Java——AtomicReference原子引用

缺乏、安全感 2023-06-09 10:28 24阅读 0赞

AtomicReference<V>（在java.util.concurrent.atomic.AtomicReference<V>中）可以原子更新的对象引用。

### 构造方法   ###

<table>  &nbsp;  
 <tbody> 
  <tr> 
   <th>构造方法和描述</th> 
  </tr> 
  <tr> 
   <td><code><a rel="nofollow">AtomicReference</a>()</code> <p>使用null初始值创建新的AtomicReference。</p> </td> 
  </tr> 
  <tr> 
   <td><code><a rel="nofollow">AtomicReference</a>(<a rel="nofollow">V</a>&nbsp;initialValue)</code> <p>用给定的初始值创建一个新的AtomicReference。</p> </td> 
  </tr> 
 </tbody> 
</table>

### 方法摘要 ###

<table>  &nbsp;  
 <tbody> 
  <tr> 
   <th>修饰和类型</th> 
   <th>方法和描述</th> 
  </tr> 
  <tr> 
   <td><code><a rel="nofollow">V</a></code></td> 
   <td><code><a rel="nofollow">accumulateAndGet</a>(<a rel="nofollow">V</a>&nbsp;x, <a rel="nofollow">BinaryOperator</a>&lt;<a rel="nofollow">V</a>&gt;&nbsp;accumulatorFunction)</code> <p>使用将给定函数应用于当前值和给定值的结果原子更新当前值，返回更新后的值。</p> </td> 
  </tr> 
  <tr> 
   <td><code>boolean</code></td> 
   <td><code><a rel="nofollow">compareAndSet</a>(<a rel="nofollow">V</a>&nbsp;expect, <a rel="nofollow">V</a>&nbsp;update)</code> <p>如果当前值 <code>==</code>为预期值，则将值设置为给定的更新值。</p> </td> 
  </tr> 
  <tr> 
   <td><code><a rel="nofollow">V</a></code></td> 
   <td><code><a rel="nofollow">get</a>()</code> <p>获取当前值。</p> </td> 
  </tr> 
  <tr> 
   <td><code><a rel="nofollow">V</a></code></td> 
   <td><code><a rel="nofollow">getAndAccumulate</a>(<a rel="nofollow">V</a>&nbsp;x, <a rel="nofollow">BinaryOperator</a>&lt;<a rel="nofollow">V</a>&gt;&nbsp;accumulatorFunction)</code> <p>使用给定函数应用给当前值和给定值的结果原子更新当前值，返回上一个值。</p> </td> 
  </tr> 
  <tr> 
   <td><code><a rel="nofollow">V</a></code></td> 
   <td><code><a rel="nofollow">getAndSet</a>(<a rel="nofollow">V</a>&nbsp;newValue)</code> <p>将原子设置为给定值并返回旧值。</p> </td> 
  </tr> 
  <tr> 
   <td><code><a rel="nofollow">V</a></code></td> 
   <td><code><a rel="nofollow">getAndUpdate</a>(<a rel="nofollow">UnaryOperator</a>&lt;<a rel="nofollow">V</a>&gt;&nbsp;updateFunction)</code> <p>用应用给定函数的结果原子更新当前值，返回上一个值。</p> </td> 
  </tr> 
  <tr> 
   <td><code>void</code></td> 
   <td><code><a rel="nofollow">lazySet</a>(<a rel="nofollow">V</a>&nbsp;newValue)</code> <p>最终设定为给定值。</p> </td> 
  </tr> 
  <tr> 
   <td><code>void</code></td> 
   <td><code><a rel="nofollow">set</a>(<a rel="nofollow">V</a>&nbsp;newValue)</code> <p>设置为给定值。</p> </td> 
  </tr> 
  <tr> 
   <td><code><a rel="nofollow">String</a></code></td> 
   <td><code><a rel="nofollow">toString</a>()</code> <p>返回当前值的String表示形式。</p> </td> 
  </tr> 
  <tr> 
   <td><code><a rel="nofollow">V</a></code></td> 
   <td><code><a rel="nofollow">updateAndGet</a>(<a rel="nofollow">UnaryOperator</a>&lt;<a rel="nofollow">V</a>&gt;&nbsp;updateFunction)</code> <p>使用给定函数的结果原子更新当前值，返回更新的值。</p> </td> 
  </tr> 
  <tr> 
   <td><code>boolean</code></td> 
   <td><code><a rel="nofollow">weakCompareAndSet</a>(<a rel="nofollow">V</a>&nbsp;expect, <a rel="nofollow">V</a>&nbsp;update)</code> <p>如果当前值为 <code>==</code> ，则将原值设置为给定的更新值。</p> </td> 
  </tr> 
 </tbody> 
</table>

代码

public class AtomicReference<V>  implements java.io.Serializable {
        private static final long serialVersionUID = -1848883965231344442L;
     
        // 获取Unsafe对象，Unsafe的作用是提供CAS操作
        private static final Unsafe unsafe = Unsafe.getUnsafe();
        private static final long valueOffset;
     
        static {
          try {
            valueOffset = unsafe.objectFieldOffset
                (AtomicReference.class.getDeclaredField("value"));
          } catch (Exception ex) { throw new Error(ex); }
        }
     
        // volatile类型
        private volatile V value;
     
        public AtomicReference(V initialValue) {
            value = initialValue;
        }
     
        public AtomicReference() {
        }
     
        public final V get() {
            return value;
        }
     
        public final void set(V newValue) {
            value = newValue;
        }
     
        public final void lazySet(V newValue) {
            unsafe.putOrderedObject(this, valueOffset, newValue);
        }
     
        public final boolean compareAndSet(V expect, V update) {
            return unsafe.compareAndSwapObject(this, valueOffset, expect, update);
        }
     
        public final boolean weakCompareAndSet(V expect, V update) {
            return unsafe.compareAndSwapObject(this, valueOffset, expect, update);
        }
     
        public final V getAndSet(V newValue) {
            while (true) {
                V x = get();
                if (compareAndSet(x, newValue))
                    return x;
            }
        }
     
        public String toString() {
            return String.valueOf(get());
        }
    }

public class Main{
    	public static void main(String[] args) {
     
    		// 创建两个Person对象，它们的id分别是101和102。
            Person2 p1 = new Person2(101);
            Person2 p2 = new Person2(102);
            // 新建AtomicReference对象，初始化它的值为p1对象
            AtomicReference ar = new AtomicReference(p1);
            //更改p1的id.
            p1.setId(106);
            // 通过CAS设置ar。如果ar的值为p1的话，则将其设置为p2。
            ar.compareAndSet(p1, p2);
     
            Person2 p3 = (Person2)ar.get();
            System.out.println("p3 is "+p3);
            System.out.println("p3.equals(p1)="+p3.equals(p1));
        }
     
    }
    class Person2 {
        volatile long id;
        public Person2(long id) {
            this.id = id;
        }
        public String toString() {
            return "id:"+id;
        }
        public void setId(long id){
        	this.id=id;
        }
    }

运行结果如下：

p3 is id:102  
p3.equals(p1)=false

可以看出，修改了p1的id对compareAndSet()并没有影响，因为修改id仅仅改变了p1的成员变量，而AtomicReference底层使用的是compareAndSwapObject实现CAS，比较的是两个对象的地址是否相等。也就是它可以保证你在修改对象引用时的线程安全性。上面代码成功的将原子类中的引用从p1变成p2，而且是线程安全的。