SoftReference

Class Overview

A reference that is cleared when its referent is not strongly reachable and there is memory pressure.

Avoid Soft References for Caching

In practice, soft references are inefficient for caching. The runtime doesn't have enough information on which references to clear and which to keep. Most fatally, it doesn't know what to do when given the choice between clearing a soft reference and growing the heap.

The lack of information on the value to your application of each reference limits the usefulness of soft references. References that are cleared too early cause unnecessary work; those that are cleared too late waste memory.

Most applications should use an android.util.LruCache instead of soft references. LruCache has an effective eviction policy and lets the user tune how much memory is allotted.

Garbage Collection of Soft References

When the garbage collector encounters an object obj that is softly-reachable, the following happens:

• A set refs of references is determined. refs contains the following elements:
  • All soft references pointing to obj.
  • All soft references pointing to objects from which obj is strongly reachable.
• All references in refs are atomically cleared.
• At the same time or some time in the future, all references in refs will be enqueued with their corresponding reference queues, if any.

The system may delay clearing and enqueueing soft references, yet all SoftReferences pointing to softly reachable objects will be cleared before the runtime throws an OutOfMemoryError.

Unlike a WeakReference, a SoftReference will not be cleared and enqueued until the runtime must reclaim memory to satisfy an allocation.

Summary

Public Constructors
	SoftReference(T r) Constructs a new soft reference to the given referent.
	SoftReference(T r, ReferenceQueue<? super T> q) Constructs a new soft reference to the given referent.

[Expand] Inherited Methods
From class java.lang.ref.Reference void clear() Makes the referent null. boolean enqueue() Forces the reference object to be enqueued if it has been associated with a queue. T get() Returns the referent of the reference object. boolean isEnqueued() Checks whether the reference object has been enqueued.
From class java.lang.Object Object clone() Creates and returns a copy of this Object. boolean equals( Object o) Compares this instance with the specified object and indicates if they are equal. void finalize() Invoked when the garbage collector has detected that this instance is no longer reachable. final Class<?> getClass() Returns the unique instance of Class that represents this object's class. int hashCode() Returns an integer hash code for this object. final void notify() Causes a thread which is waiting on this object's monitor (by means of calling one of the wait() methods) to be woken up. final void notifyAll() Causes all threads which are waiting on this object's monitor (by means of calling one of the wait() methods) to be woken up. String toString() Returns a string containing a concise, human-readable description of this object. final void wait() Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object. final void wait(long millis, int nanos) Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object or until the specified timeout expires. final void wait(long millis) Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object or until the specified timeout expires.