原子更新基本類型

原子更新數組

原子更新抽象類型

原子更新字段

原子更新基本類型：

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);//給下標為2的元素加1
        a.getAndAdd(2, 10);//獲取下標為2的元素，並且加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();
    }
}

運行結果：

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<>(); //對user的set和get執行原子操作
    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();
    }
}

運行結果：

0
1
2
Thread-0 0

對CAS的源碼理解：--初步理解

在AtomicInteger中有這樣一段源碼：

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的縮寫。如果prev和next不相等，則返回true。否則，返回false。最終是通過unsafe來實現的。

以上代碼表示，如果compareAndSet返回true，則while條件為false，退出循環，返回prev值。如果compareAndSet返回false，則while為true，繼續執行循環體，重新獲取prev的值，重新獲取更新值，直到返回的compareAndSet值為true。

演示源碼大致步驟如下：

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

參考資料：

《java並發編程與實戰》龍果學院

Java並發編程原理與實戰十三：JDK提供的原子類原理與使用