Added in API level 1

Summary: Methods | Protected Methods | Inherited Methods

Reference

public abstract class Reference
extends Object

java.lang.Object
↳	java.lang.ref.Reference<T>

Known direct subclasses

PhantomReference<T>, SoftReference<T>, WeakReference<T>

PhantomReference<T>	Phantom reference objects, which are enqueued after the collector determines that their referents may otherwise be reclaimed.
SoftReference<T>	Soft reference objects, which are cleared at the discretion of the garbage collector in response to memory demand.
WeakReference<T>	Weak reference objects, which do not prevent their referents from being made finalizable, finalized, and then reclaimed.

Abstract base class for reference objects. This class defines the operations common to all reference objects. Because reference objects are implemented in close cooperation with the garbage collector, this class may not be subclassed directly.

Summary

Public methods
`void`	`clear()` Clears this reference object.
`boolean`	`enqueue()` Adds this reference object to the queue with which it is registered, if any.
`T`	`get()` Returns this reference object's referent.
`boolean`	`isEnqueued()` This method was deprecated in API level 31. This method was never implemented to test if a reference object has been cleared and enqueued as it was previously specified since 1.2. This method could be misused due to the inherent race condition or without an associated `ReferenceQueue`. An application relying on this method to release critical resources could cause serious performance issue. An application should use `ReferenceQueue` to reliably determine what reference objects that have been enqueued or `refersTo(null)` to determine if this reference object has been cleared.
`static void`	`reachabilityFence(Object ref)` Ensures that the object referenced by the given reference remains strongly reachable, regardless of any prior actions of the program that might otherwise cause the object to become unreachable; thus, the referenced object is not reclaimable by garbage collection at least until after the invocation of this method.
`final boolean`	`refersTo(T obj)` Tests if the referent of this reference object is `obj`.

Protected methods
`Object`	`clone()` Throws `CloneNotSupportedException`.

Inherited methods

From class


        
          java.lang.Object

`Object`	`clone()` Creates and returns a copy of this object.
`boolean`	`equals(Object obj)` Indicates whether some other object is "equal to" this one.
`void`	`finalize()` Called by the garbage collector on an object when garbage collection determines that there are no more references to the object.
`final Class<?>`	`getClass()` Returns the runtime class of this `Object`.
`int`	`hashCode()` Returns a hash code value for the object.
`final void`	`notify()` Wakes up a single thread that is waiting on this object's monitor.
`final void`	`notifyAll()` Wakes up all threads that are waiting on this object's monitor.
`String`	`toString()` Returns a string representation of the object.
`final void`	`wait(long timeoutMillis, int nanos)` Causes the current thread to wait until it is awakened, typically by being notified or interrupted, or until a certain amount of real time has elapsed.
`final void`	`wait(long timeoutMillis)` Causes the current thread to wait until it is awakened, typically by being notified or interrupted, or until a certain amount of real time has elapsed.
`final void`	`wait()` Causes the current thread to wait until it is awakened, typically by being notified or interrupted.

Public methods

clear

Added in API level 1

public void clear ()

Clears this reference object. Invoking this method will not cause this object to be enqueued.

This method is invoked only by Java code; when the garbage collector clears references it does so directly, without invoking this method.

enqueue

Added in API level 1

public boolean enqueue ()

Adds this reference object to the queue with which it is registered, if any.

This method is invoked only by Java code; when the garbage collector enqueues references it does so directly, without invoking this method.

Returns
`boolean`	`true` if this reference object was successfully enqueued; `false` if it was already enqueued or if it was not registered with a queue when it was created

get

Added in API level 1

public T get ()

Returns this reference object's referent. If this reference object has been cleared, either by the program or by the garbage collector, then this method returns null.

Returns
`T`	The object to which this reference refers, or `null` if this reference object has been cleared

isEnqueued

Added in API level 1
Deprecated in API level 31

public boolean isEnqueued ()

This method was deprecated in API level 31.
This method was never implemented to test if a reference object has been cleared and enqueued as it was previously specified since 1.2. This method could be misused due to the inherent race condition or without an associated ReferenceQueue. An application relying on this method to release critical resources could cause serious performance issue. An application should use ReferenceQueue to reliably determine what reference objects that have been enqueued or refersTo(null) to determine if this reference object has been cleared.

Tests if this reference object is in its associated queue, if any. This method returns true only if all of the following conditions are met:

this reference object was registered with a queue when it was created; and
the garbage collector has added this reference object to the queue or enqueue() is called; and
this reference object is not yet removed from the queue.

Otherwise, this method returns false. This method may return false if this reference object has been cleared but not enqueued due to the race condition.

Returns
`boolean`	`true` if and only if this reference object is in its associated queue (if any).

reachabilityFence

Added in API level 28

public static void reachabilityFence (Object ref)

Ensures that the object referenced by the given reference remains strongly reachable, regardless of any prior actions of the program that might otherwise cause the object to become unreachable; thus, the referenced object is not reclaimable by garbage collection at least until after the invocation of this method. Invocation of this method does not itself initiate garbage collection or finalization.

This method establishes an ordering for strong reachability with respect to garbage collection. It controls relations that are otherwise only implicit in a program -- the reachability conditions triggering garbage collection. This method is designed for use in uncommon situations of premature finalization where using synchronized blocks or methods, or using other synchronization facilities are not possible or do not provide the desired control. This method is applicable only when reclamation may have visible effects, which is possible for objects with finalizers (See Section 12.6 17 of The Java™ Language Specification) that are implemented in ways that rely on ordering control for correctness.

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.

Parameters
`ref`	`Object`: the reference. If `null`, this method has no effect.

refersTo

Added in API level 33

public final boolean refersTo (T obj)

Tests if the referent of this reference object is obj. Using a null obj returns true if the reference object has been cleared. Prefer this to a comparison with the result of get.

Parameters
`obj`	`T`: the object to compare with this reference object's referent

Returns
`boolean`	`true` if `obj` is the referent of this reference object

Protected methods

clone

Added in API level 1

protected Object clone ()

Throws CloneNotSupportedException. A Reference cannot be meaningfully cloned. Construct a new Reference instead.

Returns
`Object`	never returns normally

Throws
`CloneNotSupportedException`	always