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); }}