Java中四种引用：强、软、弱、虚引用

这篇文章非常棒：http://alinazh.blog.51cto.com/5459270/1276173

Java中四种引用：强、软、弱、虚引用

1.1、强引用
当我们使用new 这个关键字创建对象时，被创建的对象就是强引用，如Object object = new Object() 这个Object()就是一个强引用。如果一个对象具有强引用，垃圾回收器就不会去回收有强引用的对象，如当jvm内存不足时，具备强引用的对象，虚拟机宁可会报内存空间不足的异常来终止程序，也不会靠垃圾回收器去回收该对象来解决内存。

1.2、软引用

如果一个对象只具有软引用，则内存空间足够，垃圾回收器就不会回收它；

如果内存空间不足了，就会回收这些对象的内存。

只要垃圾回收器没有回收它，该对象就可以被程序使用。

软引用可用来实现内存敏感的高速缓存（下文给出示例）。

软引用可以和一个引用队列（ReferenceQueue）关联使用：

String str = new String("test");
ReferenceQueue<String> rq = new ReferenceQueue<>();
SoftReference sf = new SoftReference(str,rq);

如果软引用(sf)所引用的对象(str)被垃圾回收器回收，Java虚拟机就会把这个软引用(sf)加入到与之关联的引用队列(rq)中,之后可以通过循环调用rq的poll()方法，来清除无用的软引用对象(sf)

如果软引用所引用的对象(str)还没有被垃圾回收器回收，则可以通过这个引用(sf)的get()方法重新获得对引用对象(str)的强引用.

1.3、弱引用
如果一个对象只具有弱引用，只要垃圾回收器在自己的内存空间中线程检测到了，就会立即被回收，对应内存也会被释放掉。相比软引用弱引用的生命周期要比软引用短很多。不过，如果垃圾回收器是一个优先级很低的线程，也不一定会很快就会释放掉软引用的内存。

通俗的来说就是：发生GC时必定回收弱引用指向的内存空间。

示例代码：

public static void main(String[] args) {
	String string = new String("test");
	ReferenceQueue<String> rq = new ReferenceQueue<>();
	WeakReference<String> wr = new WeakReference<String>(string,rq);
	System.out.println(wr.get());
	string = null;
	System.gc();
	System.out.println(wr.get());
}

运行结果：

test
null

1.4、虚引用

又称为幽灵引用或幻影引用，虚引用既不会影响对象的生命周期，也无法通过虚引用来获取对象实例，仅用于在发生GC时接收一个系统通知。

相关问题一、若一个对象的引用类型有多个，那到底如何判断它的可达性呢？其实规则如下：

　　1. 单条引用链的可达性以最弱的一个引用类型来决定；
　　2. 多条引用链的可达性以最强的一个引用类型来决定；

我们假设图2中引用①和③为强引用，⑤为软引用，⑦为弱引用，对于对象5按照这两个判断原则，路径①-⑤取最弱的引用⑤，因此该路径对对象5的引用为软引用。同样，③-⑦为弱引用。在这两条路径之间取最强的引用，于是对象5是一个软可及对象（当将要发生OOM时则会被回收掉）。

相关问题二、与引用相关联的引用队列的作用是什么

引用队列的作用：使用ReferenceQueue清除失去了引用对象的Reference（SoftReference，WeakReference等）

作为一个Java对象，SoftReference对象除了具有保存软引用的特殊性之外，也具有Java对象的一般性。所以，当软可及对象被回收之后，虽然这个SoftReference对象的get()方法返回null,但这个SoftReference对象已经不再具有存在的价值，需要一个适当的清除机制，避免大量SoftReference对象带来的内存泄漏。在java.lang.ref包里还提供了ReferenceQueue。如果在创建SoftReference对象的时候，使用了一个ReferenceQueue对象作为参数提供给SoftReference的构造方法，如:

ReferenceQueue queue = new ReferenceQueue();
SoftReference ref=new SoftReference(aMyObject, queue);

