CON40-J. Do not synchronize on a collection view if the backing collection is accessible

The java.util.Collections interface's documentation [[API 06]] warns about the consequences of failing to synchronize on an accessible collection object when iterating over its view:

It is imperative that the user manually synchronize on the returned map when iterating over any of its collection views... Failure to follow this advice may result in non-deterministic behavior.

If the collection view is synchronized instead of the backing collection, the class may end up with two different locking strategies. In this case, if the backing collection is accessible to multiple threads, the class is not thread-safe.

Noncompliant Code Example (collection view)

This noncompliant code example creates two views, a synchronized view of an empty map encapsulated by the map field, and a set view of the map's keys encapsulated by the set field. Furthermore, this code synchronizes on the set view instead of the more accessible map view [[Tutorials 08]].

// map has package-private accessibility
final Map<Integer, String> map = Collections.synchronizedMap(new HashMap<Integer, String>());
private final Set<Integer> set = map.keySet();

public void doSomething() {
  synchronized(set) {  // Incorrectly synchronizes on set
    for (Integer k : set) { 
      // ...
    }
  }
}

If the set is synchronized instead of the map, another thread may modify the contents of the map, and invalidate the k iterator.

Compliant Solution (collection lock object)

This compliant solution synchronizes on the map view instead of the set view.

// map has package-private accessibility
final Map<Integer, String> map = Collections.synchronizedMap(new HashMap<Integer, String>());
private final Set<Integer> set = map.keySet();

public void doSomething() {
  synchronized(map) {  // Synchronize on map, not set
    for (Integer k : set) { 
      // ...
    }
  }
}

This is compliant because the iterator cannot fail as a result of changes in the map's structure when an iteration is in progress.

Risk Assessment

Synchronizing on a collection view instead of the collection object may cause non-deterministic behavior.

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

References

[[API 06]] Class Collections
[[Tutorials 08]] Wrapper Implementations

Issue Tracking

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