condition.await()会释放锁

是的，condition.await()会释放锁。当线程调用condition.await()方法时，它会释放当前持有的锁，并进入等待状态，直到其他线程调用condition.signal()或condition.signalAll()方法唤醒它。在唤醒后，线程会重新尝试获取锁并继续执行。

相关问题

package com.company.calcul; import java.util.concurrent.locks.Condition; import java.util.concurrent.locks.ReentrantLock; public class Thread1 { public static void main(String[] args) { ReentrantLock reentrantLock = new ReentrantLock(); Condition condition1 = reentrantLock.newCondition(); Condition condition2 = reentrantLock.newCondition(); Condition condition3 = reentrantLock.newCondition(); Thread threadA = new Thread(() -> { reentrantLock.lock(); try { condition1.await(); System.out.println("A"); condition2.notify(); } catch (InterruptedException e) { e.printStackTrace(); } reentrantLock.unlock(); }); Thread threadB = new Thread(() -> { reentrantLock.lock(); try { condition2.await(); System.out.println("B"); condition3.notify(); } catch (InterruptedException e) { e.printStackTrace(); } reentrantLock.unlock(); }); Thread threadC = new Thread(() -> { reentrantLock.lock(); try { condition3.await(); System.out.println("C"); } catch (InterruptedException e) { e.printStackTrace(); } reentrantLock.unlock(); }); threadA.start(); threadB.start(); threadC.start(); condition1.notify(); } }

这段代码使用了ReentrantLock和Condition来实现线程间的通信。在这个例子中，有三个线程A、B、C，它们依次打印字母"A"、"B"、"C"。

代码中首先创建了一个ReentrantLock对象reentrantLock，以及三个Condition对象condition1、condition2、condition3。然后创建了三个线程，分别对应字母"A"、"B"、"C"。

在线程A的run方法中，先调用reentrantLock.lock()获取锁，然后调用condition1.await()释放锁并进入等待状态，直到被其他线程调用condition1.notify()唤醒。唤醒后打印字母"A"，并调用condition2.notify()唤醒线程B，最后调用reentrantLock.unlock()释放锁。

线程B和线程C的逻辑类似，先获取锁，然后进入等待状态，直到被唤醒后打印相应的字母，并唤醒下一个线程。

最后，在主线程中启动三个子线程，并调用condition1.notify()唤醒线程A开始执行。

需要注意的是，notify()方法只能唤醒一个等待的线程，所以在这里使用了三个Condition对象来实现三个线程的顺序执行。

package com.company.calcul; import java.util.concurrent.locks.Condition; import java.util.concurrent.locks.ReentrantLock; public class Thread1 { public static void main(String[] args) { ReentrantLock reentrantLock = new ReentrantLock(); Condition condition1 = reentrantLock.newCondition(); Condition condition2 = reentrantLock.newCondition(); Condition condition3 = reentrantLock.newCondition(); Thread threadA = new Thread(() -> { reentrantLock.lock(); try { condition1.await(); System.out.println("A"); condition2.signal(); } catch (InterruptedException e) { e.printStackTrace(); } reentrantLock.unlock(); }); Thread threadB = new Thread(() -> { reentrantLock.lock(); try { condition2.await(); System.out.println("B"); condition3.signal(); } catch (InterruptedException e) { e.printStackTrace(); } reentrantLock.unlock(); }); Thread threadC = new Thread(() -> { reentrantLock.lock(); try { condition3.await(); System.out.println("C"); } catch (InterruptedException e) { e.printStackTrace(); } reentrantLock.unlock(); }); threadA.start(); threadB.start(); threadC.start(); condition1.signal(); } } 程序无法执行，为什么？

这段代码无法正确执行是因为在调用Condition的signal()方法之前还没有获取到锁。

在代码中，线程A、B、C都会先调用reentrantLock.lock()获取锁，然后进入等待状态，并在等待状态中调用conditionX.await()方法来释放锁并等待被唤醒。

但是，在主线程中启动三个子线程之后，立即调用了condition1.signal()方法来唤醒线程A，这时候线程A还没有开始运行，也没有获取到锁，所以调用signal()方法会抛出IllegalMonitorStateException异常。

为了解决这个问题，可以在启动子线程之后，先调用reentrantLock.lock()获取锁，然后再调用condition1.signal()方法唤醒线程A，最后释放锁。这样可以确保线程A在调用signal()方法之前已经获取到了锁。

修正后的代码如下：

package com.company.calcul;

import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.ReentrantLock;

public class Thread1 {
    public static void main(String[] args) {

        ReentrantLock reentrantLock = new ReentrantLock();
        Condition condition1 = reentrantLock.newCondition();
        Condition condition2 = reentrantLock.newCondition();
        Condition condition3 = reentrantLock.newCondition();

        Thread threadA = new Thread(() -> {
            reentrantLock.lock();
            try {
                condition1.await();
                System.out.println("A");
                condition2.signal();
            } catch (InterruptedException e) {
                e.printStackTrace();
            } finally {
                reentrantLock.unlock();
            }
        });

        Thread threadB = new Thread(() -> {
            reentrantLock.lock();
            try {
                condition2.await();
                System.out.println("B");
                condition3.signal();
            } catch (InterruptedException e) {
                e.printStackTrace();
            } finally {
                reentrantLock.unlock();
            }
        });

        Thread threadC = new Thread(() -> {
            reentrantLock.lock();
            try {
                condition3.await();
                System.out.println("C");
            } catch (InterruptedException e) {
                e.printStackTrace();
            } finally {
                reentrantLock.unlock();
            }
        });

        reentrantLock.lock();
        try {
            threadA.start();
            threadB.start();
            threadC.start();
            condition1.signal();
        } finally {
            reentrantLock.unlock();
        }
    }
}

在修正后的代码中，主线程在启动子线程之前先获取锁，然后再启动子线程并调用condition1.signal()方法唤醒线程A，最后释放锁。这样就能保证线程A在调用signal()方法之前已经获取到了锁。