Re-entrant Locks

来源：互联网发布：lgd和淘宝编辑：程序博客网时间：2024/06/05 14:30

1. 可以多次重入，只不过也要多次释放锁而已

2. unlock最好放在finally块里面，这样就可以保证即使获得锁后运行抛异常，也可以释放锁

3. lock里面Condition的await和signal对应synchronized的wait notify

4. signal运行后只有unlock了另外一个线程才能被唤醒

5. 使用Lock而不是synchronized的好处是可以使用try lock，这样就可以避免死锁，具体见下一篇博客

/* * Re-entrant Locks */public class App {public static void main(String[] args) throws InterruptedException {final Runner runner = new Runner();Thread t1 = new Thread(new Runnable(){public void run() {try {runner.firstThread();} catch (InterruptedException e) {e.printStackTrace();}}});Thread t2 = new Thread(new Runnable(){public void run() {try {runner.secondRunner();} catch (InterruptedException e) {e.printStackTrace();}}});t1.start();t2.start();t1.join();t2.join();runner.finished();}}

import java.util.Scanner;import java.util.concurrent.locks.Condition;import java.util.concurrent.locks.Lock;import java.util.concurrent.locks.ReentrantLock;/* *Re-entrant Locks */public class Runner {private int count = 0;private Lock lock = new ReentrantLock();private Condition cond = lock.newCondition();private void increment() {for(int i=0; i<1000; i++) {count ++;}}public void firstThread() throws InterruptedException {lock.lock();System.out.println("waiting ...");cond.await();System.out.println("wake up ... ");try {increment();} finally {lock.unlock();}}public void secondRunner() throws InterruptedException {Thread.sleep(1000);lock.lock();System.out.println("press the return key.");new Scanner(System.in).nextLine();System.out.println("get return key.");cond.signal();try {increment();} finally {lock.unlock();}}public void finished() {System.out.println("count is:" + count);}}

如果把secondRunner中的unlock注释掉那么就不会唤醒firstThread

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