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The @Region and @RegionLock annotations document the locking policy upon which the promise of thread-safety is predicated.

Even when one or more @RegionLock or @GuardedBy annotations have been used to document the locking policy of a class, the @ThreadSafe annotation provides an intuitive way for reviewers to learn that the class is thread-safe.

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For each mutable state variable that may be accessed by more than one thread, all accesses to that variable must be performed with the same lock held. In this case, we say that the variable is guarded by that lock. (p. 28)

JCIP provides the @GuardedBy annotation for this purpose, and SureLogic provides the @RegionLock annotation. The field or method to which the @GuardedBy annotation is applied can be accessed only when holding a particular lock. It may be an intrinsic lock or a dynamic lock such as java.util.concurrent.Lock.

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@ThreadSafe
public final class MovablePoint {

  @GuardedBy("this")
  double xPos = 1.0;
  @GuardedBy("this")
  double yPos = 1.0;
  @GuardedBy("itself")
  static final List<MovablePoint> memo
    = new ArrayList<MovablePoint>();

  public void move(double slope, double distance) {
    synchronized (this) {
      rememberPoint(this);
      xPos += (1 / slope) * distance;
      yPos += slope * distance;
    }
  }

  public static void rememberPoint(MovablePoint value) {
    synchronized (memo) {
      memo.add(value);
    }
  }
}

The @GuardedBy annotations on the xPos and yPos fields indicate that access to these fields is protected by holding a lock on this. The move() method also synchronizes on this, which modifies these fields. The @GuardedBy annotation on the memo list indicates that a lock on the ArrayList object protects its contents. The rememberPoint() method also synchronizes on the memo list.

One issue with the @GuardedBy annotation is that it fails to indicate when there is a relationship between the fields of a class. This limitation can be overcome by using the SureLogic @RegionLock annotation, which declares a new region lock for the class to which this annotation is applied. This declaration creates a new named lock that associates a particular lock object with a region of the class. The region may be accessed only when the lock is held. For example, the SimpleLock locking policy indicates that synchronizing on the instance protects all of its state:

 @RegionLock("SimpleLock is this protects Instance")
 class Simple { ... }

Unlike @GuardedBy, the @RegionLock annotation allows the programmer to give an explicit, and hopefully meaningful, name to the locking policy.

In addition to naming the locking policy, the @Region annotation allows a name to be given to the region of the state that is being protected. That name makes it clear that the state and locking policy belong together, as demonstrated in the following example:

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For example, in the following code, the @Unique("return") annotation documents that the object returned from the constructor is a unique reference:

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