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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.

Synchronize on the original collection object when using synchronization wrappers to enforce atomicity (CON07-J. Do not assume that a group of calls to independently atomic methods is atomic).

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

// 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) { 
      // ...
    }
  }
}

Risk Assessment

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

Rule

Severity

Likelihood

Remediation Cost

Priority

Level

CON40-J

medium

probable

medium

P8

L2

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

100%

Review List

  1. handler

    "warns about the consequences of synchronizing on any view over a collection object, rather than synchronizing on the collection object" ... it doesn't warn against it...it just says you should synchronize the collection...suggest reverting the change

    Priority MEDIUM
    dmohindr
    Mar 24, 2010
  2. handler

    The title could do away with "still"

    Priority MEDIUM
    rcs
    Mar 24, 2010

VOID CON00-J. Synchronize access to shared mutable variables      11. Concurrency (CON)      CON03-J. Do not use background threads during class initialization

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