volatile的原理实现可以看这篇文章，真的是从硬件层面上说明了volatile怎样保证可见性。volatile有两个作用，一是保证变量的可见性，二是防止指令重排序。

下面这个实例，如果没有设置成volatile关键字，那么线程读的isRunning永远都是自己私有内存中的，线程将会一直在while循环中

public class RunThread extends Thread{

    private volatile boolean isRunning = true;
    private void setRunning(boolean isRunning){
        this.isRunning = isRunning;
    }
    
    public void run(){
        System.out.println("进入run方法..");
        int i = 0;
        while(isRunning == true){
            //..
        }
        System.out.println("线程停止");
    }
    
    public static void main(String[] args) throws InterruptedException {
        RunThread rt = new RunThread();
        rt.start();
        Thread.sleep(1000);
        rt.setRunning(false);
        System.out.println("isRunning的值已经被设置了false");
    }
    
    
}

这是展示volatile虽然有可见性，但是没有原子性：

/**
 * volatile关键字不具备synchronized关键字的原子性（同步）
 * @author alienware
 *
 */
public class VolatileNoAtomic extends Thread{
    private static volatile int count = 0;
    
    //这个被注释的代码可以保证结果正确
    //private static  AtomicInteger count = new AtomicInteger(0); 
    
    private static void addCount(){
        for (int i = 0; i < 1000; i++) {
            count++ ;
            
            //这个被注释的代码可以保证结果正确
            //count.incrementAndGet();
        }
        System.out.println(count);
    }
    
    public void run(){
        addCount();
    }
    
    public static void main(String[] args) {
        
        VolatileNoAtomic[] arr = new VolatileNoAtomic[100];
        for (int i = 0; i < 10; i++) {
            arr[i] = new VolatileNoAtomic();
        }
        
        for (int i = 0; i < 10; i++) {
            arr[i].start();
        }
    }
    
}

这是使用atomic，保证原子性的代码：

public class AtomicUse {

    private static AtomicInteger count = new AtomicInteger(0);
    
    //多个addAndGet在一个方法内是非原子性的，需要加synchronized进行修饰，保证4个addAndGet整体原子性
    /**synchronized*/
    public synchronized int multiAdd(){
            try {
                Thread.sleep(100);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            count.addAndGet(1);
            count.addAndGet(2);
            count.addAndGet(3);
            count.addAndGet(4); //+10
            return count.get();
    }
    
    
    public static void main(String[] args) {
        
        final AtomicUse au = new AtomicUse();

        List<Thread> ts = new ArrayList<Thread>();
        for (int i = 0; i < 100; i++) {
            ts.add(new Thread(new Runnable() {
                @Override
                public void run() {
                    System.out.println(au.multiAdd());
                }
            }));
        }

        for(Thread t : ts){
            t.start();
        }
    }
}

线程通信

ListAdd2.java，可以看出本来list已经到5了，那么t2应该出while循环抛异常，但是因为它执行了wait方法，释放锁了。而t1得到锁一直执行，虽然t1执行了notify方法，但是只是发出通知而已，只有它的方法执行完才释放锁让t2执行。

package com.bjsxt.base.conn008;

import java.util.ArrayList;
import java.util.List;
import java.util.Queue;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.LinkedBlockingDeque;
import java.util.concurrent.LinkedBlockingQueue;
/**
 * wait notfiy 方法，wait释放锁，notfiy不释放锁
 * @author alienware
 *
 */
public class ListAdd2 {
    private volatile static List list = new ArrayList();    
    
    public void add(){
        list.add("bjsxt");
    }
    public int size(){
        return list.size();
    }
    
    public static void main(String[] args) {
        
        final ListAdd2 list2 = new ListAdd2();
        
        // 1 实例化出来一个 lock
        // 当使用wait 和 notify 的时候 ， 一定要配合着synchronized关键字去使用
        final Object lock = new Object();
        
//      final CountDownLatch countDownLatch = new CountDownLatch(1);
        
        Thread t1 = new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    synchronized (lock) {
                        for(int i = 0; i <10; i++){
                            list2.add();
                            System.out.println("当前线程：" + Thread.currentThread().getName() + "添加了一个元素..");
                            Thread.sleep(500);
                            if(list2.size() == 5){
                                System.out.println("已经发出通知..");
//                              countDownLatch.countDown();
                                lock.notify();
                            }
                        }                       
                    }
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }

            }
        }, "t1");
        
        Thread t2 = new Thread(new Runnable() {
            @Override
            public void run() {
                synchronized (lock) {
                    if(list2.size() != 5){
                        try {
                            System.out.println("t2进入...");
                            lock.wait();
//                          countDownLatch.await();
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                    }
                    System.out.println("当前线程：" + Thread.currentThread().getName() + "收到通知线程停止..");
                    throw new RuntimeException();
                }
            }
        }, "t2");   
        
        t2.start();
        t1.start();
        
    }
    
}

package com.xushu.multi;

import java.util.LinkedList;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicInteger;

public class MyQueue {

    private LinkedList<Object> list = new LinkedList<Object>();

    private AtomicInteger count = new AtomicInteger(0);

    private final int minSize = 0;

    private final int maxSize;

    public MyQueue(int size) {
        this.maxSize = size;
    }

    private final Object lock = new Object();

    public void put(Object obj) {
        synchronized (lock) {
            if (count.get() == this.maxSize) {
                try {
                    lock.wait();
                } catch (InterruptedException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }
            list.add(obj);
            count.incrementAndGet();
            lock.notify();
            System.out.println("新加入的元素为：" + obj);
        }
    }

    public Object take() {
        Object ret = null;
        synchronized (lock) {
            if (count.get() == minSize) {
                try {
                    lock.wait();
                } catch (InterruptedException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }
            ret = list.removeFirst();
            count.decrementAndGet();
            lock.notify();
        }
        return ret;
    }

    public int getSize() {
        return this.count.get();
    }

    public static void main(String[] args) {

        final MyQueue mq = new MyQueue(5);
        mq.put("a");
        mq.put("b");
        mq.put("c");
        mq.put("d");

        System.out.println("当前容器的长度:" + mq.getSize());

        Thread t1 = new Thread(new Runnable() {
            @Override
            public void run() {
                mq.put("f");
                mq.put("g");
                mq.put("e");
            }
        }, "t1");

        t1.start();

        Thread t2 = new Thread(new Runnable() {
            @Override
            public void run() {
                Object o1 = mq.take();
                System.out.println("移除的元素为:" + o1);
                Object o2 = mq.take();
                System.out.println("移除的元素为:" + o2);
            }
        }, "t2");

        try {
            TimeUnit.SECONDS.sleep(2);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        t2.start();
    }
}