java并发J.U.C AtomicReference VS Volatile

SpinLock实现,摘自并发编程网

package com.juc.simple;
import java.util.concurrent.atomic.AtomicReference;
/**
 * a implement of spinlock based on  cas
 *
 */
public class SpinLock {
	private AtomicReference<Thread> sign = new AtomicReference<>();
	/**
	 * 让当前线程不停地的在循环体内执
	 * 当循环的条件被其他线程改变时 才能进入临界区
	 */
	public void lock() {
		Thread current = Thread.currentThread();
		//自旋,如果sign为null才能设置为current or memcachedb: checkandset
		while (!sign.compareAndSet(null, current)) {
		}
	}
	public void unlock() {
		Thread current = Thread.currentThread();
		sign.compareAndSet(current, null);
	}
}

这里是通过 AtomicReference实现的，而AtomicReference是通过volatile与Unsafe的CAS实现

volatile保证可见性，所有的线程看到的都是最新（主内存？）的值,但是volatile的操作不具有原子性，这样像（a++)这种操作就不能保证安全了，实际是读--修改--写操作的过程

AtomicReference的CAS操作更新值具有原子性

先来一段AtomicReference与volatile的比较

package com.juc.simple;
import java.util.concurrent.atomic.AtomicReference;
public class VolatileVsAtomicReference {
	private static volatile Integer volatileVar = 0;
	//初始值value为0
	private static AtomicReference<Integer> atomicReference=new AtomicReference<Integer>(volatileVar);
	public static void main(String[] args) {
		 try {
			testAtomicReference();
			testVolatile();
		} catch (InterruptedException e) {
			System.out.println(e.getMessage());
		}
	}
	public static void testAtomicReference() throws InterruptedException{
	    for (int i = 0; i < 1000; i++) {
	        new Thread(new Runnable(){
	            @Override
	            public void run() {
	                for (int i = 0; i < 1000; i++)
	                    while(true){
	                        Integer temp=atomicReference.get();
	                        if(atomicReference.compareAndSet(temp, temp+1)){
	                        	break;
	                        }
	                    }
	            }
	        }).start();
	    }
	    Thread.sleep(1000);
	    System.out.println("atomicReference "+atomicReference.get()); //1000000
	}
	public static void testVolatile() throws InterruptedException{
	    for (int i = 0; i < 1000; i++) {
	        new Thread(new Runnable(){
	            @Override
	            public void run() {
	                for (int i = 0; i < 1000; i++) {
	                	volatileVar=volatileVar++;
	                }
	            }
	        }).start();
	    }
	    Thread.sleep(1000);
	    System.out.println("volatileVar "+volatileVar); //may 8545
	}
}

结果：试过好几次volatileVar值都是不一样的

atomicReference 1000000
volatileVar 8545

这里可以看出如果用了CAS的话AtomicReference是可以保证原子性的，但如果只是简单的get()和set()方法是不行的，其实就等同于volatile，因为这货本身就是volatile实现的

参考

Java volatile reference vs. AtomicReference

oracle官方链接地址

get has the memory effects of reading a volatile variable.
set has the memory effects of writing (assigning) a volatile variable.

Java 理论与实践: 正确使用 Volatile 变量

AtomicReference的原代码实现

package java.util.concurrent.atomic;
import sun.misc.Unsafe;
/**
 * An object reference that may be updated atomically. See the {@link
 * java.util.concurrent.atomic} package specification for description
 * of the properties of atomic variables.
 * @since 1.5
 * @author Doug Lea
 * @param <V> The type of object referred to by this reference
 */
public class AtomicReference<V>  implements java.io.Serializable {
    private static final long serialVersionUID = -1848883965231344442L;
    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 变量
    private volatile V value;
    /**
     * Creates a new AtomicReference with the given initial value.
     *
     * @param initialValue the initial value
     */
    public AtomicReference(V initialValue) {
        value = initialValue;
    }
    /**
     * Creates a new AtomicReference with null initial value.
     */
    public AtomicReference() {
    }
    /**
     * Gets the current value.
     *
     * @return the current value
     */
    public final V get() {
        return value;
    }
    /**
     * Sets to the given value.
     *
     * @param newValue the new value
     */
    //volatile变量赋值
    public final void set(V newValue) {
        value = newValue;
    }
    /**
     * Eventually sets to the given value.
     *
     * @param newValue the new value
     * @since 1.6
     */
    //设置为新的值
    public final void lazySet(V newValue) {
        unsafe.putOrderedObject(this, valueOffset, newValue);
    }
    /**
     * Atomically sets the value to the given updated value
     * if the current value {@code ==} the expected value.
     * @param expect the expected value
     * @param update the new value
     * @return true if successful. False return indicates that
     * the actual value was not equal to the expected value.
     */
    //如果当前值与期望值一致，则设置为新的值
    public final boolean compareAndSet(V expect, V update) {
        return unsafe.compareAndSwapObject(this, valueOffset, expect, update);
    }
    /**
     * Atomically sets the value to the given updated value
     * if the current value {@code ==} the expected value.
     *
     * <p>May <a href="package-summary.html#Spurious">fail spuriously</a>
     * and does not provide ordering guarantees, so is only rarely an
     * appropriate alternative to {@code compareAndSet}.
     *
     * @param expect the expected value
     * @param update the new value
     * @return true if successful.
     */
    public final boolean weakCompareAndSet(V expect, V update) {
        return unsafe.compareAndSwapObject(this, valueOffset, expect, update);
    }
    /**
     * Atomically sets to the given value and returns the old value.
     *
     * @param newValue the new value
     * @return the previous value
     */
    //以原子方式设置为新的值，并且返回旧值
    public final V getAndSet(V newValue) {
        while (true) {
            V x = get();
            if (compareAndSet(x, newValue))
                return x;
        }
    }
    /**
     * Returns the String representation of the current value.
     * @return the String representation of the current value.
     */
    public String toString() {
        return String.valueOf(get());
    }
}