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.

Do you want to know how to develop your skillset to become a Java Rockstar?

Subscribe to our newsletter to start Rocking right now!

To get you started we give you two of our best selling eBooks for FREE!

JPA Mini Book

Learn how to leverage the power of JPA in order to create robust and flexible Java applications. With this Mini Book, you will get introduced to JPA and smoothly transition to more advanced concepts.

JVM Troubleshooting Guide

The Java virtual machine is really the foundation of any Java EE platform. Learn how to master it with this advanced guide!

Given email address is already subscribed, thank you!
Oops. Something went wrong. Please try again later.
Please provide a valid email address.
Thank you, your sign-up request was successful! Please check your e-mail inbox.
Please complete the CAPTCHA.
Please fill in the required fields.

Leave a Reply


1 × one =



Java Code Geeks and all content copyright © 2010-2014, Exelixis Media Ltd | Terms of Use | Privacy Policy | Contact
All trademarks and registered trademarks appearing on Java Code Geeks are the property of their respective owners.
Java is a trademark or registered trademark of Oracle Corporation in the United States and other countries.
Java Code Geeks is not connected to Oracle Corporation and is not sponsored by Oracle Corporation.
Do you want to know how to develop your skillset and become a ...
Java Rockstar?

Subscribe to our newsletter to start Rocking right now!

To get you started we give you two of our best selling eBooks for FREE!

Get ready to Rock!
You can download the complementary eBooks using the links below:
Close