Java 官方死鎖示例

Question

我從 Oracle 官方網站閱讀了教程並看到了這個例子

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) throws Exception{
        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();
    }
}

我不明白為什么會發生死鎖？ 據我了解，線程 alphonse 執行完成然后釋放鎖，然后線程 gaston 將繼續執行，沒有死鎖。

Answer 1

你應該運行它看看。 它永遠不會退出。

當你得到一個線程轉儲時，你會看到兩個線程：

"Thread-0" #14 prio=5 os_prio=31 cpu=16.42ms elapsed=8.92s tid=0x00007fb3e6000000 nid=0x6603 waiting for monitor entry  [0x0000700004ef3000]
   java.lang.Thread.State: BLOCKED (on object monitor)
    at Deadlock$Friend.bowBack(Deadlock.java:17)
    - waiting to lock <0x000000061fff4b78> (a Deadlock$Friend)
    at Deadlock$Friend.bow(Deadlock.java:14)
    - locked <0x000000061fff4b38> (a Deadlock$Friend)
    at Deadlock$1.run(Deadlock.java:29)
    at java.lang.Thread.run(java.base@15-ea/Thread.java:832)

和

"Thread-1" #15 prio=5 os_prio=31 cpu=0.71ms elapsed=8.92s tid=0x00007fb3e587ee00 nid=0x8e03 waiting for monitor entry  [0x0000700004ff6000]
   java.lang.Thread.State: BLOCKED (on object monitor)
    at Deadlock$Friend.bowBack(Deadlock.java:17)
    - waiting to lock <0x000000061fff4b38> (a Deadlock$Friend)
    at Deadlock$Friend.bow(Deadlock.java:14)
    - locked <0x000000061fff4b78> (a Deadlock$Friend)
    at Deadlock$2.run(Deadlock.java:35)
    at java.lang.Thread.run(java.base@15-ea/Thread.java:832)

查看每個已鎖定並正在等待的監視器的 ID。

您可以看到第一個線程正在等待第二個線程持有的鎖，反之亦然。