关于对象锁加final

使用synchronized锁的时候，作为锁的对象最好要加上final修饰符，因为可能线程会改变锁变量持有的具体的对象。demo如下：

public class Test02 {
    static Object lock = new Object();

    public static void main(String[] args) {
        Thread t1 = new Thread(() -> {
            lock = new Object();
            synchronized (lock) {
                for (int i = 0; i < 10; i++) {
                    try {
                        Thread.sleep(5);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                    System.out.println("A");
                }
            }
        });

        Thread t2 = new Thread(() -> {
            lock = new Object();
            synchronized (lock) {
                for (int i = 0; i < 10; i++) {
                    try {
                        Thread.sleep(5);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                    System.out.println("B");
                }
            }
        });

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

上述程序的打印输出为：

A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B

但是要是把锁改成final的。代码如下：

public class Test02 {
    static final Object lock = new Object();

    public static void main(String[] args) {
        Thread t1 = new Thread(() -> {
//            lock = new Object(); // 编译出错，final不能修改
            synchronized (lock) {
                for (int i = 0; i < 10; i++) {
                    try {
                        Thread.sleep(5);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                    System.out.println("A");
                }
            }
        });

        Thread t2 = new Thread(() -> {
//            lock = new Object();
            synchronized (lock) {
                for (int i = 0; i < 10; i++) {
                    try {
                        Thread.sleep(5);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                    System.out.println("B");
                }
            }
        });

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

打印的结果为：

A
A
A
A
A
A
A
A
A
A
B
B
B
B
B
B
B
B
B
B

综述，以后锁对象加final修饰。