Question

The following code throws a NullPointerException.

import java.io.*;

public class NullFinalTest {
    public static void main(String[] args) throws IOException, ClassNotFoundException {
        Foo foo = new Foo();
        foo.useLock();
        ByteArrayOutputStream buffer = new ByteArrayOutputStream();
        new ObjectOutputStream(buffer).writeObject(foo);
        foo = (Foo) new ObjectInputStream(new ByteArrayInputStream(buffer.toByteArray())).readObject();
        foo.useLock();
    }

    public static class Foo implements Serializable {
        private final String lockUsed = "lock used";
        private transient final Object lock = new Object();
        public void useLock() {
            System.out.println("About to synchronize");
            synchronized (lock) { // <- NullPointerException here on 2nd call
                System.out.println(lockUsed);
            }
        }
    }
}

Here is the output:

About to synchronize
lock used
About to synchronize
Exception in thread "main" java.lang.NullPointerException
    at NullFinalTest$Foo.useLock(NullFinalTest.java:18)
    at NullFinalTest.main(NullFinalTest.java:10)

How can lock possibly be null?

Answer 1

A transient final field used as a lock is null

Here are few facts about the transient variable:

– Transient keyword when used on an instance variable, will prevent that instance variable to be serialized.

– On De-serialization, the transient variable get to their Default values…..

Eg:

• Object Reference Variable to null
• int to 0
• boolean to false, etc…….

So thats the reason you are getting a NullPointerException, when deserializing it…