（Java多线程系列二）线程间同步

Java多线程间同步

1、什么是线程安全

通过一个案例了解线程安全

案例:需求现在有100张火车票，有两个窗口同时抢火车票，请使用多线程模拟抢票效果。

先来看一个线程不安全的例子

class SellTicketRunnable implements Runnable {

    public int count = 100;

    @Override
    public void run() {
        while (count > 0) {
            try {
                Thread.sleep(500);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            int index = 100 - count + 1;
            System.out.println(Thread.currentThread().getName() + "卖出第" + index + "张票");
            count--;
        }
    }
}

public class JavaSyncDemo {

    public static void main(String[] args) {
        SellTicketRunnable runnable = new SellTicketRunnable();
        Thread sellThread1 = new Thread(runnable);
        Thread sellThread2 = new Thread(runnable);
        sellThread1.start();
        sellThread2.start();
    }
}

可以看到两个线程同时卖票的时候，会出现漏卖，多卖同一张票，还会出现超卖的问题，这就是线程不安全的问题。

当多个线程同时共享，同一个全局变量或静态变量，做写的操作时，可能会发生数据冲突问题，也就是线程安全问题。但是做读操作是不会发生数据冲突问题。

2、线程安全问题的解决办法

(1)使用同步代码块

class SellTicketRunnable implements Runnable {

    public int count = 100;

    private Object lock = new Object();

    @Override
    public void run() {
        while (count > 0) {
            try {
                Thread.sleep(500);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            synchronized (lock) {
                if (count > 0) {
                    int index = 100 - count + 1;
                    System.out.println(Thread.currentThread().getName() + "卖出第" + index + "张票");
                    count--;
                }
            }
        }
    }
}

public class JavaSyncDemo {

    public static void main(String[] args) {
        SellTicketRunnable runnable = new SellTicketRunnable();
        Thread sellThread1 = new Thread(runnable);
        Thread sellThread2 = new Thread(runnable);
        sellThread1.start();
        sellThread2.start();
    }
}

从上面的案例可以看出，使用synchronized同步代码块包裹住写操作，每个线程在调用同步代码块中逻辑的时候，都需要先获取同步锁，所以避免了多线程写操作数据的冲突问题。

(2)使用同步函数

class SellTicketRunnable01 implements Runnable {

    public int count = 100;

    @Override
    public void run() {
        while (count > 0) {
            try {
                Thread.sleep(500);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            this.sale();
        }
    }

    synchronized void sale() {
        if (count > 0) {
            int index = 100 - count + 1;
            System.out.println(Thread.currentThread().getName() + "卖出第" + index + "张票");
            count--;
        }
    }
}

public class JavaSyncDemo01 {

    public static void main(String[] args) {
        SellTicketRunnable01 runnable = new SellTicketRunnable01();
        Thread sellThread1 = new Thread(runnable);
        Thread sellThread2 = new Thread(runnable);
        sellThread1.start();
        sellThread2.start();
    }
}

synchronized包裹的函数，其实就是给该函数块添加了一把this锁。

注意：synchronized 修饰静态方法使用锁是当前类的字节码文件（即类名.class），同理，如果在静态方法中添加个同步代码块，可以获取类名.class为代码块加锁

class SellTicketRunnable02 implements Runnable {

    public static int count = 100;

    @Override
    public void run() {
        while (count > 0) {
            try {
                Thread.sleep(500);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            SellTicketRunnable02.sale();
        }
    }

    static void sale() {
        synchronized (SellTicketRunnable02.class) {
            if (count > 0) {
                int index = 100 - count + 1;
                System.out.println(Thread.currentThread().getName() + "卖出第" + index + "张票");
                count--;
            }
        }
    }
}

public class JavaSyncDemo02 {

    public static void main(String[] args) {
        SellTicketRunnable02 runnable = new SellTicketRunnable02();
        Thread sellThread1 = new Thread(runnable);
        Thread sellThread2 = new Thread(runnable);
        sellThread1.start();
        sellThread2.start();
    }
}

