Java n个线程轮流打印数字的问题

Posted 2020-09-25 cynchanpin

一. 实现两个线程。轮流打印出数字。例如以下：

bThread --> 10
aThread --> 9
bThread --> 8
aThread --> 7
bThread --> 6
aThread --> 5
bThread --> 4
aThread --> 3
bThread --> 2
aThread --> 1

用java中的Lock类实现：

package com.yjq.thread_demo;

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

public class TwoThreadPrinter {

	private Lock threadLock = new ReentrantLock();

	private boolean flag = false;
	
	int count =10;

	Thread aThread = new Thread(new Runnable() {
		public void run() {
			while (true) {
					// 锁定
					threadLock.lock();
					try {
						if ( count < 1) {
							return;
						}
						if (flag) {
							// aThread的任务
							System.out.println("aThread --> " + (count--));
							flag = !flag;
						}
					} catch (Exception e) {
						// TODO: handle exception
					} finally {
						// 释放锁
						threadLock.unlock();
					}
				}
		}
	});

	Thread bThread = new Thread(new Runnable() {
		public void run() {
			while (true) {
					// 锁定
					threadLock.lock();
					try {
						if ( count < 1) {
							return;
						}
						if (!flag) {
							// aThread的任务
							System.out.println("bThread --> " + (count--));
							flag = !flag;
						}
					} catch (Exception e) {
						// TODO: handle exception
					} finally {
						// 释放锁
						threadLock.unlock();
				}
			}
		}
	});

	public void startTwoThread() {
		aThread.start();
		bThread.start();
	}

	public static void main(String[] args) {
		TwoThreadPrinter twoThreadPrinter = new TwoThreadPrinter();
		twoThreadPrinter.startTwoThread();
	}

}

用synchronized实现：

package com.yjq.thread_demo;

public class TwoThreadPrinter2 {
	

	
private Object threadLock = new Object();

	int count =10;

	Thread aThread = new Thread(new Runnable() {
		public void run() {
			while (true) {
					// 锁定
					synchronized (threadLock) {
						if ( count < 1) {
							return;
						}

//							// aThread的任务
							System.out.println("aThread --> " + (count--));

						threadLock.notify();
						try {
							threadLock.wait();
						} catch (Exception e) {
							// TODO: handle exception
						}
					}
				}
		}
	});

	Thread bThread = new Thread(new Runnable() {
		public void run() {
			while (true) {
					// 锁定
					synchronized (threadLock) {
						if ( count < 1) {
							return;
						}

//							// aThread的任务
							System.out.println("bThread --> " + (count--));

						threadLock.notify();
						try {
							threadLock.wait();
						} catch (Exception e) {
							// TODO: handle exception
						}
					}
				}
		}
	});

	public void startTwoThread() {
		aThread.start();
		bThread.start();
	}

	public static void main(String[] args) {
		TwoThreadPrinter twoThreadPrinter = new TwoThreadPrinter();
		twoThreadPrinter.startTwoThread();
	}

}

用Lock类的方法比較easy理解。 lock() 和 unlock()之间的块是被锁定的

用synchronize的方法少用了个flag来标志轮到哪个线程来打印，这是由于线程锁的notifity( )方法会释放锁并唤醒其它线程 ，线程锁的wait( )方法则是释放锁并休眠当前线程,假设刚好仅仅有两个线程，那么自然就是開始还有一个线程而休眠本线程。

二.扩展到n个线程轮流打印数字的问题

n个线程轮流打印数字。用Lock类是比較easy扩展的

比方三个线程轮流打印数字

package com.myexample.test;


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

public class ThreeThreadPrinter {

	private Lock threadLock = new ReentrantLock();

	private int flag = 0;
	
	int count =10;

	Thread aThread = new Thread(new Runnable() {
		public void run() {
			while (true) {
					// 锁定
					threadLock.lock();
					try {
						if ( count < 1) {
							return;
						}
						if (count%3 == 0 ) {
							// aThread的任务
							System.out.println("aThread --> " + count);
							count--;
						}
					} catch (Exception e) {
						// TODO: handle exception
					} finally {
						// 释放锁
						threadLock.unlock();
					}
				}
		}
	});

	Thread bThread = new Thread(new Runnable() {
		public void run() {
			while (true) {
					// 锁定
					threadLock.lock();
					try {
						if ( count < 1) {
							return;
						}
						if (count%3 == 1 ) {
							// aThread的任务
							System.out.println("bThread --> " + count);
							count--;
						}
					} catch (Exception e) {
						// TODO: handle exception
					} finally {
						// 释放锁
						threadLock.unlock();
				}
			}
		}
	});

	Thread cThread = new Thread(new Runnable() {
		public void run() {
			while (true) {
					// 锁定
					threadLock.lock();
					try {
						if ( count < 1) {
							return;
						}
						if (count%3 == 2 ) {
							// aThread的任务
							System.out.println("cThread --> " + count);
							count--;
						}
					} catch (Exception e) {
						// TODO: handle exception
					} finally {
						// 释放锁
						threadLock.unlock();
				}
			}
		}
	});
	
	public void startTwoThread() {
		aThread.start();
		bThread.start();
		cThread.start();
	}

	public static void main(String[] args) {
		ThreeThreadPrinter twoThreadPrinter = new ThreeThreadPrinter();
		twoThreadPrinter.startTwoThread();
	}

}

输出结果

bThread --> 10
aThread --> 9
cThread --> 8
bThread --> 7
aThread --> 6
cThread --> 5
bThread --> 4
aThread --> 3
cThread --> 2
bThread --> 1