Java并发编程原理与实战十三:JDK提供的原子类原理与使用

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原子更新基本类型

原子更新数组

原子更新抽象类型

原子更新字段

原子更新基本类型:

 
package com.roocon.thread.t8;

import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.atomic.AtomicIntegerArray;

public class Sequence {
private AtomicInteger value = new AtomicInteger(0);
private int [] s = {2,1,4,6};
AtomicIntegerArray a = new AtomicIntegerArray(s);

public int getNext(){
a.getAndIncrement(2);//给下标为2的元素加1
a.getAndAdd(2, 10);//获取下标为2的元素,并且加10
for (int i=0; i < a.length(); i++){
System.out.println(a.get(i));
}
return value.getAndIncrement();
}

public static void main(String[] args) {
Sequence sequence = new Sequence();
new Thread(new Runnable() {
@Override
public void run() {
while (true){
System.out.println(Thread.currentThread().getName()+" "+sequence.getNext());
try {
Thread.sleep(100);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}).start();
new Thread(new Runnable() {
@Override
public void run() {
while (true){
System.out.println(Thread.currentThread().getName()+" "+sequence.getNext());
try {
Thread.sleep(100);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}).start();
new Thread(new Runnable() {
@Override
public void run() {
while (true){
System.out.println(Thread.currentThread().getName()+" "+sequence.getNext());
try {
Thread.sleep(100);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}).start();
}
}
 

运行结果:

 
Thread-0 0
Thread-1 1
Thread-2 2
Thread-0 3
Thread-1 4
Thread-2 5
...
 
 
package com.roocon.thread.t8;

import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.atomic.AtomicIntegerArray;
import java.util.concurrent.atomic.AtomicIntegerFieldUpdater;
import java.util.concurrent.atomic.AtomicReference;

public class Sequence {
    private AtomicInteger value = new AtomicInteger(0);
    AtomicReference<User> user = new AtomicReference<>(); //对user的set和get执行原子操作
    AtomicIntegerFieldUpdater<User> old =  AtomicIntegerFieldUpdater.newUpdater(User.class, "old");

    public int getNext(){
        User user = new User();
        System.out.println(old.getAndIncrement(user));
        System.out.println(old.getAndIncrement(user));
        System.out.println(old.getAndIncrement(user));
        return value.getAndIncrement();
    }

    public static void main(String[] args) {
        Sequence sequence = new Sequence();
        new Thread(new Runnable() {
            @Override
            public void run() {
                   System.out.println(Thread.currentThread().getName()+" "+sequence.getNext());
                   try {
                       Thread.sleep(100);
                   } catch (InterruptedException e) {
                       e.printStackTrace();
                   }
               }
        }).start();
    }
}
 

运行结果:

0
1
2
Thread-0 0

对CAS的源码理解:--初步理解

在AtomicInteger中有这样一段源码:

 
public final int getAndUpdate(IntUnaryOperator updateFunction) {
        int prev, next;
        do {
            prev = get();
            next = updateFunction.applyAsInt(prev);
        } while (!compareAndSet(prev, next));
        return prev;
    }

public final boolean compareAndSet(int expect, int update) {
return unsafe.compareAndSwapInt(this, valueOffset, expect, update);
}
 
 

其中,compareAndSwap就是CAS的缩写。如果prev和next不相等,则返回true。否则,返回false。最终是通过unsafe来实现的。

以上代码表示,如果compareAndSet返回true,则while条件为false,退出循环,返回prev值。如果compareAndSet返回false,则while为true,继续执行循环体,重新获取prev的值,重新获取更新值,直到返回的compareAndSet值为true。

演示源码大致步骤如下:


 Object prev = get(); //1
        next = prev + 1;
        boolean flag = cas(prev, next);
        if (flag) {
            return prev;
        }else {
            go to 1
        } Object pre

 

参考资料:

《java并发编程与实战》龙果学院

 

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