java - ReadWriteLock 不会被解锁

Question

我在这段代码中有一个死锁，但我不明白为什么。

有3个班和主班。

import java.util.concurrent.locks.ReentrantReadWriteLock;

public class TestThread {
private static final ReentrantReadWriteLock lock = new ReentrantReadWriteLock(true);
private static final String message = "a";

public static void main(String[] args) throws InterruptedException {
    Thread thread1 = new Thread(new WriterA(lock, message), "Writer A");
    Thread thread2 = new Thread(new WriterB(lock, message), "Writer B");
    Thread thread3 = new Thread(new Reader(lock), "Reader");

    thread1.start();
    thread2.start();
    thread3.start();

    thread1.join();
    thread2.join();
    thread3.join();
}
}

这是一个阅读器类：

import java.util.concurrent.locks.ReentrantReadWriteLock;

public class Reader implements Runnable {

private final ReentrantReadWriteLock lock;

public Reader(ReentrantReadWriteLock lock) {
    this.lock = lock;
}

@Override
public void run() {
    if (lock.isWriteLocked()) {
        System.out.println("Write lock present.");
    }
    lock.readLock().lock();
    System.out.println("Readlock lock");

    try {
        Long duration = (long) (Math.random() * 10000);
        System.out.println(Thread.currentThread().getName() + " taken " + (duration/1000) + " seconds.");
        Thread.sleep(duration);
    } catch (InterruptedException e) {
        e.printStackTrace();
    } finally {
        System.out.println("Unlock readlock");
        lock.readLock().unlock();
    }
}
}

这是作家B：

import java.util.concurrent.locks.ReentrantReadWriteLock;

public class WriterB implements Runnable {

private final ReentrantReadWriteLock lock;
private String message;

public WriterB(ReentrantReadWriteLock lock, String message) {
    this.lock = lock;
    this.message = message;
}

@Override
public void run() {
    while(true) {
        if (lock.writeLock().tryLock()) {
            lock.writeLock().lock();
            System.out.println(Thread.currentThread().getName() + ": Locking now");
            try {
                long duration = (long) (Math.random() * 10000);
                System.out.println(Thread.currentThread().getName() + " taken " + (duration/1000) + " seconds.");
                Thread.sleep(duration);
            } catch (InterruptedException e) {
                e.printStackTrace();
            } finally {
                message = message.concat("b");
                System.out.println(Thread.currentThread().getName() + ": Unlock now");
                lock.writeLock().unlock();
            }
            break;
        } else {
            System.out.println(Thread.currentThread().getName() + ": Write lock is present. Waiting 1000 ms.");
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
}
}

这是作家A：

import java.util.concurrent.locks.ReentrantReadWriteLock;

public class WriterA implements Runnable {

private final ReentrantReadWriteLock lock;
private String message;

public WriterA(ReentrantReadWriteLock lock, String message) {
    this.lock = lock;
    this.message = message;
}

@Override
public void run() {
    lock.writeLock().lock();
    System.out.println(Thread.currentThread().getName() + ": Locking now");
    try {
        Long duration = (long) (Math.random() * 10000);
        System.out.println(Thread.currentThread().getName() + " taken " + (duration/1000) + " seconds.");
        Thread.sleep(duration);
    } catch (InterruptedException e) {
        e.printStackTrace();
    } finally {
        message = message.concat("a");
        System.out.println(Thread.currentThread().getName() + ": Unlock now");
        lock.writeLock().unlock();
    }
}
}

当我运行主类时，有时所有类都在那里工作，但是当 Writer B 启动时它陷入死锁。

坏情况控制台输出：

Writer B: Locking now
Writer B taken 4 seconds.
Writer B: Unlock now

好案例控制台输出：

Writer A: Locking now
Writer B: Write lock is present. Waiting 1000 ms.
Writer A taken 5 seconds.
Writer B: Write lock is present. Waiting 1000 ms.
Writer B: Write lock is present. Waiting 1000 ms.
Writer B: Write lock is present. Waiting 1000 ms.
Writer B: Write lock is present. Waiting 1000 ms.
Writer B: Write lock is present. Waiting 1000 ms.
Writer A: Unlock now
Readlock lock
Reader taken 3 seconds.
Writer B: Write lock is present. Waiting 1000 ms.
Writer B: Write lock is present. Waiting 1000 ms.
Writer B: Write lock is present. Waiting 1000 ms.
Writer B: Write lock is present. Waiting 1000 ms.
Unlock readlock
Writer B: Locking now
Writer B taken 7 seconds.
Writer B: Unlock now