多线程的线程间通信

一、两个线程间的通信

• 1.什么时候需要通信
  • 多个线程并发执行时, 在默认情况下CPU是随机切换线程的
  • 如果我们希望他们有规律的执行, 就可以使用通信, 例如每个线程执行一次打印
• 2.怎么通信
  • 如果希望线程等待, 就调用wait()
  • 如果希望唤醒等待的线程, 就调用notify();
  • 这两个方法必须在同步代码中执行, 并且使用同步锁对象来调用

public class Demo1_Notify {

	/**
	 * @param args
	 * 等待唤醒机制
	 */
	public static void main(String[] args) {
		final Printer p = new Printer();
		
		new Thread() {
			public void run() {
				while(true) {
					try {
						p.print1();
					} catch (InterruptedException e) {
						
						e.printStackTrace();
					}
				}
			}
		}.start();
		
		new Thread() {
			public void run() {
				while(true) {
					try {
						p.print2();
					} catch (InterruptedException e) {
						
						e.printStackTrace();
					}
				}
			}
		}.start();
	}

}

//等待唤醒机制
class Printer {
	private int flag = 1;
	public void print1() throws InterruptedException {							
		synchronized(this) {
			if(flag != 1) {
				this.wait();					//当前线程等待
			}
			System.out.print("黑");
			System.out.print("马");
			System.out.print("程");
			System.out.print("序");
			System.out.print("员");
			System.out.print("\r\n");
			flag = 2;
			this.notify();						//随机唤醒单个等待的线程
		}
	}
	
	public void print2() throws InterruptedException {
		synchronized(this) {
			if(flag != 2) {
				this.wait();
			}
			System.out.print("传");
			System.out.print("智");
			System.out.print("播");
			System.out.print("客");
			System.out.print("\r\n");
			flag = 1;
			this.notify();
		}
	}
}

二、三个或三个以上线程通信

• 如果多个线程之间通信, 需要使用notifyAll()通知所有线程, 用while来反复判断条件

public class Demo2_NotifyAll {

	/**
	 * @param args
	 */
	public static void main(String[] args) {
		final Printer2 p = new Printer2();
		new Thread() {
			public void run() {
				while(true) {
					try {
						p.print1();
					} catch (InterruptedException e) {
						
						e.printStackTrace();
					}
				}
			}
		}.start();
		
		new Thread() {
			public void run() {
				while(true) {
					try {
						p.print2();
					} catch (InterruptedException e) {
						
						e.printStackTrace(); 
					}
				}
			}
		}.start();
		
		new Thread() {
			public void run() {
				while(true) {
					try {
						p.print3();
					} catch (InterruptedException e) {
						
						e.printStackTrace();
					}
				}
			}
		}.start();
	}

}
/*1,在同步代码块中,用哪个对象锁,就用哪个对象调用wait方法
 * 2,为什么wait方法和notify方法定义在Object这类中?
 * 	因为锁对象可以是任意对象,Object是所有的类的基类,所以wait方法和notify方法需要定义在Object这个类中
 * 3,sleep方法和wait方法的区别?
 * a,sleep方法必须传入参数,参数就是时间,时间到了自动醒来
 *   wait方法可以传入参数也可以不传入参数,传入参数就是在参数的时间结束后等待,不传入参数就是直接等待
 * b,sleep方法在同步函数或同步代码块中,不释放锁,睡着了也抱着锁睡
 * 	wait方法在同步函数或者同步代码块中,释放锁
 */ 
class Printer2 {
	private int flag = 1;
	public void print1() throws InterruptedException {							
		synchronized(this) {
			while(flag != 1) {
				this.wait();					//当前线程等待
			}
			System.out.print("黑");
			System.out.print("马");
			System.out.print("程");
			System.out.print("序");
			System.out.print("员");
			System.out.print("\r\n");
			flag = 2;
			//this.notify();						//随机唤醒单个等待的线程
			this.notifyAll();
		}
	}
	
	public void print2() throws InterruptedException {
		synchronized(this) {
			while(flag != 2) {
				this.wait();					//线程2在此等待
			}
			System.out.print("传");
			System.out.print("智");
			System.out.print("播");
			System.out.print("客");
			System.out.print("\r\n");
			flag = 3;
			//this.notify();
			this.notifyAll();
		}
	}
	
	public void print3() throws InterruptedException {
		synchronized(this) {
			while(flag != 3) {
				this.wait();						//线程3在此等待,if语句是在哪里等待,就在哪里起来
													//while循环是循环判断,每次都会判断标记
			}
			System.out.print("i");
			System.out.print("t");
			System.out.print("h");
			System.out.print("e");
			System.out.print("i");
			System.out.print("m");
			System.out.print("a");
			System.out.print("\r\n");
			flag = 1;
			//this.notify();
			this.notifyAll();
		}
	}
}

三、互斥锁

• 1.同步
  • 使用ReentrantLock类的lock()和unlock()方法进行同步
• 2.通信
  • 使用ReentrantLock类的newCondition()方法可以获取Condition对象
  • 需要等待的时候使用Condition的await()方法, 唤醒的时候用signal()方法
  • 不同的线程使用不同的Condition, 这样就能区分唤醒的时候找哪个线程了

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

public class Demo3_ReentrantLock {

	/**
	 * @param args
	 */
	public static void main(String[] args) {
		final Printer3 p = new Printer3();
		
		new Thread() {
			public void run() {
				while(true) {
					try {
						p.print1();
					} catch (InterruptedException e) {
						
						e.printStackTrace();
					}
				}
			}
		}.start();
		
		new Thread() {
			public void run() {
				while(true) {
					try {
						p.print2();
					} catch (InterruptedException e) {
						
						e.printStackTrace();
					}
				}
			}
		}.start();
		
		new Thread() {
			public void run() {
				while(true) {
					try {
						p.print3();
					} catch (InterruptedException e) {
						
						e.printStackTrace();
					}
				}
			}
		}.start();
	}

}

class Printer3 {
	private ReentrantLock r = new ReentrantLock();
	private Condition c1 = r.newCondition();
	private Condition c2 = r.newCondition();
	private Condition c3 = r.newCondition();
	
	private int flag = 1;
	public void print1() throws InterruptedException {							
		r.lock();								//获取锁
			if(flag != 1) {
				c1.await();
			}
			System.out.print("黑");
			System.out.print("马");
			System.out.print("程");
			System.out.print("序");
			System.out.print("员");
			System.out.print("\r\n");
			flag = 2;
			//this.notify();						//随机唤醒单个等待的线程
			c2.signal();
		r.unlock();								//释放锁
	}
	
	public void print2() throws InterruptedException {
		r.lock();
			if(flag != 2) {
				c2.await();
			}
			System.out.print("传");
			System.out.print("智");
			System.out.print("播");
			System.out.print("客");
			System.out.print("\r\n");
			flag = 3;
			//this.notify();
			c3.signal();
		r.unlock();
	}
	
	public void print3() throws InterruptedException {
		r.lock();
			if(flag != 3) {
				c3.await();
			}
			System.out.print("i");
			System.out.print("t");
			System.out.print("h");
			System.out.print("e");
			System.out.print("i");
			System.out.print("m");
			System.out.print("a");
			System.out.print("\r\n");
			flag = 1;
			c1.signal();
		r.unlock();
	}
}