271        6225312  <instanceKlassKlass>
       7:          6830        5632192  <constantPoolCacheKlass>
       8:         86717        2081208  java.lang.String
       9:          2264        1311720  <methodDataKlass>
      10:         10880         870400  java.lang.reflect.Method
      11:          8987         869888  java.lang.Class
      12:         13330         747264  [[I
      13:         11808         733872  [S
      14:         20110         643520  java.util.concurrent.ConcurrentHashMap$HashEntry
      15:         18574         594368  java.util.HashMap$Entry
      16:          3668         504592  [Ljava.util.HashMap$Entry;
      17:         30698         491168  java.lang.Integer
      18:          2247         486864  [I
      19:          7486         479104  java.net.URL
      20:          8032         453616  [Ljava.lang.Object;
      21:         10259         410360  java.util.LinkedHashMap$Entry
      22:           699         380256  <objArrayKlassKlass>
      23:          5782         277536  org.apache.catalina.loader.ResourceEntry
      24:          8327         266464  java.lang.ref.WeakReference
      25:          2374         207928  [Ljava.util.concurrent.ConcurrentHashMap$HashEntry;
      26:          3440         192640  java.util.LinkedHashMap
      27:          4779         191160  java.lang.ref.SoftReference
      28:          3576         171648  java.util.HashMap
      29:         10080         161280  java.lang.Object

jmap -histo:live 24971 | grep com.yuhuo 查询类名包含com.yuhuo的信息

jmap -histo:live 24971 | grep com.yuhuo > histo.txt 保存信息到histo.txt文件

**附 - jmap输出中class name非自定义类的说明：**

<table> 
 <thead> 
  <tr> 
   <th>BaseType&nbsp;Character</th> 
   <th>Type</th> 
   <th>Interpretation</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>B</td> 
   <td>byte</td> 
   <td>signed&nbsp;byte</td> 
  </tr> 
  <tr> 
   <td>C</td> 
   <td>char</td> 
   <td>Unicode&nbsp;character</td> 
  </tr> 
  <tr> 
   <td>D</td> 
   <td>double</td> 
   <td>double-precision&nbsp;floating-point&nbsp;value</td> 
  </tr> 
  <tr> 
   <td>F</td> 
   <td>float</td> 
   <td>single-precision&nbsp;floating-point&nbsp;value</td> 
  </tr> 
  <tr> 
   <td>I</td> 
   <td>int</td> 
   <td>integer</td> 
  </tr> 
  <tr> 
   <td>J</td> 
   <td>long</td> 
   <td>long&nbsp;integer</td> 
  </tr> 
  <tr> 
   <td>L;</td> 
   <td>reference</td> 
   <td>an&nbsp;instance&nbsp;of&nbsp;class</td> 
  </tr> 
  <tr> 
   <td>S</td> 
   <td>short</td> 
   <td>signed&nbsp;short</td> 
  </tr> 
  <tr> 
   <td>Z</td> 
   <td>boolean</td> 
   <td>true&nbsp;or&nbsp;false</td> 
  </tr> 
  <tr> 
   <td>[</td> 
   <td>reference</td> 
   <td>one&nbsp;array&nbsp;dimension，[I表示int[]</td> 
  </tr> 
 </tbody> 
</table>

**-permstat**

打印Java堆内存的永久区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@localhost jdk1.7.0_79]# jmap -permstat 24971

**-F**

强制模式。如果指定的pid没有响应，请使用jmap -dump或jmap -histo选项。此模式下，不支持live子选项。