Page History

private final Map<Integer, String> mapView =
    

The {{java.util.Collections}} interface's documentation \[[API 2006|AA. Java References#API 06]\] warns about the consequences of failing to synchronize on an accessible collection object when iterating over its view:
{quote}
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.
{quote}

A class that uses a collection view instead of the backing collection as the lock object 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.

{mc} I don't see the point of this statement 
To make a group of statements atomic, synchronize on the original collection object when using synchronization wrappers.([CON07-J. Do not assume that a group of calls to independently atomic methods is atomic]). 
{mc}


h2. Noncompliant Code Example (Collection View)

This noncompliant code example creates two views: a synchronized view of an empty {{HashMap}} encapsulated by the {{map}} field and a set view of the map's keys encapsulated by the {{set}} field. This example synchronizes on the {{set}} view \[[Tutorials 2008|AA. Java References#Tutorials 08]\].

{code:bgColor=#FFcccc}
// map has package-private accessibility
final Map<Integer, String> map = Collections.synchronizedMap(new HashMap<Integer, String>());
private final Set<Integer> setsetView = mapmapView.keySet();

public Map<Integer, String> getMap() {
  return mapView;
}

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

private  for {{Map}}, which provides the backing collection for {{Set}}, as shown in the following figure:

!con06-j-backing-collection.JPG!

{{HashMap}} is not accessible, but the {{Map}} view is.  Because the {{Set}} view is synchronized instead of {{Map}} view, another thread can modify the contents of {{map}} and invalidate the {{k}} iterator.

h2. Compliant Solution (Collection Lock Object)

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

{code:bgColor=#ccccff}
// map has package-private accessibility
final Map<Integer, String> mapmapView =
    Collections.synchronizedMap(new HashMap<Integer, String>());
private final Set<Integer> setsetView = mapmapView.keySet();

public Map<Integer, String> getMap() {
  return mapView;
}

public void doSomething() {
  synchronized (mapmapView) {  // Synchronize on map, rather notthan set
    for (Integer k : setsetView) {
      // ...
    }
  }
}
{code}

||Completed||Priority||Locked||CreatedDate||CompletedDate||Assignee||Name||
|F|M|F|1270825291208|          |dmohindr|suggested => "HashMap is not accessible, but the Map view is. Because the set view is synchronized instead of the map view, another thread can modify the contents of map and invalidate the k iterator."|