多线程与锁

1 多线程

1.1 实现多线程

写法 1：继承 Thread 类

class Worker extends Thread {
    @Override
    public void run() {
        for (int i = 0; i < 10; i ++ ) {
            System.out.println("Hello! " + this.getName());
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                throw new RuntimeException(e);
            }
        }
    }
}

public class Main {
    public static void main(String[] args) {
        Worker worker1 = new Worker();
        Worker worker2 = new Worker();
        worker1.setName("thread-1");
        worker2.setName("thread-2");
        worker1.start();
        worker2.start();
    }
}

写法 2：实现 Runnable 接口

class Worker1 implements Runnable {
    @Override
    public void run() {
        for (int i = 0; i < 10; i ++ ) {
            System.out.println("Hello! " + "thread-1");
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                throw new RuntimeException(e);
            }
        }
    }
}

class Worker2 implements Runnable {
    @Override
    public void run() {
        for (int i = 0; i < 10; i ++ ) {
            System.out.println("Hello! " + "thread-2");
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                throw new RuntimeException(e);
            }
        }
    }
}

public class Main {
    public static void main(String[] args) {
        new Thread(new Worker1()).start();
        new Thread(new Worker2()).start();
    }
}

1.2 常用 API

1. start()：开启一个线程
2. Thread.sleep()：休眠一个线程
3. join()：等待线程执行结束
4. interrupt()：从休眠中中断线程
5. setDaemon()：将线程设置为守护线程。当只剩下守护线程时，程序自动退出

---

2 锁

• lock：获取锁，如果锁已经被其他线程获取，则阻塞
• unlock：释放锁，并唤醒被该锁阻塞的其他线程

import java.util.concurrent.locks.ReentrantLock;
class Worker extends Thread {
    public static int cnt = 0;
    private static final ReentrantLock lock = new ReentrantLock();


    @Override
    public void run() {
        for (int i = 0; i < 100000; i ++ ) {
            lock.lock();
            try {
                cnt ++ ;
            } finally {
                lock.unlock();
            }
        }
    }
}

public class Main {
    public static void main(String[] args) throws InterruptedException {
        Worker worker1 = new Worker();
        Worker worker2 = new Worker();

        worker1.start();
        worker2.start();
        worker1.join();
        worker2.join();

        System.out.println(Worker.cnt);
    }
}

---

3 同步（Synchronized）

写法 1：将 Synchronized 加到代码块上

class Count {
    public int cnt = 0;
}

class Worker extends Thread {
    public final Count count;

    public Worker(Count count) {
        this.count = count;
    }

    @Override
    public void run() {
        synchronized (count) {
            for (int i = 0; i < 100000; i ++ ) {
                count.cnt ++ ;
            }
        }
    }
}

public class Main {
    public static void main(String[] args) throws InterruptedException {
        Count count = new Count();

        Worker worker1 = new Worker(count);
        Worker worker2 = new Worker(count);

        worker1.start();
        worker2.start();
        worker1.join();
        worker2.join();

        System.out.println(count.cnt);
    }
}

写法 2：将 Synchronized 加到函数上（锁加到了 this 对象上）

class Worker implements Runnable {
    public static int cnt = 0;

    private synchronized void work() {
        for (int i = 0; i < 100000; i ++ ) {
            cnt ++ ;
        }
    }

    @Override
    public void run() {
        work();
    }
}

public class Main {
    public static void main(String[] args) throws InterruptedException {
        Worker worker = new Worker();
        Thread worker1 = new Thread(worker);
        Thread worker2 = new Thread(worker);

        worker1.start();
        worker2.start();
        worker1.join();
        worker2.join();

        System.out.println(Worker.cnt);
    }
}

3.1 wait 与 notify

package org.yxc;

class Worker extends Thread {
    private final Object object;
    private final boolean needWait;

    public Worker(Object object, boolean needWait) {
        this.object = object;
        this.needWait = needWait;
    }

    @Override
    public void run() {
        synchronized (object) {
            try {
                if (needWait) {
                    object.wait();
                    System.out.println(this.getName() + ": 被唤醒啦！");
                } else {
                    object.notifyAll();
                }
            } catch (InterruptedException e) {
                throw new RuntimeException(e);
            }
        }
    }
}

public class Main {
    public static void main(String[] args) throws InterruptedException {
        Object object = new Object();
        for (int i = 0; i < 5; i ++ ) {
            Worker worker = new Worker(object, true);
            worker.setName("thread-" + i);
            worker.start();
        }

        Worker worker = new Worker(object, false);
        worker.setName("thread-" + 5);
        Thread.sleep(1000);
        worker.start();
    }
}

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