Reference

API Note:

• Finalization may occur whenever the virtual machine detects that no reference to an object will ever be stored in the heap: The garbage collector may reclaim an object even if the fields of that object are still in use, so long as the object has otherwise become unreachable. This may have surprising and undesirable effects in cases such as the following example in which the bookkeeping associated with a class is managed through array indices. Here, method action uses a reachabilityFence to ensure that the Resource object is not reclaimed before bookkeeping on an associated ExternalResource has been performed; in particular here, to ensure that the array slot holding the ExternalResource is not nulled out in method Object#finalize, which may otherwise run concurrently.

   class Resource {
     private static ExternalResource[] externalResourceArray = ...
  
     int myIndex;
     Resource(...) {
       myIndex = ...
       externalResourceArray[myIndex] = ...;
       ...
     }
     protected void finalize() {
       externalResourceArray[myIndex] = null;
       ...
     }
     public void action() {
       try {
         // ...
         int i = myIndex;
         Resource.update(externalResourceArray[i]);
       } finally {
         Reference.reachabilityFence(this);
       }
     }
     private static void update(ExternalResource ext) {
       ext.status = ...;
     }
   }

  Here, the invocation of reachabilityFence is nonintuitively placed after the call to update, to ensure that the array slot is not nulled out by Object#finalize before the update, even if the call to action was the last use of this object. This might be the case if, for example a usage in a user program had the form new Resource().action(); which retains no other reference to this Resource. While probably overkill here, reachabilityFence is placed in a finally block to ensure that it is invoked across all paths in the method. In a method with more complex control paths, you might need further precautions to ensure that reachabilityFence is encountered along all of them.

  It is sometimes possible to better encapsulate use of reachabilityFence. Continuing the above example, if it were acceptable for the call to method update to proceed even if the finalizer had already executed (nulling out slot), then you could localize use of reachabilityFence:

   public void action2() {
     // ...
     Resource.update(getExternalResource());
   }
   private ExternalResource getExternalResource() {
     ExternalResource ext = externalResourceArray[myIndex];
     Reference.reachabilityFence(this);
     return ext;
   }

  Method reachabilityFence is not required in constructions that themselves ensure reachability. For example, because objects that are locked cannot, in general, be reclaimed, it would suffice if all accesses of the object, in all methods of class Resource (including finalize) were enclosed in synchronized (this) blocks. (Further, such blocks must not include infinite loops, or themselves be unreachable, which fall into the corner case exceptions to the "in general" disclaimer.) However, method reachabilityFence remains a better option in cases where this approach is not as efficient, desirable, or possible; for example because it would encounter deadlock.