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Background
相关概念
什么是多线程
我们把组成程序(Program)各个部分称为线程(Thread)。也可以说，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，它可以允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.
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线程的状态
轮询
Samples
我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，叫做轮询(Polling)。轮询是会浪费一定的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.
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下面提供一个轮询的实现示例。
Message：
isAvailable初始值是false，设置为true以后执行轮询体。
注意要使用线程安全的AtomicBoolean，如果使用boolean，在多线程情况下会有意想不到的结果。
import lombok.Getter;import lombok.Setter;import java.util.concurrent.atomic.AtomicBoolean;@Setter@Getterpublic class Message { private AtomicBoolean isAvailable = new AtomicBoolean(false); private String msg; public Message(String str) { this.msg = str; }} 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()); } }} 执行测试：
休眠3秒以后，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"); }} 输出结果：
PollingWaiter : waiter starting at time: 14:26:08.482overPollingWaiter : msg is available at time: 14:26:11.402PollingWaiter : msg is available after count: -69547606PollingWaiter : processed: process it
wait 和 notify
除了轮询，Java通过wait 和 notify机制实现了线程间的通信。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重新获得CPU资源，再次进入运行状态。
由于wait 和 notify相关的方法实现在了java.lang.Object类中，因此所有的子类都可以使用这些方法。
wait 和 notify相关的方法需要在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()方法会导致当前线程从执行状态改为待执行状态，一直到另外一个线程为当前对象执行notify()或者notifyAll()方法。
- wait(long timeout)
与wait()方法的不同点是，如果timeout时间到了以后，还没有前对象执行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()的效果是一样的。
- 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所有线程，执行notify()会随机唤醒一个线程。
- notifyAll()
相比与notify()，此方法会唤醒所有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，增加如下代码实现。
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()); } }} 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(); } }} 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"); }} 在启动两个线程同时执行wait方法的时候，会发现notify以后只有一个线程被唤醒了，而另一个线程则陷入了无尽地等待之中。
Links
仓库地址
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完整示例代码请参考：
参考链接
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