Java強引用、軟引用、弱引用、虛引用詳解

Java中沒有指針的概念，而援用就是1個弱化的指針，保證開發不能任意操作內存。最近整理了1下之前不明白的各種級別援用：強援用、軟援用、弱援用、虛援用，它們的特點和利用場景匯總以下： 
1、強援用 
如果1個對象具有強援用，GC絕不會回收它；當內存空間不足，JVM寧愿拋出OutOfMemoryError毛病。1般new出來的對象都是強援用，以下

//強援用 
User strangeReference=new User();  
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2、軟援用 
如果1個對象具有軟援用，當內存空間不足，GC會回收這些對象的內存，使用軟援用構建敏感數據的緩存。 
在JVM中，軟援用是以下定義的，可以通過1個時間戳來回收，下面引自JVM：

    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.
     *
     * @param referent object the new soft reference will refer to
     */
    public SoftReference(T referent) {
        super(referent);
        this.timestamp = clock;
    }

    /**
     * Creates a new soft reference that refers to the given object and is
     * registered with the given queue.
     *
     * @param referent object the new soft reference will refer to
     * @param q the queue with which the reference is to be registered,
     *          or <tt>null</tt> if registration is not required
     *
     */
    public SoftReference(T referent, ReferenceQueue<? super T> q) {
        super(referent, q);
        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>.
     *
     * @return   The object to which this reference refers, or
     *           <code>null</code> if this reference object has been cleared
     */
    public T get() {
        T o = super.get();
        if (o != null && this.timestamp != clock)
            this.timestamp = clock;
        return o;
    }

}
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軟援用的聲明的借助強援用或匿名對象，使用泛型SoftReference；可以通過get方法取得強援用。具體以下：

//軟援用
SoftReference<User>softReference=new SoftReference<User>(new User());
strangeReference=softReference.get();//通過get方法取得強援用
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3、弱援用 
如果1個對象具有弱援用，在GC線程掃描內存區域的進程中，不管當前內存空間足夠與否，都會回收內存，使用弱援用 構建非敏感數據的緩存。 
在JVM中，弱援用是以下定義的，下面引自JVM：

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.
     *
     * @param referent object the new weak reference will refer to
     */
    public WeakReference(T referent) {
        super(referent);
    }

    /**
     * Creates a new weak reference that refers to the given object and is
     * registered with the given queue.
     *
     * @param referent object the new weak reference will refer to
     * @param q the queue with which the reference is to be registered,
     *          or <tt>null</tt> if registration is not required
     */
    public WeakReference(T referent, ReferenceQueue<? super T> q) {
        super(referent, q);
    }

}
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弱援用的聲明的借助強援用或匿名對象，使用泛型WeakReference<T>，具體以下：

//弱援用
WeakReference<User>weakReference=new WeakReference<User>(new User());
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4、虛援用 
如果1個對象僅持有虛援用，在任什么時候候都可能被垃圾回收，虛援用與軟援用和弱援用的1個區分在于：虛援用必須和援用隊列聯合使用，虛援用主要用來跟蹤對象 被垃圾回收的活動。 
在JVM中，虛援用是以下定義的，下面引自JVM：

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

    /**
     * Returns this reference object's referent.  Because the referent of a
     * phantom reference is always inaccessible, this method always returns
     * <code>null</code>.
     *
     * @return  <code>null</code>
     */
    public T get() {
        return null;
    }

    /**
     * Creates a new phantom reference that refers to the given object and
     * is registered with the given queue.
     *
     * <p> It is possible to create a phantom reference with a <tt>null</tt>
     * queue, but such a reference is completely useless: Its <tt>get</tt>
     * method will always return null and, since it does not have a queue, it
     * will never be enqueued.
     *
     * @param referent the object the new phantom reference will refer to
     * @param q the queue with which the reference is to be registered,
     *          or <tt>null</tt> if registration is not required
     */
    public PhantomReference(T referent, ReferenceQueue<? super T> q) {
        super(referent, q);
    }

}
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虛援用PhantomReference<T>的聲明的借助強援用或匿名對象,結合泛型ReferenceQueue<T>初始化，具體以下：

//虛援用
PhantomReference<User> phantomReference=new PhantomReference<User>(new User(),new ReferenceQueue<User>());
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5、總結 
下面是1段關于強援用、軟援用、弱援用、虛援用的程序：

import java.lang.ref.*;
import java.util.HashSet;
import java.util.Set;

class User {

    private String name;

    public User()
    {}

    public User(String name)
    {
        this.name=name;
    }

    @Override
    public String toString() {
        return name;
    }

    public void finalize(){
        System.out.println("Finalizing ... "+name);
    }
}

/**
 * Created by jinxu on 15⑷⑵5.
 */
public class ReferenceDemo {

    private static ReferenceQueue<User> referenceQueue = new ReferenceQueue<User>();
    private static final int size = 10;

    public static void checkQueue(){
       /* Reference<? extends User> reference = null;
        while((reference = referenceQueue.poll())!=null){
            System.out.println("In queue : "+reference.get());
        }*/
        Reference<? extends User> reference = referenceQueue.poll();
        if(reference!=null){
            System.out.println("In queue : "+reference.get());
        }
    }

    public static void testSoftReference()
    {
        Set<SoftReference<User>> softReferenceSet = new HashSet<SoftReference<User>>();
        for (int i = 0; i < size; i++) {
            SoftReference<User> ref = new SoftReference<User>(new User("Soft " + i), referenceQueue);
            System.out.println("Just created: " + ref.get());
            softReferenceSet.add(ref);
        }
        System.gc();
        checkQueue();
    }

    public static void testWeaKReference()
    {
        Set<WeakReference<User>> weakReferenceSet = new HashSet<WeakReference<User>>();
        for (int i = 0; i < size; i++) {
            WeakReference<User> ref = new WeakReference<User>(new User("Weak " + i), referenceQueue);
            System.out.println("Just created: " + ref.get());
            weakReferenceSet.add(ref);
        }
        System.gc();
        checkQueue();
    }

    public static void testPhantomReference()
    {
        Set<PhantomReference<User>> phantomReferenceSet = new HashSet<PhantomReference<User>>();
        for (int i = 0; i < size; i++) {
            PhantomReference<User> ref =
                    new PhantomReference<User>(new User("Phantom " + i), referenceQueue);
            System.out.println("Just created: " + ref.get());
            phantomReferenceSet.add(ref);
        }
        System.gc();
        checkQueue();
    }

    public static void main(String[] args) {
        testSoftReference();
        testWeaKReference();
        testPhantomReference();
    }
}
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