garbage collection - How to ensure finalize() is always called (Thinking in Java exercise)

Question

I'm slowly working through Bruce Eckel's Thinking in Java 4th edition, and the following problem has me stumped:

Create a class with a finalize( ) method that prints a message. In main( ), create an object of your class. Modify the previous exercise so that your finalize( ) will always be called.

This is what I have coded:

public class Horse {
    boolean inStable;
    Horse(boolean in){
        inStable = in;
    }   
    public void finalize(){
        if (!inStable) System.out.print("Error: A horse is out of its stable!");
    }
}
public class MainWindow {
    public static void main(String[] args) {
        Horse h = new Horse(false);
        h = new Horse(true);
        System.gc();
    }
}

It creates a new Horse object with the boolean inStable set to false. Now, in the finalize() method, it checks to see if inStable is false. If it is, it prints a message.

Unfortunately, no message is printed. Since the condition evaluates to true, my guess is that finalize() is not being called in the first place. I have run the program numerous times, and have seen the error message print only a couple of times. I was under the impression that when System.gc() is called, the garbage collector will collect any objects that aren't referenced.

Googling a correct answer gave me this link, which gives much more detailed, complicated code. It uses methods I haven't seen before, such as System.runFinalization(), Runtime.getRuntime(), and System.runFinalizersOnExit().

Is anybody able to give me a better understanding of how finalize() works and how to force it to run, or walk me through what is being done in the solution code?

Answer 1

When the garbage collector finds an object that is eligible for collection but has a finalizer it does not deallocate it immediately. The garbage collector tries to complete as quickly as possible, so it just adds the object to a list of objects with pending finalizers. The finalizer is called later on a separate thread.

You can tell the system to try to run pending finalizers immediately by calling the method System.runFinalization after a garbage collection.

But if you want to force the finalizer to run, you have to call it yourself. The garbage collector does not guarantee that any objects will be collected or that the finalizers will be called. It only makes a "best effort". However it is rare that you would ever need to force a finalizer to run in real code.