java多线程 线程安全问题

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

模拟线程安全问题

public class SafeThread implements Runnable {

    private int ticketCount = 50;

    @Override
    public void run() {
        while (ticketCount > 0) {
            try {
                Thread.sleep(50);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println(Thread.currentThread().getName() + ",出售第" + (50 - ticketCount + 1) + "张票");
            ticketCount--;
        }
    }
}

@RequestMapping("test-safe")
    public void testSafe() {
        SafeThread safeThread = new SafeThread();
        Thread t1 = new Thread(safeThread, "thread-1");
        Thread t2 = new Thread(safeThread, "thread-2");
        t1.start();
        t2.start();
    }

结果：火车票会重复出售

解决办法

使用多线程之间同步synchronized或使用锁(lock)
1.同步代码块

public class SafeThread implements Runnable {

    private int ticketCount = 50;

    @Override
    public void run() {
        while (ticketCount > 0) {
            try {
                Thread.sleep(50);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            synchronized (this) {
                System.out.println(Thread.currentThread().getName() + ",出售第" + (50 - ticketCount + 1) + "张票");
                ticketCount--;
            }
        }
    }
}

2.同步方法

public class SafeThread implements Runnable {

    private int ticketCount = 50;

    @Override
    public void run() {
        while (ticketCount > 0) {
            try {
                Thread.sleep(50);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
//            synchronized (this) {
//                System.out.println(Thread.currentThread().getName() + ",出售第" + (50 - ticketCount + 1) + "张票");
//                ticketCount--;
//            }
            sale();
        }
    }

    private synchronized void sale() {
        System.out.println(Thread.currentThread().getName() + ",出售第" + (50 - ticketCount + 1) + "张票");
        ticketCount--;
    }
}

注意：同步函数使用this锁

3.静态同步函数
方法上加上static关键字，使用synchronized 关键字修饰或者使用类.class文件。
静态的同步函数使用的锁是该函数所属字节码文件对象
可以用 getClass方法获取，也可以用当前类名.class 表示

public class SafeThread implements Runnable {

    private int ticketCount = 50;

    @Override
    public void run() {
        while (ticketCount > 0) {
            try {
                Thread.sleep(50);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
//            synchronized (this) {
//                System.out.println(Thread.currentThread().getName() + ",出售第" + (50 - ticketCount + 1) + "张票");
//                ticketCount--;
//            }
//            sale();
            sale2();
        }
    }

    private synchronized void sale() {
        System.out.println(Thread.currentThread().getName() + ",出售第" + (50 - ticketCount + 1) + "张票");
        ticketCount--;
    }


    private void sale2() {
        synchronized (SafeThread.class) {
            System.out.println(Thread.currentThread().getName() + ",出售第" + (50 - ticketCount + 1) + "张票");
            ticketCount--;
        }
    }
}

死锁问题

同步中嵌套同步,导致锁无法释放
解决办法：尽量避免同步中嵌套同步