(3)使用lock锁

class SellTicketRunnable03 implements Runnable {

    public int count = 100;

    private Lock lock = new ReentrantLock();

    @Override
    public void run() {
        while (count > 0) {
            try {
                Thread.sleep(500);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            lock.lock();
            if (count > 0) {
                int index = 100 - count + 1;
                System.out.println(Thread.currentThread().getName() + "卖出第" + index + "张票");
                count--;
            }
            lock.unlock();
        }
    }
}

public class JavaSyncDemo03 {

    public static void main(String[] args) {
        SellTicketRunnable03 runnable = new SellTicketRunnable03();
        Thread sellThread1 = new Thread(runnable);
        Thread sellThread2 = new Thread(runnable);
        sellThread1.start();
        sellThread2.start();
    }
}

lock和synchronized的区别

①lock在使用时需要手动的获取锁和释放锁;
②lock可以尝试非阻塞的获取锁，如果这一时刻锁没有被其他线程获取到，则成功获取并持有锁；
③lock锁可以响应中断，当获取到锁的线程被中断时，中断异常会被抛出，同时锁被释放；
④lock在指定截至时间之前获取锁，如果解释时间到了依旧无法获取锁，就返回。

// lock锁的安全使用方法
class lockDemo {
    Lock lock = new ReentrantLock();
    void demoFun() {
        lock.lock();
        try {
            // 可能出现线程安全的操作
        } finally {
            lock.unlock();
        }
    }
}

(4)使用Java原子类

java.util.concurrent.atomic.AtomicBoolean;

java.util.concurrent.atomic.AtomicInteger;

java.util.concurrent.atomic.AtomicLong;

java.util.concurrent.atomic.AtomicReference;

class SellTicketRunnable04 implements Runnable {

   public AtomicInteger count = new AtomicInteger(100);

   @Override
   public void run() {
       while (true) {
           try {
               Thread.sleep(50);
           } catch (InterruptedException e) {
               e.printStackTrace();
           }
           if (count.get() > 0) {
               int index = 100 - count.getAndDecrement() + 1;
               System.out.println(Thread.currentThread().getName() + "卖出第" + index + "张票");
           }
       }
   }
}

public class JavaSyncDemo04 {

   public static void main(String[] args) {
       SellTicketRunnable04 runnable = new SellTicketRunnable04();
       Thread sellThread1 = new Thread(runnable);
       Thread sellThread2 = new Thread(runnable);
       sellThread1.start();
       sellThread2.start();
   }
}

3、死锁

先看一个死锁的示例

public class DeadLockDemo01 {

    private static Object lock1 = new Object();
    private static Object lock2 = new Object();

    public static void main(String[] args) {
        new Thread() { //线程1
            public void run() {
                while (true) {
                    synchronized (lock1) {
                        System.out.println(this.getName() + ":获取lock1锁");
                        synchronized (lock2) {
                            System.out.println(this.getName() + ":获取lock2锁");
                        }
                    }
                }
            }
        }.start();

        new Thread() { //线程2
            public void run() {
                while (true) {
                    synchronized (lock2) {
                        System.out.println(this.getName() + ":获取lock2锁");
                        synchronized (lock1) {
                            System.out.println(this.getName() + "::获取lock1锁");
                        }
                    }
                }
            }
        }.start();
    }
}

运行上面的代码，可以观察到线程卡死，就是出现了死锁

线程1先拿到lock1锁，再拿到lock2锁，执行完成后才能释放所有锁；
线程2先拿到lock2锁，再拿到lock1锁，执行完成后才能释放所有锁。
如果在线程1获取到lock1锁的时候，线程2获取到lock2还没释放，线程1无法获取lock2锁，也就无法释放lock2锁，这时系统就会出现死锁。

线程死锁的避免办法：不要在同步中嵌套同步

源码地址