Java 學習記錄105 — Deadlock

今天要學的是 Deadlock

Deadlocks.java

import java.util.Random;
public class Deadlocks {
    public static void main(String[] args) {
        Message message = new Message();
        (new Thread(new Writer(message))).start();
        (new Thread(new Reader(message))).start();
    }
}
class Message {
    private String message;
    private boolean empty = true;
    public synchronized String read() {
        while (empty) {
        }
        empty = true;
        return message;
    }
    public synchronized void write(String message) {
        while (!empty) {
        }
        empty = false;
        this.message = message;
    }

}
class Writer implements Runnable {
    private Message message;
    public Writer(Message message) {
        this.message = message;
    }
    public void run() {
        String messages[] = {
                "Humpty Dumpty sat on a wall",
                "Humpty Dumpty had a great fall",
                "All the king's horse and all the king's men",
                "Couldn't put Humpty together again."
        };
        Random random = new Random();
        for (int i = 0; i < messages.length; i++) {
            message.write(messages[i]);
            try {
                Thread.sleep(random.nextInt(2000));
            } catch (InterruptedException e) {
            }
        }
        message.write("Finished");
    }
}
class Reader implements Runnable {
    private Message message;
    public Reader(Message message) {
        this.message = message;
    }
    public void run() {
        Random ramdom = new Random();
        for (String latestMessage = message.read(); !latestMessage.equals("Finished");
             latestMessage = message.read()) {
            System.out.println(latestMessage);
            try {
                Thread.sleep(random.nextInt(2000));
            } catch (InterruptedException e) {
            }
        }
    }
}

輸出結果：

Humpty Dumpty sat on a wall

這裡做了兩個 Thread，有 implements Runnable 的兩個就是，一個是讀取，一個是寫入

可以看到在 Message 裡面的兩個 Method 都有使用 synchronized

而輸出結果只顯示了一句，照理說要四句都顯示出來

這裡的條件是要等待 empty 的 bool 改變才能跳出 while

但因為使用了 synchronized 造成第一個 Thread 進去後，其他 Thread 在外面等

但因為沒有達成退出條件，所以就一直卡在 while 裡面，程序就進入永遠的循環

這就是所謂的 Deadlock

資料補充：Deadlock

修改 Deadlocks.java

import java.util.Random;
public class Deadlocks {
    public static void main(String[] args) {
        Message message = new Message();
        (new Thread(new Writer(message))).start();
        (new Thread(new Reader(message))).start();
    }
}
class Message {
    private String message;
    private boolean empty = true;
    public synchronized String read() {
        while (empty) {
            try {
                wait();
            } catch (InterruptedException e) {
            }
        }
        empty = true;
        notifyAll();
        return message;
    }
    public synchronized void write(String message) {
        while (!empty) {
            try {
                wait();
            } catch (InterruptedException e) {
            }
        }
        empty = false;
        this.message = message;
        notifyAll();
    }

}
class Writer implements Runnable {
    private Message message;
    public Writer(Message message) {
        this.message = message;
    }
    public void run() {
        String messages[] = {
                "Humpty Dumpty sat on a wall",
                "Humpty Dumpty had a great fall",
                "All the king's horse and all the king's men",
                "Couldn't put Humpty together again."
        };
        Random random = new Random();
        for (int i = 0; i < messages.length; i++) {
            message.write(messages[i]);
            try {
                Thread.sleep(random.nextInt(2000));
            } catch (InterruptedException e) {
            }
        }
        message.write("Finished");
    }
}
class Reader implements Runnable {
    private Message message;
    public Reader(Message message) {
        this.message = message;
    }
    public void run() {
        Random random = new Random();
        for (String latestMessage = message.read(); !latestMessage.equals("Finished");
             latestMessage = message.read()) {
            System.out.println(latestMessage);
            try {
                Thread.sleep(random.nextInt(2000));
            } catch (InterruptedException e) {
            }
        }
    }
}

輸出結果：

Humpty Dumpty sat on a wall

Humpty Dumpty had a great fall

All the king’s horse and all the king’s men

Couldn’t put Humpty together again.

因此這邊使用了 wait() 跟 notifyAll() 來解決此問題

前者可以當作是先在這邊暫停並等待喚醒而後者就是喚醒前者

一開始沒有使用這兩者的時候，程序卡在 read() 裡面

當第二次 for 的時候，因為第一次已經把 empty 改成 true 了

也沒有外力可以改變，所以就卡死在那邊

當這次使用了這兩者，就變成到 wait() 之時，先暫停執行並解鎖，好讓程序繼續執行

而此時 write() 那邊，因為可以繼續往下走就把 empty 改成 false 了

之後又走到 notifyAll() 把 read() 那邊喚醒

此時一看 empty 已經改變就順利跳出 while 了，所以不會繼續卡死在那邊

上面代碼全都紀錄在我的 Github