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Finally, it is important to recognize the entities with which synchronization is required rather than indiscreetly scavenging for variables or objects to synchronize on.

Noncompliant Code Example

When using synchronization wrappers, the synchronization object needs to be the {{Collection}} object. The synchronization is necessary to enforce atomicity ([CON38-J. Ensure atomicity of thread-safe code]). This noncompliant example demonstrates inappropriate synchronization resulting from locking on a {{Collection}} view instead of the Collection itself \[[Tutorials 08|AA. Java References#Tutorials 08]\]. 

Map<Integer, String> m = Collections.synchronizedMap(new HashMap<Integer, String>());
Set<Integer> s = m.keySet();
synchronized(s) {  // Incorrectly synchronizes on s
  for(Integer k : s) { /* do something */ }
}

Compliant Solution

This compliant solution correctly synchronizes on the Collection object instead of the Collection view.

// ...
synchronized(m) {  // Synchronize on m, not s
  for(Integer k : s) { /* do something */ }
}

Risk Assessment

Synchronizing on an incorrect variable can provide a false sense of thread safety and result in nondeterministic behavior.

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\[[API 06|AA. Java References#API 06]\] Class String
\[[Pugh 08|AA. Java References#Pugh 08]\] "Synchronization"
\[[Tutorials 08|AA. Java References#Tutorials 08]\] [Wrapper Implementations|http://java.sun.com/docs/books/tutorial/collections/implementations/wrapper.html]

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CON35-J. Do not try to force thread shutdown      08. Concurrency (CON)      08. Concurrency (CON)