java - 为什么线程在引用更改时不释放锁

Question

public class UThread implements Runnable {

    Object o;

    UThread(Object o) {
        this.o = o;
    }

    @Override
    public void run() {
        synchronized (o) {
            System.out.println("inside before change" + Thread.currentThread().getName());
            try {
                Thread.sleep(5000);
            } catch (Exception er) {
            }
            o = new Object();
            System.out.println("inside after change" + Thread.currentThread().getName());
            try {
                Thread.sleep(5000);
            } catch (Exception er) {
            }

        }
        System.out.println("outside " + Thread.currentThread().getName());

    }


}


public class main {
    public static void main(String str[]) {

        Object o = new Object();
        UThread uThread = new UThread(o);
        Thread th = new Thread(uThread);

        UThread uThread2 = new UThread(o);
        Thread th2 = new Thread(uThread2);

        th.start();
        try {
            Thread.sleep(1000);
        } catch (Exception er) {
        }
        th2.start();
    }
}

当我们执行它打印的代码时

在 changeThread-0 之前里面
changeThread-0 之后的内部
线程 0 外
在 changeThread-1 之前里面
changeThread-1 之后的内部
在 Thread-1 之外

那么，当我更改对象时，为什么第二个线程没有锁定 o。

Answer 1

分配一个引用对底层对象没有任何作用。它所做的只是复制引用值，而不触及或改变对象本身。例如，它不会释放锁，也不应该。

例如

Object o2 = o;
synchronized(o) {
    o = new Object();
    // o2 is still locked
}
// o2 is not locked