JDK提供的原子类原理与使用

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
        }