Thursday, April 30, 2009

Synchronized Statement and Nested Classes in Java

I just encountered something in Java that I thought was worth mentioning. It's not exactly a gotcha, but it's good to be aware of.

If you have a nested instance class, synchronizing on this in the inner class is not the same as synchronizing on this in the outer class. This may seem obvious, but it is worth noting because the inner class can refer to methods of either the inner or the outer class in a seemingly ambiguous manner (without explicit qualification). Here's an example program, and its output:

public class WhatIsSyncQualThis {
 
class IJustDriftedAway {
void superFoo() {
synchronized(IJustDriftedAway.this) {
if( Thread.holdsLock(WhatIsSyncQualThis.this) )
System.out.println("1 - held");
}
synchronized(this) {
if( Thread.holdsLock(WhatIsSyncQualThis.this) )
System.out.println("2 - held");
}
synchronized(WhatIsSyncQualThis.this) {
if( Thread.holdsLock(this) )
System.out.println("3 - held");
}
synchronized(this) {
if( Thread.holdsLock(this) )
System.out.println("4 - held");
}
}
}
void fooIt() {
(new IJustDriftedAway()).superFoo();
}
public static void main(String[] args) {
System.out.println("Running");
(new WhatIsSyncQualThis()).fooIt();
System.out.println("Done");
}
}

Output:
Running
4 - held
Done

If you know how nested instance classes are implemented, this is not at all surprising. A separate class is created that takes as a constructor argument a reference to an instance of the outer class. So there are two separate instances upon which we are synchronizing.


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