那么当这个SoftReference所软引用的aMyOhject被垃圾收集器回收的同时，ref所强引用的SoftReference对象被列入ReferenceQueue。也就是说，ReferenceQueue中保存的对象是Reference对象，而且是已经失去了它所软引用的对象的Reference对象。另外从ReferenceQueue这个名字也可以看出，它是一个队列，当我们调用它的poll()方法的时候，如果这个队列中不是空队列，那么将返回队列前面的那个Reference对象。

在任何时候，我们都可以调用ReferenceQueue的poll()方法来检查是否有它所关心的非强可及对象被回收。如果队列为空，将返回一个null,否则该方法返回队列中前面的一个Reference对象。利用这个方法，我们可以检查哪个SoftReference所软引用的对象已经被回收。于是我们可以把这些失去所软引用的对象的SoftReference对象清除掉。常用的方式为:

SoftReference ref = null;
while ((ref = (EmployeeRef) q.poll()) != null) {
// 清除ref
}

理解了ReferenceQueue的工作机制之后，我们就可以开始构造一个Java对象的高速缓存器了。下面是一个高速缓存器的代码：

EmployeeCache 类：

import java.lang.ref.ReferenceQueue;
import java.lang.ref.SoftReference;
import java.util.Hashtable;

public class EmployeeCache {
	static private EmployeeCache cache;
	private Hashtable<String, EmployeeRef> employeeRefs;
	private ReferenceQueue<Employee> q;
	private class EmployeeRef extends SoftReference<Employee>{
		private String _key="";
		public EmployeeRef(Employee em,ReferenceQueue<Employee> q) {
			super(em,q);
			_key=em.getId();
		}
	}
	private EmployeeCache() {
		employeeRefs = new Hashtable<>();
		q = new ReferenceQueue<>();
	}

	public static EmployeeCache getInstance(){
		if(cache==null){
			cache = new EmployeeCache();
		}
		return cache;
	}

	private void cacheEmployee(Employee em){
		cleanCache();
		EmployeeRef ref = new EmployeeRef(em, q);
		employeeRefs.put(em.getId(), ref);
	}

	public Employee getEmployee(String id){
		Employee employee = null;
		if(employeeRefs.contains(id)){
			EmployeeRef ref = employeeRefs.get(id);
			employee = ref.get();
		}

		if(employee == null){
			employee = new Employee(id);
			System.out.println("从信息中心获取新的对象");
			this.cacheEmployee(employee);
		}
		return employee;
	}
	private void cleanCache() {
		EmployeeRef ref = null;
		while((ref=(EmployeeRef) q.poll())!=null){
			employeeRefs.remove(ref._key);
		}
	}
	public void clearCache(){
		cleanCache();
		employeeRefs.clear();
		System.gc();
		System.runFinalization();
	}
}

Employee类：

import java.lang.ref.Reference;
import java.lang.ref.ReferenceQueue;
import java.lang.ref.SoftReference;

public class Employee {
	private String id;
	private String name;
	private String department;
	private String phone;
	private int salary;
	private String origin;

	public Employee(String id) {
		this.id = id;
		getDataFromInfoCenter();
	}

	private void getDataFromInfoCenter() {

	}

	public String getId() {
		String id = (int) (Math.random() * 10 + 1) + "";
		return id;
	}
}

 

2、jdk中的引用实现类

代表软引用的类：java.lang.ref.SoftReference
代表弱引用的类：java.lang.ref.WeakReference
代表虚引用的类：java.lang.ref.PhantomReference
他们同时继承了：java.lang.ref.Reference
引用队列：java.lang.ref.ReferenceQueue,这个引用队列是可以三种引用类型联合使用的，以便跟踪java虚拟机回收所引用对象的活动。

附：相关源码

 /*
  * @(#)ReferenceQueue.java    1.23 03/12/19
  *
  * Copyright 2004 Sun Microsystems, Inc. All rights reserved.
  * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
  */

 package java.lang.ref;

