Java synchronization question

Question

I was going through some code snippets looking at the synchronization aspect. I believe locking happens on objects. In the case of java we only have references to objects. Java should use the reference to procure the object and lock it.

What happens if the reference happens to be null? I feel this would break. If this were to work the locking needs to happen on references as well (which beats me). Can someone clarify this for me.

    synchronized (formatters) {
        if(formatters == null) {
            prepareCache(feedId);
        }
    }

You always can try it in code :) But yes, syncrhonized locks on the object instance (not the reference), because if you have 3 different references to the same instance, syncrhonizing on each one will block between the, — helios
– helios, Commented Feb 11, 2010 at 9:16

Hans W · Accepted Answer · 2010-02-11 09:13:12Z

5

You get a NullPointerException. For example:

class A {
    public static void main(String[] ss) {
        Object o = null;
        synchronized (o) {
        }
    }
}

Gives you:

Exception in thread "main" java.lang.NullPointerException
    at A.main(A.java:4)

answered Feb 11, 2010 at 9:13

Hans W

3,8911 gold badge24 silver badges21 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

reevesy · Accepted Answer · 2012-06-14 18:00:04Z

5

From The synchronised Statement section in the Java Language Specification:

"SynchronizedStatement: synchronized ( Expression ) Block" ... Otherwise, if the value of the Expression is null, a NullPointerException is thrown."

edited Jun 14, 2012 at 18:00

reevesy

3,4721 gold badge28 silver badges23 bronze badges

answered Feb 11, 2010 at 9:17

Joe

9235 silver badges13 bronze badges

Comments

monksy · Accepted Answer · 2010-02-11 09:08:53Z

2

According to this forum, and many other related links. If formatters is null, then the NullPointerException will be thrown.

answered Feb 11, 2010 at 9:08

monksy

14.2k19 gold badges78 silver badges128 bronze badges

Comments

Nate W. · Accepted Answer · 2010-02-11 09:15:53Z

1

Sssh, you're not supposed to know those are actually references to objects! Think of them as they're presented - like an object - and not how they're implemented. The Object class provides a single lock, so your formatters object will have inherited it. If formatters happens to be null, then synchronizing on it will throw a NullPointerException.

answered Feb 11, 2010 at 9:15

Nate W.

9,2776 gold badges46 silver badges66 bronze badges

Comments

Thirler · Accepted Answer · 2010-02-11 09:31:22Z

1

Where possible don't synchronize on objects that are actually used. Synchronize on private final Objects that you create within the class that does the locking. Reason for this is that others might choose the same object to synchronize on, which means you have no idea what kind of side effects this locking has.

answered Feb 11, 2010 at 9:31

Thirler

20.8k14 gold badges68 silver badges98 bronze badges

Comments

mfx · Accepted Answer · 2010-02-11 09:34:45Z

0

It won't work. You synchronize on objects, not on variables. When the variable is null, there is no object to synchronize on, so an Exception is thrown.

answered Feb 11, 2010 at 9:34

mfx

7,4281 gold badge30 silver badges29 bronze badges

Collectives™ on Stack Overflow

Java synchronization question

6 Answers 6

Comments

Comments

Comments

Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

6 Answers 6

Comments

Comments

Comments

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related