Java多线程系列--“JUC原子类”04之 AtomicReference原子类 (r)

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

《Java多线程系列--“JUC原子类”04之 AtomicReference原子类 (r)》

// 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)

《Java多线程系列--“JUC原子类”04之 AtomicReference原子类 (r)》

 

AtomicReference源码分析(基于JDK1.7.0_40)

在JDK1.7.0_40中AtomicReference.java的源码如下:

《Java多线程系列--“JUC原子类”04之 AtomicReference原子类 (r)》

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());
    }
}

《Java多线程系列--“JUC原子类”04之 AtomicReference原子类 (r)》

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

《Java多线程系列--“JUC原子类”04之 AtomicReference原子类 (r)》

// 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;
    }
}

《Java多线程系列--“JUC原子类”04之 AtomicReference原子类 (r)》

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

1. Java多线程系列–“JUC原子类”01之 框架

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

4. Java多线程系列目录(共xx篇)

 

    原文作者:JUC
    原文地址: https://blog.csdn.net/wangtaomtk/article/details/52280562
    本文转自网络文章,转载此文章仅为分享知识,如有侵权,请联系博主进行删除。
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