 /**
  * Reference queues, to which registered reference objects are appended by the
  * garbage collector after the appropriate reachability changes are detected.
  *
  * @version  1.23, 12/19/03
  * @author   Mark Reinhold
  * @since    1.2
  */

 public class ReferenceQueue<T> {

     /**
      * Constructs a new reference-object queue.
      */
     public ReferenceQueue() { }

     private static class Null extends ReferenceQueue {
     boolean enqueue(Reference r) {
         return false;
     }
     }

     static ReferenceQueue NULL = new Null();
     static ReferenceQueue ENQUEUED = new Null();

     static private class Lock { };
     private Lock lock = new Lock();
     private Reference<? extends T> head = null;
     private long queueLength = 0;

     boolean enqueue(Reference<? extends T> r) {    /* Called only by Reference class */
     synchronized (r) {
         if (r.queue == ENQUEUED) return false;
         synchronized (lock) {
         r.queue = ENQUEUED;
         r.next = (head == null) ? r : head;
         head = r;
         queueLength++;
                 if (r instanceof FinalReference) {
                     sun.misc.VM.addFinalRefCount(1);
                 }
         lock.notifyAll();
         return true;
         }
     }
     }

     private Reference<? extends T> reallyPoll() {    /* Must hold lock */
     if (head != null) {
         Reference<? extends T> r = head;
         head = (r.next == r) ? null : r.next;
         r.queue = NULL;
         r.next = r;
         queueLength--;
             if (r instanceof FinalReference) {
                 sun.misc.VM.addFinalRefCount(-1);
             }
         return r;
     }
     return null;
     }

     /**
      * Polls this queue to see if a reference object is available.  If one is
      * available without further delay then it is removed from the queue and
      * returned.  Otherwise this method immediately returns <tt>null</tt>.
      *
      * @return  A reference object, if one was immediately available,
      *          otherwise <code>null</code>
      */
     public Reference<? extends T> poll() {
     synchronized (lock) {
         return reallyPoll();
     }
     }

     /**
      * Removes the next reference object in this queue, blocking until either
      * one becomes available or the given timeout period expires.
      *
      * <p> This method does not offer real-time guarantees: It schedules the
      * timeout as if by invoking the {@link Object#wait(long)} method.
      *
      * @param  timeout  If positive, block for up <code>timeout</code>
      *                  milliseconds while waiting for a reference to be
      *                  added to this queue.  If zero, block indefinitely.
      *
      * @return  A reference object, if one was available within the specified
      *          timeout period, otherwise <code>null</code>
      *
      * @throws  IllegalArgumentException
      *          If the value of the timeout argument is negative
      *
      * @throws  InterruptedException
      *          If the timeout wait is interrupted
      */
     public Reference<? extends T> remove(long timeout)
     throws IllegalArgumentException, InterruptedException
     {
     if (timeout < 0) {
         throw new IllegalArgumentException("Negative timeout value");
     }
     synchronized (lock) {
         Reference<? extends T> r = reallyPoll();
         if (r != null) return r;
         for (;;) {
         lock.wait(timeout);
         r = reallyPoll();
         if (r != null) return r;
         if (timeout != 0) return null;
         }
     }
     }

     /**
      * Removes the next reference object in this queue, blocking until one
      * becomes available.
      *
      * @return A reference object, blocking until one becomes available
      * @throws  InterruptedException  If the wait is interrupted
      */
     public Reference<? extends T> remove() throws InterruptedException {
     return remove(0);
     }

 }

ReferenceQueue

 /*
  * @(#)SoftReference.java    1.34 03/12/19
  *
  * Copyright 2004 Sun Microsystems, Inc. All rights reserved.
  * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
  */

 package java.lang.ref;

 /**
  * 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.
  *
  * <p> Suppose that the garbage collector determines at a certain point in time
  * that an object is <a href="package-summary.html#reachability">softly
  * reachable</a>.  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.
  *
  * <p> 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.
  *
  * <p> 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.
  *
  * @version  1.34, 12/19/03
  * @author   Mark Reinhold
  * @since    1.2
  */

 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;
     return o;
     }

 }

SoftReference