The java.util.Collections interface's documentation [[API 06]] warns about the consequences of synchronizing on any view over a collection object, rather than synchronizing on the collection object itself.
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, which is held by the map field, and a set view of the map's keys, which is held by the set field. Furthermore, this code synchronizes on the set view instead of the 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 safe because the origianl map (created by new HashMap()) is not accessible and hence unmodifiable except via the map field.
// 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
50%
Review List
VOID CON00-J. Synchronize access to shared mutable variables 11. Concurrency (CON) CON03-J. Do not use background threads during class initialization