2024-01-15  阅读(71)
原文作者:Java 充电社 原文地址: http://www.itsoku.com/course/9/182

jmap

jdk安装后会自带一些小工具,jmap命令(Memory Map for Java)是其中之一。主要用于打印指定Java进程(或核心文件、远程调试服务器)的共享对象内存映射或堆内存细节。

jmap命令可以获得运行中的jvm的堆的快照,从而可以离线分析堆,以检查内存泄漏,检查一些严重影响性能的大对象的创建,检查系统中什么对象最多,各种对象所占内存的大小等等。可以使用jmap生成Heap Dump。

如果不想使用jmap命令,要想获取Java堆转储快照还有一些比较“暴力”的手段:譬如在前面用过的 -XX:+HeapDumpOnOutOfMemoryError参数,可以让虚拟机在OOM异常出现之后自动生成dump文件,通过-XX:+HeapDumpOnCtrlBreak参数可以使用[ctrl]+[Break]键让虚拟机生成dump文件,又或者在Linux系统下通过Kill -3 命令发送进程退出信息“恐吓”一下虚拟机,也能拿到dump文件。

jmap的作用并不仅仅是为了获取dump文件,他还可以查询finalize执行队列,java堆和永久代的详细信息,如空间使用率、当前用的是哪种收集器等。

jmap命令格式

    [root@ady01 ~]# jmap
    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)
    
    where <option> is one of:
        <none>               to print same info as Solaris pmap
        -heap                to print java heap summary
        -histo[:live]        to print histogram of java object heap; if the "live"
                             suboption is specified, only count live objects
        -clstats             to print class loader statistics
        -finalizerinfo       to print information on objects awaiting finalization
        -dump:<dump-options> to dump java heap in hprof binary format
                             dump-options:
                               live         dump only live objects; if not specified,
                                            all objects in the heap are dumped.
                               format=b     binary format
                               file=<file>  dump heap to <file>
                             Example: jmap -dump:live,format=b,file=heap.bin <pid>
        -F                   force. Use with -dump:<dump-options> <pid> or -histo
                             to force a heap dump or histogram when <pid> does not
                             respond. The "live" suboption is not supported
                             in this mode.
        -h | -help           to print this help message
        -J<flag>             to pass <flag> directly to the runtime system

主要选项:

选项 作用
-dump 生成java堆转储快照,格式为:-dump:[live,]format=b,file=&lt;filename>,其中live子参数说明是否只dump出存活对象
-finalizerinfo 显示在F-Queue中等待Finalizer线程执行finalize方法的对象,只在linux/solaris平台下有效
-heap 显示堆详细信息,如使用哪种回收期、参数配置、分带状况等,只在linux/solaris平台下有效
-histo 显示堆中对象统计信息,包括类、实例数量和合计容量
-permstat 以ClassLoader为统计口径显示永久代内存状况,只在linux/solaris平台下有效
-F 当虚拟机进程对-dump选项没有响应时,可以使用这个选项强制生成dump快照,只在linux/solaris平台下有效

jmap -dump:生成java堆转储快照

生成java对转存快照,格式:jmap -dump:[live,]format=b,file=文件名

    C:\Users\Think>jmap -dump:live,format=b,file=D:/1.hprof 24956
    Dumping heap to D:\1.hprof ...
    Heap dump file created

可以使用jdk提供的jvisualvm.exe查看hprof文件

202401151947373721.png

jmap -heap:显示堆详细信息

显示堆详细信息

命令格式 :jmap -heap

    [root@ady01 ~]# jmap -heap 25867
    Attaching to process ID 25867, please wait...
    Debugger attached successfully.
    Server compiler detected.
    JVM version is 25.181-b13
    
    using thread-local object allocation.
    Parallel GC with 2 thread(s)
    
