在JVM发生FGC前后dump内存

一.需求

有时候web应用经常会发生FGC,我们想知道FGC把那些对象给回收了，思路很简单就是看看FGC之前内存中有那些实例，FGC之后内存中又有那些实例，通过前后的比较，我们就能很容易知道FGC回收了那些实例，当然我们可以手工去dump内存，在FGC发生之前dump一下内存，再在FGC发生之后dump一下内存，但是这dump的时间点不好把握，能否让JVM自动去dump就更好了。

二.手工dump内存的方法

jmap -dump:format=b,file=/home/admin/xxx.bin PID
其中PID是java的进程ID

三.让JVM自己dump内存

在JVM启动的时候增加下面两个参数 -XX:+HeapDumpBeforeFullGC -XX:+HeapDumpAfterFullGC就可以在FGC的前后dump一下内存。不过增加这两个参数会在每次FGC的时候dump内存，dump内存本身对应用有影响，但是不会导致JVM停机。特别是在web应用启动的时候，就会不断的去dump内存，因为web应用启动的时候会频繁发生FGC。

四.使用Java代码来dump内存

import java.lang.management.ManagementFactory;

import javax.management.MBeanServer;

import com.sun.management.HotSpotDiagnosticMXBean;

public class HeapDumper {
 // This is the name of the HotSpot Diagnostic MBean
    private static final String HOTSPOT_BEAN_NAME =
         "com.sun.management:type=HotSpotDiagnostic";

    // field to store the hotspot diagnostic MBean 
    private static volatile HotSpotDiagnosticMXBean hotspotMBean;

     /*
      * Call this method from your application whenever you 
      * want to dump the heap snapshot into a file.
      *
      * @param fileName name of the heap dump file
      * @param live flag that tells whether to dump
      *             only the live objects
      */
    static void dumpHeap(String fileName, boolean live) {
        // initialize hotspot diagnostic MBean
        initHotspotMBean();
        try {
            hotspotMBean.dumpHeap(fileName, live);
        } catch (RuntimeException re) {
            throw re;
        } catch (Exception exp) {
            throw new RuntimeException(exp);
        }
    }

    // initialize the hotspot diagnostic MBean field
    private static void initHotspotMBean() {
        if (hotspotMBean == null) {
            synchronized (HeapDumper.class) {
                if (hotspotMBean == null) {
                    hotspotMBean = getHotspotMBean();
                }
            }
        }
    }

    // get the hotspot diagnostic MBean from the
    // platform MBean server
    private static HotSpotDiagnosticMXBean getHotspotMBean() {
        try {
            MBeanServer server = ManagementFactory.getPlatformMBeanServer();
            HotSpotDiagnosticMXBean bean = 
                ManagementFactory.newPlatformMXBeanProxy(server,
                HOTSPOT_BEAN_NAME, HotSpotDiagnosticMXBean.class);
            return bean;
        } catch (RuntimeException re) {
            throw re;
        } catch (Exception exp) {
            throw new RuntimeException(exp);
        }
    }

    public static void main(String[] args) {
        // default heap dump file name
        String fileName = "heap.bin";
        // by default dump only the live objects
        boolean live = true;

        // simple command line options
        switch (args.length) {
            case 2:
                live = args[1].equals("true");
            case 1:
                fileName = args[0];
        }

        // dump the heap
        dumpHeap(fileName, live);
    }
}