并发编程之Wait和Notify

Background

相关概念

什么是多线程

咱们把组成程序(Program)各个部分称为线程(Thread)。也能够说，线程就是程序中轻量级的进程(Process)。

多线程(Multithreading)是Java的一个特性，它能够容许一个程序的多个部分(也就是线程)并发地执行，以达到最大程度利用CPU的目的。

Multithreading is a Java feature that allows concurrent execution of two or more parts of a program for maximum utilization of CPU. Each part of such program is called a thread. So, threads are light-weight processes within a process.

-- www.geeksforgeeks.org/multithread…

线程的状态

轮询

Samples

咱们把循环执行某个逻辑判断，直到判断条件为true才执行判断体中的逻辑，叫作轮询(Polling)。轮询是会浪费必定的CPU资源的。

The process of testing a condition repeatedly till it becomes true is known as polling.Polling is usually implemented with the help of loops to check whether a particular condition is true or not. If it is true, certain action is taken. This waste many CPU cycles and makes the implementation inefficient.

-- www.geeksforgeeks.org/inter-threa…

下面提供一个轮询的实现示例。

Message：

isAvailable初始值是false，设置为true之后执行轮询体。

注意要使用线程安全的AtomicBoolean，若是使用boolean，在多线程状况下会有意想不到的结果。

import lombok.Getter;
import lombok.Setter;
import java.util.concurrent.atomic.AtomicBoolean;

@Setter
@Getter
public class Message {
    private AtomicBoolean isAvailable = new AtomicBoolean(false);
    private String msg;
    public Message(String str) {
        this.msg = str;
    }
}
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PollingWaiter：

import java.time.LocalDateTime;
import java.time.format.DateTimeFormatter;

public class PollingWaiter implements Runnable {
    private Message msg;
    public PollingWaiter(Message m) {
        this.msg = m;
    }

    @Override
    public void run() {
        String name = Thread.currentThread().getName();
        synchronized (msg) {
            int count = 0;
            System.out.println(name + " : waiter starting at time: " + LocalDateTime.now().format(DateTimeFormatter.ISO_TIME));
            while (!msg.getIsAvailable().get()) {
                count++;
            }
            System.out.println(name + " : msg is available at time: " + LocalDateTime.now().format(DateTimeFormatter.ISO_TIME));
            System.out.println(name + " : msg is available after count: " + count);
            System.out.println(name + " : processed: " + msg.getMsg());
        }
    }
}

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执行测试：

休眠3秒之后，再执行轮询体内的代码。

import java.util.concurrent.atomic.AtomicBoolean;

public class WaitNotifyTest {

    public static void main(String[] args) {
        testPolling();
    }

    public static void testPolling() {

        Message msg = new Message("process it");

        PollingWaiter waiter = new PollingWaiter(msg);

        new Thread(waiter, "PollingWaiter").start();

        try {
            Thread.sleep(3000);
        } catch (Exception e) {
            e.printStackTrace();
        }
        msg.setIsAvailable(new AtomicBoolean(true));
        System.out.println("over");
    }

}
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输出结果：

PollingWaiter : waiter starting at time: 14:26:08.482
over
PollingWaiter : msg is available at time: 14:26:11.402
PollingWaiter : msg is available after count: -69547606
PollingWaiter : processed: process it
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wait 和 notify

除了轮询，Java经过wait 和 notify机制实现了线程间的通讯。wait就是让执有某个对象的线程处于等待阻塞状态，而notify就是让等待阻塞中的线程从新得到CPU资源，再次进入运行状态。

因为wait 和 notify相关的方法实如今了java.lang.Object类中，所以全部的子类均可以使用这些方法。

wait 和 notify相关的方法须要在synchronized代码块中执行。

方法介绍

下面简要介绍一下这些方法：

• wait()

wait()方法会致使当前线程从执行状态改成待执行状态，一直到另一个线程为当前对象执行notify()或者notifyAll()方法。

• wait(long timeout)

