原文  http://itindex.net/detail/46666-jvm-fgc-dump

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);
}
}