Garbage collection notification?

Question

I\'d like to register a callback with the JVM so I know when garbage collection is happening. Is there any way to do this?

EDIT: I want to do this so I can log out

Answer 1

As of Java7u4, you can get notifications from the GarbageCollectorMXBean. See http://docs.oracle.com/javase/7/docs/jre/api/management/extension/com/sun/management/GarbageCollectionNotificationInfo.html

Answer 2

Another use case for getting impending GC notifications: if your app is load balanced then you can then notify the load balancer to take your node out of the pool when a GC is about to start to that it does not receive requests that would have to wait for a full GC to get dealt with.

That doesn't help in-flight requests that arrived just before the GC kicked in but, in my case at least, most requests are subsecond and major GCs are 5-10s, every few minutes. We can tweak the NewGen ratios etc but the general point still applies: major GC may be much longer than typical response times so you may want to premptively stop a node starting a major GC from receiving requests.

When GC is over, a thread in the JVM can send a notification to the load balancer to let it know its back in business or the LB can rely on its usual keepalive.

Answer 3

There's an interesting article on Javalobby discussing one method of doing this.

Answer 4

If you're looking at this as a diagnostic tool, I recommend redirecting your application log to StdOut and then redirecting both StdOut and StdErr into a file. This will give you the detail of JVM logging, without forcing you to change your application code.

Answer 5

I think that the standard way is to use the JVM Tool Interface (JVM TI) to write an agent with a GC start callback and to log the time from it (see GetTime). Note that a Garbage Collection Start event is sent for full GCs only.

Sample JVM TI agents is available in the demo directory of the JDK 5.0 or the JDK 6 download. The tech article The JVM Tool Interface (JVM TI): How VM Agents Work is another very good resource. Also have a look at Creating a Debugging and Profiling Agent with JVMTI.