线程的三种同步机制

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我是靠谱客的博主跳跃龙猫，这篇文章主要介绍线程的三种同步机制，现在分享给大家，希望可以做个参考。

由一个例子引入今天的内容
例子，创建三个窗口买票，总票数为100张，使用实现Runnable接口的方式
问题：卖票过程中，出现了重票、错票 --> 出现了线程安全问题
问题出现原因：当某个线程操作车票的过程中，尚未操作完成时，其它线程参与进来，也操作了该张票
如何解决：当一个线程在操作共享数据时，其他线程不能参与进来，直到线程A操作完共享数据，其他线程才可以开始操作
这种情况即使线程A出现了阻塞，也不能改变
在Java中，我们通过同步机制，来解决线程的安全问题

1.同步代码块

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synchronized(同步监视器){
//需要被同步的代码
}

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public class WindowTest1 {
    public static void main(String[] args) {
        Window1 window1 = new Window1();
        Thread thread = new Thread(window1);
        Thread thread1 = new Thread(window1);
        Thread thread2 = new Thread(window1);

thread.setName("线程1");
        thread1.setName("线程2");
        thread2.setName("线程3");

thread.start();
        thread1.start();
        thread2.start();
    }
}

class Window1 implements Runnable {
    private int ticket = 100;
    Object object = new Object();

@Override
    public void run() {
        while (true) {
//            synchronized (object) {
            synchronized (this) {
                if (ticket > 0) {
                    try {
                        Thread.sleep(10);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                    System.out.println(Thread.currentThread().getName() + ":卖票，票号为：" + ticket);
                    ticket--;
                } else {
                    break;
                }
            }
        }
    }
}

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public class WindowTest1 {
    public static void main(String[] args) {
        Window1 window1 = new Window1();
        Thread thread = new Thread(window1);
        Thread thread1 = new Thread(window1);
        Thread thread2 = new Thread(window1);

        thread.setName("线程1");
        thread1.setName("线程2");
        thread2.setName("线程3");

        thread.start();
        thread1.start();
        thread2.start();
    }
}

class Window1 implements Runnable {
    private int ticket = 100;
    Object object = new Object();

    @Override
    public void run() {
        while (true) {
//            synchronized (object) {
            synchronized (this) {
                if (ticket > 0) {
                    try {
                        Thread.sleep(10);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                    System.out.println(Thread.currentThread().getName() + ":卖票，票号为：" + ticket);
                    ticket--;
                } else {
                    break;
                }
            }
        }
    }
}

说明：

操作共享数据的代码即为需要被同步的代码
共享数据：多个线程共同操作的变量，比如：ticket就是共享数据
同步监视器：俗称锁，任何一个类的对象都可以充当锁
要求：多个线程必须要共用同一把锁
补充：在实现Runnable接口创建多线程的方式中，我们可以考虑使用this充当同步监视器

2.同步方法

在需要同步的方法上添加synchronized关键字

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public class WindowTest3 {
    public static void main(String[] args) {
        Window3 window3 = new Window3();
        Thread thread1 = new Thread(window3);
        Thread thread2 = new Thread(window3);
        Thread thread3 = new Thread(window3);

        thread1.setName("线程1");
        thread2.setName("线程2");
        thread3.setName("线程3");

        thread1.start();
        thread2.start();
        thread3.start();
    }
}

class Window3 implements Runnable {
    private int ticket = 100;

    @Override
    public void run() {
        while (true) {
            show();
            if (ticket <= 0) break;
        }
    }

    private synchronized void show() {//同步监视器:this
        if (ticket > 0) {
            try {
                Thread.sleep(10);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println(Thread.currentThread().getName() + ":卖票，票号为：" + ticket);
            ticket--;
        }
    }
}

3.Lock锁 – JDK 5.0 新增

创建锁对象
在需要同步的代码块前调用lock()加锁
同步代码块后调用unlock()释放锁

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class Window implements Runnable {
    private int ticket = 100;
    private ReentrantLock lock = new ReentrantLock(true);//1.创建锁对象

@Override
    public void run() {
        while (true) {
            try {
                lock.lock();//2.调用lock()加锁
                if (ticket > 0) {
                    try {
                        Thread.sleep(100);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                    System.out.println(Thread.currentThread().getName() + "：售票，票号为：" + ticket);
                    ticket--;
                } else {
                    break;
                }
            } finally {
                lock.unlock();//3.调用unlock()释放锁
            }
        }
    }
}

public class LockTest {
    public static void main(String[] args) {
        Window window = new Window();

Thread thread1 = new Thread(window);
        Thread thread2 = new Thread(window);
        Thread thread3 = new Thread(window);

thread1.setName("线程1");
        thread2.setName("线程2");
        thread3.setName("线程3");

thread1.start();
        thread2.start();
        thread3.start();
    }
}

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class Window implements Runnable {
    private int ticket = 100;
    private ReentrantLock lock = new ReentrantLock(true);//1.创建锁对象

    @Override
    public void run() {
        while (true) {
            try {
                lock.lock();//2.调用lock()加锁
                if (ticket > 0) {
                    try {
                        Thread.sleep(100);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                    System.out.println(Thread.currentThread().getName() + "：售票，票号为：" + ticket);
                    ticket--;
                } else {
                    break;
                }
            } finally {
                lock.unlock();//3.调用unlock()释放锁
            }
        }
    }
}

public class LockTest {
    public static void main(String[] args) {
        Window window = new Window();

        Thread thread1 = new Thread(window);
        Thread thread2 = new Thread(window);
        Thread thread3 = new Thread(window);

        thread1.setName("线程1");
        thread2.setName("线程2");
        thread3.setName("线程3");

        thread1.start();
        thread2.start();
        thread3.start();
    }
}