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From "Wendy Feng (JIRA)" <j...@apache.org>
Subject [jira] Created: (HARMONY-6653) Use instance lock to protect static share data in ObjID.java
Date Sun, 19 Sep 2010 02:28:33 GMT
Use instance lock to protect static share data in ObjID.java
------------------------------------------------------------

                 Key: HARMONY-6653
                 URL: https://issues.apache.org/jira/browse/HARMONY-6653
             Project: Harmony
          Issue Type: Bug
          Components: Classlib
    Affects Versions: 6.0M1
         Environment: Window XP
            Reporter: Wendy Feng


I found unsafe synchonrization in ObjID.java as follow:

private static class NumberGenerator {
 static long numCounter = 65536; 
...
synchronized long nextLong() {
            return useRandom ? sGenerator.nextLong() : numCounter++;
        }
...
}

numCounter is a static class field. querySerialNumber increment (non-atomic) operation is
in a synchronized method which means it is protected by "this".

Consequence: 
Different object instances will synchronize on different "this", numCounter increment is not
protected by a uniform lock. It results in race condition in numCounter increment, thus having
incorrect numCounter value.

It would be more proper to write the code as follows:
private static class NumberGenerator {
 static long numCounter = 65536; 
private static final Object monitor = new Object()
...
long nextLong() {
            synchornized(monitor)
                {
                    return useRandom ? sGenerator.nextLong() : numCounter++;
                  }
        }
...
}


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