Reference

概述

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.

引用对象的基类；

定义了所有引用对象的基本行为；

SoftReference

概述

Soft reference objects, which are cleared at the discretion of the garbage collector in response to memory demand.
Soft references are most often used to implement memory-sensitive caches.

软引用对象，GC根据内存需求自行决定clear；

Suppose that the garbage collector determines at a certain point in time that an object is <a href="package-summary.html#reachability">softly reachable.
At that time it may choose to clear atomically all soft references to that object and all soft references to any other softly-reachable objects from which that object is reachable through a chain of strong references.
At the same time or at some later time it will enqueue those newly-cleared soft references that are registered with reference queues.

All soft references to softly-reachable objects are guaranteed to have been cleared before the virtual machine throws an <code>OutOfMemoryError</code>.
Otherwise no constraints are placed upon the time at which a soft reference will be cleared or the order in which a set of such references to different objects will be cleared.
Virtual machine implementations are, however, encouraged to bias against clearing recently-created or recently-used soft references.

在JVM抛出OutOfMemoryError之前，软引用对象保证被clear；

Direct instances of this class may be used to implement simple caches; this class or derived subclasses may also be used in larger data structures to implement more sophisticated caches.
As long as the referent of a soft reference is strongly reachable, that is, is actually in use, the soft reference will not be cleared.
Thus a sophisticated cache can, for example, prevent its most recently used entries from being discarded by keeping strong referents to those entries, leaving the remaining entries to be discarded at the discretion of the garbage collector.

实例可以用来实现Caches;

public class SoftReference<T> extends Reference<T> {

        /**
         * Timestamp clock, updated by the garbage collector
         */
        static private long clock;

        /**
         * Timestamp updated by each invocation of the get method.
         * The VM may use this field when selecting soft references to be cleared, but it is not required to do so.
         */
        private long timestamp;

        /**
         * Creates a new soft reference that refers to the given object.
         * The new reference is not registered with any queue.
         */
        public SoftReference(T referent) {
            super(referent);
            this.timestamp = clock;
        }

        /**
         * 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 <code>null</code>.
         */
        public T get() {
            T o = super.get();
            if (o != null && this.timestamp != clock)
                this.timestamp = clock;
            return o;
        }
    }

WeakReference

概述

Weak reference objects, which do not prevent their referents from being made finalizable, finalized, and then reclaimed.
Weak references are most often used to implement canonicalizing mappings.

Suppose that the garbage collector determines at a certain point in time that an object is <a href="package-summary.html#reachability">weakly reachable</a>.
At that time it will atomically clear all weak references to that object and all weak references to any other weakly-reachable objects from which that object is reachable through a chain of strong and soft references.
At the same time it will declare all of the formerly weakly-reachable objects to be finalizable.
At the same time or at some later time it will enqueue those newly-cleared weak references that are registered with reference queues.

public class WeakReference<T> extends Reference<T> {
        /**
         * Creates a new weak reference that refers to the given object.
         * The new reference is not registered with any queue.
         */
        public WeakReference(T referent) {
            super(referent);
        }

        /**
         * Creates a new weak reference that refers to the given object and is
         * registered with the given queue.
         */
        public WeakReference(T referent, ReferenceQueue<? super T> q) {
            super(referent, q);
        }
    }

示例

public static void main(String[] args) throws InterruptedException {
        WeakReference<MyObject> weakReference = new WeakReference<>(new MyObject("jack"));

        System.out.println(weakReference.get());

        // 强制触发一次垃圾回收
        System.gc();

        Thread.sleep(100);

        System.out.println(weakReference.get());
    }

    @AllArgsConstructor
    @Data
    static class MyObject {
        private String id;
    }


结果：
WeakRefTest.MyObject(id=jack)
null