概要
本章对AtomicReference引用类型的原子类进行介绍。内容包括：
AtomicReference介绍和函数列表
AtomicReference源码分析(基于JDK1.7.0_40)
AtomicReference示例
转载请注明出处：http://www.cnblogs.com/skywang12345/p/3514623.html
AtomicReference介绍和函数列表
AtomicReference是作用是对”对象”进行原子操作。
AtomicReference函数列表
// 使用 null 初始值创建新的 AtomicReference。 AtomicReference() // 使用给定的初始值创建新的 AtomicReference。 AtomicReference(V initialValue) // 如果当前值 == 预期值，则以原子方式将该值设置为给定的更新值。 boolean compareAndSet(V expect, V update) // 获取当前值。 V get() // 以原子方式设置为给定值，并返回旧值。 V getAndSet(V newValue) // 最终设置为给定值。 void lazySet(V newValue) // 设置为给定值。 void set(V newValue) // 返回当前值的字符串表示形式。 String toString() // 如果当前值 == 预期值，则以原子方式将该值设置为给定的更新值。 boolean weakCompareAndSet(V expect, V update)
AtomicReference源码分析(基于JDK1.7.0_40)
在JDK1.7.0_40中AtomicReference.java的源码如下：
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()); } }
说明：
AtomicReference的源码比较简单。它是通过”volatile”和”Unsafe提供的CAS函数实现”原子操作。
(01) value是volatile类型。这保证了：当某线程修改value的值时，其他线程看到的value值都是最新的value值，即修改之后的volatile的值。
(02) 通过CAS设置value。这保证了：当某线程池通过CAS函数(如compareAndSet函数)设置value时，它的操作是原子的，即线程在操作value时不会被中断。
AtomicReference示例
// AtomicReferenceTest.java的源码 import java.util.concurrent.atomic.AtomicReference; public class AtomicReferenceTest { public static void main(String[] args){ // 创建两个Person对象，它们的id分别是101和102。 Person p1 = new Person(101); Person p2 = new Person(102); // 新建AtomicReference对象，初始化它的值为p1对象 AtomicReference ar = new AtomicReference(p1); // 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); Person p3 = (Person)ar.get(); System.out.println("p3 is "+p3); System.out.println("p3.equals(p1)="+p3.equals(p1)); } } class Person { volatile long id; public Person(long id) { this.id = id; } public String toString() { return "id:"+id; } }
运行结果：
p3 is id:102 p3.equals(p1)=false
结果说明：
新建AtomicReference对象ar时，将它初始化为p1。
紧接着，通过CAS函数对它进行设置。如果ar的值为p1的话，则将其设置为p2。
最后，获取ar对应的对象，并打印结果。p3.equals(p1)的结果为false，这是因为Person并没有覆盖equals()方法，而是采用继承自Object.java的equals()方法；而Object.java中的equals()实际上是调用”==”去比较两个对象，即比较两个对象的地址是否相等。
/*
* @(#)AtomicReference.java 1.12 06/06/15
*
* Copyright 2006 Sun Microsystems, Inc. All rights reserved.
* SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
*/
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; //value的偏移
static {
try {
valueOffset = unsafe.objectFieldOffset
(AtomicReference.class.getDeclaredField("value"));
} catch (Exception ex) { throw new Error(ex); }
}
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
*/
public final void set(V newValue) { //设置新值
value = newValue;
}
/**
* Eventually sets to the given value.
*
* @param newValue the new value
* @since 1.6
*/
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.
*/
public final boolean compareAndSet(V expect, V update) { //设置新值
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
*/
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());
}
}
/*
* @(#)AtomicReferenceArray.java 1.12 06/06/15
*
* Copyright 2006 Sun Microsystems, Inc. All rights reserved.
* SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
*/
package java.util.concurrent.atomic;
import sun.misc.Unsafe;
import java.util.*;
/**
* An array of object references in which elements 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 <E> The base class of elements held in this array
*/
public class AtomicReferenceArray<E> implements java.io.Serializable {
private static final long serialVersionUID = -6209656149925076980L;
private static final Unsafe unsafe = Unsafe.getUnsafe();
private static final int base = unsafe.arrayBaseOffset(Object[].class); //第一个元素地址偏移（在数组内的偏移）
private static final int scale = unsafe.arrayIndexScale(Object[].class); //数组元素增量
private final Object[] array;
//得到索引i元素的偏移量
private long rawIndex(int i) {
if (i < 0 || i >= array.length)
throw new IndexOutOfBoundsException("index " + i);
return base + i * scale;
}
/**
* Creates a new AtomicReferenceArray of given length.
* @param length the length of the array
*/
public AtomicReferenceArray(int length) {
array = new Object[length];
// must perform at least one volatile write to conform to JMM
if (length > 0) //进行一次写，保证内存可见性
unsafe.putObjectVolatile(array, rawIndex(0), null);
}
/**
* Creates a new AtomicReferenceArray with the same length as, and
* all elements copied from, the given array.
*
* @param array the array to copy elements from
* @throws NullPointerException if array is null
*/
public AtomicReferenceArray(E[] array) {
if (array == null)
throw new NullPointerException();
int length = array.length;
this.array = new Object[length];
if (length > 0) {
int last = length-1;
for (int i = 0; i < last; ++i)
this.array[i] = array[i];
// Do the last write as volatile
E e = array[last];
unsafe.putObjectVolatile(this.array, rawIndex(last), e);
}
}
/**
* Returns the length of the array.
*
* @return the length of the array
*/
public final int length() {
return array.length;
}
/**
* Gets the current value at position {@code i}.
*
* @param i the index
* @return the current value
*/
public final E get(int i) {
return (E) unsafe.getObjectVolatile(array, rawIndex(i)); //array,offset
}
/**
* Sets the element at position {@code i} to the given value.
*
* @param i the index
* @param newValue the new value
*/
public final void set(int i, E newValue) {
unsafe.putObjectVolatile(array, rawIndex(i), newValue);//array,offset,新值
}
/**
* Eventually sets the element at position {@code i} to the given value.
*
* @param i the index
* @param newValue the new value
* @since 1.6
*/
public final void lazySet(int i, E newValue) {
unsafe.putOrderedObject(array, rawIndex(i), newValue);
}
/**
* Atomically sets the element at position {@code i} to the given
* value and returns the old value.
*
* @param i the index
* @param newValue the new value
* @return the previous value
*/
public final E getAndSet(int i, E newValue) {
while (true) {
E current = get(i);
if (compareAndSet(i, current, newValue))
return current;
}
}
/**
* Atomically sets the element at position {@code i} to the given
* updated value if the current value {@code ==} the expected value.
* @param i the index
* @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.
*/
public final boolean compareAndSet(int i, E expect, E update) {
return unsafe.compareAndSwapObject(array, rawIndex(i),
expect, update);
}
/**
* Atomically sets the element at position {@code i} 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 i the index
* @param expect the expected value
* @param update the new value
* @return true if successful.
*/
public final boolean weakCompareAndSet(int i, E expect, E update) {
return compareAndSet(i, expect, update);
}
/**
* Returns the String representation of the current values of array.
* @return the String representation of the current values of array.
*/
public String toString() {
if (array.length > 0) // force volatile read
get(0);
return Arrays.toString(array);
}
}
更多内容
2. Java多线程系列–“JUC原子类”02之 AtomicLong原子类
3. Java多线程系列–“JUC原子类”03之 AtomicLongArray原子类
