如何查看JVM运行的堆内存情况

bisal的个人杂货铺 2020-07-23

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《不可不知的7个JDK命令》介绍了些jdk自带的问题排查工具，机器出现CPU飙升的情况，此时就可以借助工具，排查应用端是否存在一些潜在问题。

jmap指令可以查看JVM运行的堆内存情况，

[test@localhost]> jmap -h
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
    -permstat            to print permanent generation 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

通常，我们以hprof二进制格式dump堆的使用情况，相当于生成一个快照，然后分析这个快照文件，生成快照的指令，

[test@localhost]> jmap -dump:format=b,file=heap20200721.dump xxxxx(可以通过ps获取java进程pid号)
Dumping heap to /opt/applog/heap20200721.dump ...
Heap dump file created

在当前的路径，生成了dump文件，

-rw------- 1 jcf jcf 467M Jul 21 11:27 heap20200721.dump

然后下载文件，可以用Eclipse的MAT(Memory Analyzer)插件进行分析，如果不用插件，MAT可以直接下载，地址如下，

https://www.eclipse.org/mat/downloads.php

运行MemoryAnalyzer.exe，要求至少jdk1.8以上的环境，选择打开刚才下载的dump文件，

等待一段时间，就可以看到统计图了，

从生成的文件看其实是创建了一系列索引文件，在MAT中点击不同的选项时，应该就是从这些索引文件链接过来的，

例如，可以看下可能的泄露问题，点击饼图部分，就可以看到构成此问题的具体类，因为涉及细节，在此不做展示，

通过MAT，还可以浏览以下相关信息，

System Properties
Thread Overview
Top Consumers
Class Histogram

其实，除了MAT，还有其他能看dump文件的工具，例如jdk自带的jhat，他可以分析堆，并将堆中对象以html的形式显示出来，支持对象查询语言OQL，操作更加粗暴，参数就是任意可用的端口和dump文件名，

[test@localhost]> jhat -port 9999 heap20200721.dump
Reading from heap20200721.dump...
Dump file created Tue Jul 21 11:27:12 CST 2020
Snapshot read, resolving...
Resolving 5995070 objects...
WARNING:  Failed to resolve object id 0xe2db5378 for field annotations (signature L)
...(省略)
WARNING:  Failed to resolve object id 0xe195ce40 for field annotationDefault (signature L)
Chasing references, expect 1199 dots.............................................................................................................................
Eliminating duplicate references.............................................................................................................................
Snapshot resolved.
Started HTTP server on port 9999
Server is ready.