Java线程之间通讯（三）

使用wait和notify方法实现了线程间的通讯，都是Object 类的方法，java所有的对象都提供了这两个方法

1.wait和notify必须配合synchronized使用

2.wait方法释放锁，notify方法不释放锁

import java.util.ArrayList;
import java.util.List;

public class ListAdd1 {
    private volatile static List list = new ArrayList();

    public void add(){
        list.add("laoshi");
    }
    public int size(){
        return list.size();
    }

    public static void main(String[] args) {

        final ListAdd1 list1 = new ListAdd1();

        Thread t1 = new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    for(int i = 0; i <10; i++){
                        list1.add();
                        System.out.println("当前线程：" + Thread.currentThread().getName() + "添加了一个元素..");
                        Thread.sleep(500);
                    }
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }, "t1");

        Thread t2 = new Thread(new Runnable() {
            @Override
            public void run() {
                while(true){
                    if(list1.size() == 5){
                        System.out.println("当前线程收到通知：" + Thread.currentThread().getName() + " list size = 5 线程停止..");
                        throw new RuntimeException();
                    }
                }
            }
        }, "t2");

        t1.start();
        t2.start();
    }
}

　　

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.CountDownLatch;
/**
 * wait notfiy 方法，wait释放锁，notfiy不释放锁
 */
public class ListAdd2 {
    private volatile static List list = new ArrayList();

    public void add(){
        list.add("laoshi");
    }
    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();
    }
}

　　

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

public class MyQueue {
    //1 需要一个承装元素的集合
    private LinkedList<Object> list=new LinkedList<Object>();

    //2 需要一个计数器
    private AtomicInteger count=new AtomicInteger(0);

    //3 需要制定上限和下限
    private final int minSize=0;
    private final int maxSize ;

    //4 构造方法
    public MyQueue(int size){
        this.maxSize = size;
    }
    //5 初始化一个对象 用于加锁
    private final Object lock=new Object();

    //put(anObject): 把anObject加到BlockingQueue里,如果BlockQueue没有空间,则调用此方法的线程被阻断，
    // 直到BlockingQueue里面有空间再继续.
    public void put(Object obj){
        synchronized (lock){
            while (count.get()==this.maxSize){
                try {
                    lock.wait();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
            //1 加入元素
            list.add(obj);
            //2.计数器累加
            count.incrementAndGet();
            //3 通知另外一个线程（唤醒）
            lock.notify();
            System.out.println("新加入的元素为:" + obj);
        }
    }

    //take: 取走BlockingQueue里排在首位的对象,若BlockingQueue为空,
    // 阻断进入等待状态直到BlockingQueue有新的数据被加入.
    public Object take(){
        Object ret=null;
        synchronized (lock){
            while (count.get()==this.minSize){
                try {
                    lock.wait();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
            //1 做移除元素操作
            ret=list.removeFirst();
            //2 计数器递减
            count.decrementAndGet();
            //3 唤醒另外一个线程
            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");
        mq.put("e");

        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");
            }
        },"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();
    }
}

　　ThreadLocal：线程局部变量，是一种多线程间并发访问变量的解决方案，与synchronized枷锁的方式不同，ThreadLocal完全不提供锁，使用以空间换时间的手段，为每个线程提供变量的独立副本，以保证线程安全。

在高并发量或者竞争激烈的场景，使用ThreadLoacal可以一定程度少减少锁竞争。

public class ConnThreadLocal {
    public static  ThreadLocal<String> th=new ThreadLocal<String>();
    public void setTh(String value){
        th.set(value);
    }
    public void getTh(){
        System.out.println(Thread.currentThread().getName() + ":" + this.th.get());
    }
    public static void main(String[] args) throws InterruptedException {

        final ConnThreadLocal ct = new ConnThreadLocal();
        Thread t1 = new Thread(new Runnable() {
            @Override
            public void run() {
                ct.setTh("张三");
                ct.getTh();
            }
        }, "t1");

        Thread t2 = new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    Thread.sleep(1000);
                    ct.getTh();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }, "t2");

        t1.start();
        t2.start();
    }
}

单例模式：最常见的饥饿模式（直接实力化对象），懒汉模式（在调用方法时进行实例化对象）

在多线程中考虑性能和线程安全问题使用

dubble check instance

static inner class

public class DubbleSingleton {
    private static  DubbleSingleton ds;
    public static  DubbleSingleton getDs(){
        if(ds == null){
            try {
                //模拟初始化对象的准备时间...
                Thread.sleep(3000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            synchronized (DubbleSingleton.class) {
                if(ds == null){
                    ds = new DubbleSingleton();
                }
            }
        }
        return ds;
    }
    public static void main(String[] args) {
        Thread t1 = new Thread(new Runnable() {
            @Override
            public void run() {
                System.out.println(DubbleSingleton.getDs().hashCode());
            }
        },"t1");
        Thread t2 = new Thread(new Runnable() {
            @Override
            public void run() {
                System.out.println(DubbleSingleton.getDs().hashCode());
            }
        },"t2");
        Thread t3 = new Thread(new Runnable() {
            @Override
            public void run() {
                System.out.println(DubbleSingleton.getDs().hashCode());
            }
        },"t3");

        t1.start();
        t2.start();
        t3.start();
    }
}

　　这种方法用的多

public class Singletion {

    private static class InnerSingletion {
        private static Singletion single = new Singletion();
    }

    public static Singletion getInstance(){
        return InnerSingletion.single;
    }
}

　　

import java.util.concurrent.TimeUnit;

public class Demo3 {
	private volatile int signal;
    public synchronized void set(int value){
    	signal=1;
    	notifyAll(); //notifyAll叫醒所有的处于wait线程，争夺到时间片的线程只有一个
    	notify(); //notify方法会随机叫醒一个处于wait状态的线程
    	//notify 拿到锁
    	//this.signal=value;
    }
    public synchronized int get(){
    	System.out.println(Thread.currentThread().getName()+"方法执行了。。。");
    	if(signal!=1){
    		try {
				wait(); //释放锁
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
    	}
    	System.out.println(Thread.currentThread().getName()+"方法执行完毕。。。");
    	return signal;
    }  
    public static void main(String[] args) {
		Demo3 demo=new Demo3();
		Target t1=new Target(demo);
		Target t2=new Target(demo);
		new Thread(t2).start();
		new Thread(t2).start();
		new Thread(t2).start();
		new Thread(t2).start();
		
		try {
			TimeUnit.SECONDS.sleep(1);
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
		new Thread(t1).start();
	}
}