java - 一个线程可以同时持有两个锁吗？

Question

该示例说明了死锁是如何发生的。有一点不太明白，就是当第一个调用实例alphone的方法bow的thead即将执行gaston.bowbBack(this)时，如果如果，该线程是否会释放锁并获取实例gaston的锁，假设，它不属于第二个线程？或者它会同时持有两个锁直到该方法的所有代码被完全执行？还有一个问题，有没有办法检查线程是否持有锁？

  public class Deadlock {
        static class Friend {
            private final String name;
            public Friend(String name) {
                this.name = name;
            }
            public String getName() {
                return this.name;
            }
            public synchronized void bow(Friend bower) {
                System.out.format("%s: %s"
                    + "  has bowed to me!%n",
                    this.name, bower.getName());
                bower.bowBack(this);
            }
            public synchronized void bowBack(Friend bower) {
                System.out.format("%s: %s"
                    + " has bowed back to me!%n",
                    this.name, bower.getName());
            }
        }

        public static void main(String[] args) {
            final Friend alphonse =
                new Friend("Alphonse");
            final Friend gaston =
                new Friend("Gaston");
            new Thread(new Runnable() {
                public void run() { alphonse.bow(gaston); }
            }).start();
            new Thread(new Runnable() {
                public void run() { gaston.bow(alphonse); }
            }).start();
        }
    }

Answer 1

1)一个线程可以有多个锁，就像这样

    ...
    synchronized (obj1) {
        System.out.println(1);
        synchronized (obj2) {
            System.out.println(2);
        }
    }
    ...

2) 我们可以检查线程是否持有锁 Thread.holdsLock(Object obj)