    Heap Configuration:
       MinHeapFreeRatio         = 0
       MaxHeapFreeRatio         = 100
       MaxHeapSize              = 4164943872 (3972.0MB)
       NewSize                  = 87031808 (83.0MB)
       MaxNewSize               = 1388314624 (1324.0MB)
       OldSize                  = 175112192 (167.0MB)
       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 = 47710208 (45.5MB)
       used     = 2632072 (2.5101394653320312MB)
       free     = 45078136 (42.98986053466797MB)
       5.516790033696772% used
    From Space:
       capacity = 1048576 (1.0MB)
       used     = 770128 (0.7344512939453125MB)
       free     = 278448 (0.2655487060546875MB)
       73.44512939453125% used
    To Space:
       capacity = 524288 (0.5MB)
       used     = 0 (0.0MB)
       free     = 524288 (0.5MB)
       0.0% used
    PS Old Generation
       capacity = 220200960 (210.0MB)
       used     = 98595728 (94.02821350097656MB)
       free     = 121605232 (115.97178649902344MB)
       44.77533976236979% used
    
    38803 interned Strings occupying 4463232 bytes.

jmap -histo:显示堆中对象统计信息

显示堆中对象统计信息,包括类、实例数量和合计容量

命令格式 :jmap -histo[:live]

    C:\Users\Think>jmap -histo 28252
    
     num     #instances         #bytes  class name
    ----------------------------------------------
       1:        309006       16963968  [C
       2:          1081        7275840  [I
       3:         41164        3156952  [B
       4:         90125        2163000  java.lang.String
       5:         21000         672000  java.util.UUID
       6:         21000         336000  com.jvm.test8.Test8$User
       7:         21000         336000  com.jvm.test8.Test8$User$UserBuilder
       8:           799         300072  [Ljava.lang.Object;
       9:          7557         181368  java.lang.StringBuilder
      10:           772          87704  java.lang.Class
      11:          1026          65664  sun.nio.fs.WindowsFileAttributes
      12:          1026          49248  sun.nio.fs.WindowsPath$WindowsPathWithAttributes
      13:           837          33480  java.util.TreeMap$Entry
      14:          1032          33024  java.lang.ref.WeakReference
      15:           775          31000  sun.nio.fs.WindowsPath
      16:          1028          24672  sun.nio.fs.WindowsPathParser$Result
      17:           424          13568  java.io.File
      18:           168          12096  java.lang.reflect.Field
      19:           299          11960  java.util.LinkedHashMap$Entry
      20:           323          11200  [Ljava.lang.String;
      21:           173          11072  java.net.URL
      22:           341          10912  sun.misc.FDBigInteger
      23:           312           9984  java.util.Hashtable$Entry
      24:            66           8992  [Ljava.util.HashMap$Node;
      25:           267           8544  java.util.HashMap$Node
      26:           195           7800  java.lang.ref.Finalizer
      27:           264           6336  java.lang.StringBuffer
      28:           121           4840  java.lang.ref.SoftReference
      29:            29           4816  [Ljava.util.Hashtable$Entry;
      30:            50           4800  java.util.jar.JarFile$JarFileEntry
      31:           105           4552  [[C
      32:            53           4240  [Ljava.util.WeakHashMap$Entry;
      33:            74           4144  sun.misc.URLClassPath$JarLoader
      34:           258           4128  java.lang.Integer
      35:            50           4000  java.util.zip.ZipEntry
      36:            79           3792  java.net.NetworkInterface
      37:           150           3600  java.net.Parts
      38:           111           3552  java.util.concurrent.ConcurrentHashMap$Node
      39:           134           3216  java.security.Provider$ServiceKey
      40:            50           3200  java.util.jar.JarFile
      41:            55           3080  sun.nio.cs.UTF_8$Encoder
      42:             8           3008  java.lang.Thread
      43:            62           2976  java.util.HashMap
      44:            51           2856  java.util.zip.ZipFile$ZipFileInputStream
      45:           114           2736  java.io.ExpiringCache$Entry
      46:            53           2544  java.util.WeakHashMap
      47:            30           2400  java.lang.reflect.Constructor
      48:            56           2240  java.util.WeakHashMap$Entry
      49:            39           2184  java.util.zip.ZipFile$ZipFileInflaterInputStream
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