与wait()方法的不一样点是，若是timeout时间到了之后，尚未前对象执行notify()或者notifyAll()，则线程自动开始执行。

值得注意的是执行wait(0)和wait()的效果是同样的。

• wait(long timeout, int nanos)

与wait(long timeout)相比，此方法提供了等待超时设置的更高的精度，精确到了纳秒。

1毫秒 = 1,000,000 纳秒。

• notify()

对于等待此对象的监视器的全部线程，执行notify()会随机唤醒一个线程。

• notifyAll()

相比与notify()，此方法会唤醒全部等待该对象的监视器的线程。

示例

在上面示例代码的基础上，增长以下代码实现。

Waiter:

import java.time.LocalDateTime;
import java.time.format.DateTimeFormatter;

public class Waiter implements Runnable{

    private Message msg;

    public Waiter(Message m){
        this.msg=m;
    }

    @Override
    public void run() {
        String name = Thread.currentThread().getName();
        synchronized (msg) {
            try{
                System.out.println(name+" : waiting to get notified at time:"+ LocalDateTime.now().format(DateTimeFormatter.ISO_TIME));
                msg.wait();
            }catch(InterruptedException e){
                e.printStackTrace();
            }
            System.out.println(name+" : waiter thread got notified at time:"+LocalDateTime.now().format(DateTimeFormatter.ISO_TIME));
            //process the message now
            System.out.println(name+" : processed: "+msg.getMsg());
        }
    }

}
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Notifier：

public class Notifier implements Runnable {

    private boolean isAll = true;

    private Message msg;

    public Notifier(Message msg, boolean isAll) {
        this.msg = msg;
        this.isAll = isAll;
    }

    @Override
    public void run() {
        String name = Thread.currentThread().getName();
        System.out.println(name + " started");
        try {

            Thread.sleep(3000);

            synchronized (msg) {

                System.out.println(name + " : got the msg : "+msg.getMsg());

                msg.setMsg(name + " : Notifier work done");

                if (isAll) {
                    msg.notifyAll();
                } else {
                    msg.notify();
                }

            }
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

    }

}
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WaitNotifyTest:

import java.util.concurrent.atomic.AtomicBoolean;

public class WaitNotifyTest {

    public static void main(String[] args) {
        //testPolling();
        testNotify();
        //testNotifyAll();
    }

    public static void testPolling() {

        Message msg = new Message("process it");

        PollingWaiter waiter = new PollingWaiter(msg);

        new Thread(waiter, "PollingWaiter").start();

        try {
            Thread.sleep(3000);
        } catch (Exception e) {
            e.printStackTrace();
        }
        msg.setIsAvailable(new AtomicBoolean(true));
        System.out.println("over");
    }

    public static void testNotify() {
        Message msg = new Message("process it");

        Waiter waiter1 = new Waiter(msg);
        new Thread(waiter1, "waiter1").start();

        Waiter waiter2 = new Waiter(msg);
        new Thread(waiter2, "waiter2").start();

        Notifier notifier = new Notifier(msg, false);
        new Thread(notifier, "notifier").start();

        System.out.println("All the threads are started");
    }

    public static void testNotifyAll() {
        Message msg = new Message("process it");

        Waiter waiter1 = new Waiter(msg);
        new Thread(waiter1, "waiter1").start();

        Waiter waiter2 = new Waiter(msg);
        new Thread(waiter2, "waiter2").start();

        Notifier notifier = new Notifier(msg, false);
        new Thread(notifier, "notifier").start();

        System.out.println("All the threads are started");
    }
}

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在启动两个线程同时执行wait方法的时候，会发现notify之后只有一个线程被唤醒了，而另外一个线程则陷入了无尽地等待之中。

Links

仓库地址

• github.com/javastudyde…

• 完整示例代码请参考：

github.com/javastudyde…

参考连接

• www.geeksforgeeks.org/inter-threa…

• www.baeldung.com/java-wait-n…

• www.journaldev.com/1037/java-t…