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Java Object resurrection

Overview

After an object which overrides finalize() is collected it is added to a finalization queue to be cleaned up after calling the finalize() method of each object.  By what happens if you resurrect the object?
 
 
 
 
 
 

When is finalize called?

The finalize method is called by a single threaded system task which calls this method for each object which has been collected. Note: the nodes in the finalization queue are objects also have finalize() methods notionally. Objects cannot be cleaned up until the GC after they have been finalized.

Most objects (including the node in the finalization queue) don’t overriden finalize() and so the GC is smart enough to detect this and not add them to the queue. These obejcts can be cleaned up immediately. If you override the method, even with an empty one, it makes a difference.

What about resurrected objects?

In the finalize() method, you can resurrect the object by giving making something point to it. e.g. a static collection.  This object can no longer be collected by a GC (until it is discarded again)  So what happens then?

The object has been flags as being finalized once and is not finalized repeatedly.

static final List ZOMBIES = new ArrayList<>();
    static class Zombies {
        private int num;
        public Zombies(int num) {
            this.num = num;
        }
        @Override
        protected void finalize() throws Throwable {
            System.out.println("Resurrect " + num);
            ZOMBIES.add(this);
        }
        @Override
        public String toString() {
            return "Zombies{" + "num=" + num + '}';
        }
    }
    public static void main(String... args) throws InterruptedException {
        for (int i = 0; i < 3; i++)
            ZOMBIES.add(new Zombies(i));
        for (int j = 0; j < 5; j++) {
            System.out.println("Zombies: " + ZOMBIES);
            ZOMBIES.clear();
            System.gc();
            Thread.sleep(100);
        }
    }

prints

Zombies: [Zombies{num=0}, Zombies{num=1}, Zombies{num=2}]
Resurrect 2
Resurrect 1
Resurrect 0
Zombies: [Zombies{num=2}, Zombies{num=1}, Zombies{num=0}]
Zombies: []
Zombies: []
Zombies: []

In this example, the Zombies are added once to the collection and resurrected once by the finalize method. When they are collected a second time, they have been flagged as finalized and not queued again.

Conclusion

While it’s a good idea to avoid using finalize(), it is a small comfort to know it will only be called once it the object is resurrected.
 

Reference: Java Object resurrection from our JCG partner Peter Lawrey at the Vanilla Java blog.

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