假设有一个类 Person，定义如下：
class Person {
private String name;
private int age;
public Person(String name, int age) {
this.name = name;
this.age = age;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
public String toString() {
return "[name: " + this.name + ", age: " + this.age + "]";
}
}
如果使用普通的对象引用，在多线程情况下进行对象的更新可能会导致不一致性。例如：
一个对象的初始状态为 name=Tom, age = 18。
在 线程1 中将 name 修改为 Tom1，age + 1。
在 线程2 中将 name 修改为 Tom2，age + 2。
我们认为只会产生两种结果：
若 线程1 先执行，线程2 后执行，则中间状态为 name=Tom1, age = 19，结果状态为 name=Tom2, age = 21
若 线程2 先执行，线程1 后执行，则中间状态为 name=Tom2, age = 20，结果状态为 name=Tom1, age = 21
但是可能的输出如下：
Person is [name: Tom, age: 18]
Thread2 Values [name: Tom1, age: 21]
Thread1 Values [name: Tom1, age: 21]
Now Person is [name: Tom1, age: 21
// 普通引用
private static Person person;
public static void main(String[] args) throws InterruptedException {
person = new Person("Tom", 18);
System.out.println("Person is " + person.toString());
Thread t1 = new Thread(new Task1());
Thread t2 = new Thread(new Task2());
t1.start();
t2.start();
t1.join();
t2.join();
System.out.println("Now Person is " + person.toString());
}
static class Task1 implements Runnable {
public void run() {
person.setAge(person.getAge() + 1);
person.setName("Tom1");
System.out.println("Thread1 Values "
+ person.toString());
}
}
static class Task2 implements Runnable {
public void run() {
person.setAge(person.getAge() + 2);
person.setName("Tom2");
System.out.println("Thread2 Values "
+ person.toString());
}
}
可能的输出如下：
Atomic Person is [name: Tom, age: 18]
Thread1 Atomic References [name: Tom1, age: 19]
Thread2 Atomic References [name: Tom2, age: 21]
Now Atomic Person is [name: Tom2, age: 21]
如果使用原子性对象引用，在多线程情况下进行对象的更新可以确保一致性。例如：
// 普通引用
private static Person person;
// 原子性引用
private static AtomicReference aRperson;
public static void main(String[] args) throws InterruptedException {
person = new Person("Tom", 18);
aRperson = new AtomicReference(person);
System.out.println("Atomic Person is " + aRperson.get().toString());
Thread t1 = new Thread(new Task1());
Thread t2 = new Thread(new Task2());
t1.start();
t2.start();
t1.join();
t2.join();
System.out.println("Now Atomic Person is " + aRperson.get().toString());
}
static class Task1 implements Runnable {
public void run() {
aRperson.getAndSet(new Person("Tom1", aRperson.get().getAge() + 1));
System.out.println("Thread1 Atomic References "
+ aRperson.get().toString());
}
}
static class Task2 implements Runnable {
public void run() {
aRperson.getAndSet(new Person("Tom2", aRperson.get().getAge() + 2));
System.out.println("Thread2 Atomic References "
+ aRperson.get().toString());
}
}
AtomicReference介绍
AtomicReference是作用是对"对象"进行原子操作。
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 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。