如何查看JVM运行的堆内存情况

客官°小女子只卖身不卖艺 2023-03-01 10:18 96阅读 0赞

《[不可不知的7个JDK命令][7_JDK]》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，

![format_png][]

等待一段时间，就可以看到统计图了，

![format_png 1][]

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，

![format_png 2][]

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，

![format_png 3][]

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.

通过浏览器，访问http://10.221.165.176:9999/，就打开了网页，他是按照报名进行的分组，

![format_png 4][]

可以拉到网页的底部，支持各种检索的快捷链接，

![format_png 5][]

例如QQL语言，可以用类SQL写出检索字符串长度超过100的实例，

![format_png 6][]

得到分析文件，其实只是个工具的使用问题，如何从这些零碎的信息中，得到问题真谛，这才是关键，让我这个小白，和大家一起继续学习了。

Java性能问题排查相关的历史文章，

《[小白是怎么搞懂GC全过程？][GC]》  
《[一个Full GC次数过多导致系统CPU 100%的案例排查][Full GC_CPU 100]》  
《[Java GC的基础知识][Java GC]》

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[format_png 2]: /images/20230209/24b8d40b99fb484c8faf92388ce2fe98.png
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[format_png 6]: /images/20230209/e6504145c3914d868d503310b7565b76.png
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