为什么ReentrantLock信号()不能工作？

Question

虽然我通过信号()唤醒了线程，但是线程仍然在等待池中(我认为)，它只工作了一次。

while(true){
professor.ask();
}

因为代码，我认为它应该重复一遍又一遍。

我有两个线程“学生”，" Jogyo“，如果学生问的话，Jogyo得到信号().vice相反。

下面是我的代码

public class LockEx1 {
public static void main(String[] args) {
    Professor professor = new Professor();

    new Thread(new Student(professor), "student kim").start();
    new Thread(new JoGyo(professor), "assistant choi").start();

    try{Thread.sleep(20000);}catch (InterruptedException e){}
    System.exit(0);
}
}

class Student implements Runnable {
Professor professor; // shared resource

Student(Professor professor) {
    this.professor = professor;
}

@Override
public void run() {
    while (true) {
        professor.ask();
    }
}
}

class JoGyo implements Runnable {
Professor professor; // shared resource

JoGyo(Professor professor) {
    this.professor = professor;
}

@Override
public void run() {
    while (true) {
        try {Thread.sleep(100);} catch (InterruptedException e) {}//catch
        professor.ask();
    }
}
}


class Professor {
private final ReentrantLock lock = new ReentrantLock();

Condition forJoGyo = lock.newCondition();
Condition forStudent = lock.newCondition();

// 교수님께 질문하기
public void ask() {
    lock.lock(); // sync -- start
    String name = Thread.currentThread().getName();
    System.out.println(name + ": have a question. professor!");
    for (int i = 0; i < 5; i++) {
        try {Thread.sleep(500);} catch (InterruptedException e) {}
        System.out.println("professor: " + "explaining.." + i + " " + name + " listening.." );
    }//for
    System.out.println(name + ": thanks professor");
    if (name.contains("assistant")) {
        try{forJoGyo.await();}catch (InterruptedException e){} // 조교 재우기

        forStudent.signal(); // 학생 깨우기
    }
    else if (name.contains("student")) {
        try{forStudent.await();}catch (InterruptedException e){} // 학생 재우기

        forJoGyo.signal(); // 조교 깨우기
    }
    lock.unlock(); // sync -- end
}//ask
}

​

​

当你工作时，see..it只工作一次(每次只问教授一次)。这是因为信号()不起作用吗？为什么它不重复问教授..。

Answer 1

代码不再执行，因为它处于死锁状态。

1. 学生对“教授”对象执行ask方法。打印代码被执行，锁条件forStudent开始阻塞(await)线程直到信号。
2. 助手对ask对象执行该方法。打印代码被执行，锁条件foJoGyo开始阻塞(await)线程，直到等待信号。

两个线程现在都在等待来自另一个线程的信号。没有人能够到达信号代码，因此线程被永久锁定(死锁)-在这种情况下，直到到达从main方法返回之前的睡眠时间。

在开始等待来自另一个线程的信号之前，应该发出释放另一个线程的信号，否则两个线程都将永远等待。

if (name.contains("assistant")) {
    forStudent.signal();
    try {
        forJoGyo.await();
    } catch (InterruptedException e) {
        e.printStackTrace();
    }
} else if (name.contains("student")) {
    forJoGyo.signal();
    try {
        forStudent.await();
    } catch (InterruptedException e) {
        e.printStackTrace();
    }
}

结果：

student kim: have a question. professor!
professor: explaining..0 student kim listening..
professor: explaining..1 student kim listening..
professor: explaining..2 student kim listening..
professor: explaining..3 student kim listening..
professor: explaining..4 student kim listening..
student kim: thanks professor
assistant choi: have a question. professor!
professor: explaining..0 assistant choi listening..
professor: explaining..1 assistant choi listening..
professor: explaining..2 assistant choi listening..
professor: explaining..3 assistant choi listening..
professor: explaining..4 assistant choi listening..
assistant choi: thanks professor
student kim: have a question. professor!
professor: explaining..0 student kim listening..
professor: explaining..1 student kim listening..
professor: explaining..2 student kim listening..
professor: explaining..3 student kim listening..
professor: explaining..4 student kim listening..
student kim: thanks professor
assistant choi: have a question. professor!
professor: explaining..0 assistant choi listening..
professor: explaining..1 assistant choi listening..
professor: explaining..2 assistant choi listening..
professor: explaining..3 assistant choi listening..
professor: explaining..4 assistant choi listening..
assistant choi: thanks professor
student kim: have a question. professor!
professor: explaining..0 student kim listening..
professor: explaining..1 student kim listening..
professor: explaining..2 student kim listening..
professor: explaining..3 student kim listening..
professor: explaining..4 student kim listening..
student kim: thanks professor
assistant choi: have a question. professor!
professor: explaining..0 assistant choi listening..
professor: explaining..1 assistant choi listening..
professor: explaining..2 assistant choi listening..
professor: explaining..3 assistant choi listening..
professor: explaining..4 assistant choi listening..
assistant choi: thanks professor
student kim: have a question. professor!
professor: explaining..0 student kim listening..
professor: explaining..1 student kim listening..
professor: explaining..2 student kim listening..
professor: explaining..3 student kim listening..
professor: explaining..4 student kim listening..
student kim: thanks professor

重要的sidenote:应该将unlock finally 调用放在finally块中，其中 try 块包含 lock 后的代码，以避免在出现异常或早期返回< code >E 237时锁从未被解锁的情况。