为什么 ReentrantLock signal() 不起作用？

Question

Although i waked the Thread by signal(), Thread still in waiting pool(i think) and it works only once.

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

because of the code i thought it shoud repeat ask again and again

i have two Threads "Student","Jogyo" if Student asks then, Jogyo get signal().vice versa.

below is my code

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
}

as you see..it works(ask to professor each only onces at all) only once. is this because signal() is not working? why it doesn't repeat asking to professor..

Answer 1

The code does not execute any further because it is in a deadlock state.

1. Student executes the ask method on the Professor object. The printing code is executed and the lock condition forStudent is starting to block ( await ) the thread until a signal.
2. Assistant executes the ask method on the Professor object. The printing code is executed an the lock condition foJoGyo is starting to block ( await ) the thread until an await signal.

Both threads are now waiting for a signal from the other one. None can reach the signal code, so the threads are locked forever (deadlock) - in this case until the sleep time before returning from the main method is reached.

The signal unlocking the other thread should be emitted before starting the waiting for the signal from the other thread, otherwise both threads will wait forever.

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();
    }
}

Result:

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

Important sidenote: unlock call should be placed inside a finally block with try block containing the code after lock to avoid a situation when the lock is never unlocked in case of an exception or an early return .