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 when garbage collection happens in my application log, so I can see if it correlates to problems I'm seeing. Turning on -Xloggc is helpful, but it is a little tricky to integrate the times from the GC log (which use seconds since app start) into my main application log.

Answer 1

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

Answer 2

It looks like you could use the MemoryPoolMXBean and set the collection usage threshold to 1. This should give you a notification any time the gc runs and there is still at least a byte of memory in use.

http://java.sun.com/j2se/1.5.0/docs/api/java/lang/management/MemoryPoolMXBean.html

It looks like this doesn't work with all garbage collectors, though.

Answer 3

There is no standard way for your own program to get information from the JVM about garbage collection. Any such API is vendor specific.

Why is the facility you found insufficient?

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 resource. Also have a look at Creating a Debugging and Profiling Agent with JVMTI.