Java: Garbage Collection: Under the Hood

Can we peek under the hood at how garbage collection is done

Peeking under the hood of Java GC is tricky, and NOT 100% reliable

First, it should be stressed, and stressed again: the techniques noted below are HACKS, and are not 100% reliable.

Having said that, for simple cases, they’ve sometimes worked pretty well.

As the saying goes, your mileage may vary!

Seeing when an object becomes eligible for garbage collection

The code in this repo shows how we can peek under the hood a bit and see when objects become eligible for garbage collection.

• https://github.com/UCSB-CS56-pconrad/java-garbage-collection-demo

The basic technique is to include a static method in the class that does its best to “strongly encourage” the garbage collector to run, invoking the finalize() method of all objects eligible for garbage collection. We pass an integer to this method that corresponds to whatever line number we are trying to track. We call System.gc() and System.runFinalization() twice to really, really, strongly encourage garbage collection to happen, but I can’t stress this enough: we can’t 100% guarantee that the JVM will do as we ask.

public static void gc(int i) {
	System.out.println("Line " + i);
	System.gc();
	System.runFinalization ();
	System.gc();
	System.runFinalization ();
}

This shows how we can try to verify the answers for exam questions such as:

• Question 6 on Midterm Exam CS56-M16-E01

A slightly more sophisticated version

This version eliminates the need to manually keep track of the line numbers. It uses a hack that inspects the stack to see the current line number in the source. We need to have compiled with that tracking in place.