java 讀者寫者問題

原創

2020-02-20 18:15

讀者—寫者問題（Readers-Writers problem）也是一個經典的併發程序設計問題，是經常出現的一種同步問題。計算機系統中的數據（文件、記錄）常被多個進程共享，但其中某些進程可能只要求讀數據（稱爲讀者Reader）；另一些進程則要求修改數據（稱爲寫者Writer）。就共享數據而言，Reader和Writer是兩組併發進程共享一組數據區，要求：
（1）允許多個讀者同時執行讀操作；
（2）不允許讀者、寫者同時操作；

（3）不允許多個寫者同時操作。一次只能一個寫者

實現原理:使用Semapore 信號量分別表示讀者和寫者的信號量讀者可以多個同時讀,寫者只能有一個可以寫,使用AtomicInteger定義 readerCount 和writerCount 表示當前讀者和當前寫者的數量

package test;

import java.util.Random;
import java.util.concurrent.Semaphore;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.logging.Level;
import java.util.logging.Logger;

public class ThreadSync {

	private static ThreadSync thrdsync;
	private static Thread t1, t2, t3, t4, t5;
	private static final Random rand = new Random();
	private static Semaphore sm = new Semaphore(2);// 信號量 允許2個線程 true表示先進先出
	private static Semaphore wsm = new Semaphore(1);// 信號量 允許1個線程
	String text = "Beginning of the Book";// 代表書本
	AtomicInteger readerCount = new AtomicInteger(0); // 記錄當前讀者數量
	AtomicInteger writerCount = new AtomicInteger(0); // 記錄當前寫者數量

	// 隨機休眠一定時間
	private void busy() {
		try {
			Thread.sleep(rand.nextInt(1000) + 1000);
		} catch (InterruptedException e) {
		}
	}

	// 寫函數
	void write(String sentence) {
		System.out.println(Thread.currentThread().getName()
				+ " started to WRITE");
		text += "\n" + sentence;
		System.out.println(text);
		System.out.println("End of Book\n");
		System.out.println(Thread.currentThread().getName()
				+ " finished WRITING");
	}

	// 讀函數
	void read() {

		System.out.println("\n" + Thread.currentThread().getName()
				+ " started to READ");
		// System.out.println(text);
		// System.out.println("End of Book\n");

	}

	// 寫者
	private class Writer implements Runnable {

		ThreadSync ts;

		Writer(ThreadSync ts) {
			super();
			this.ts = ts;

		}

		public void run() {
			while (true) {
				if (readerCount.get() == 0) { // 當沒有讀者時才 可以寫
					try {
						//System.out.println("write---readerCount= "+readerCount.get());
						//System.out.println("write---writerCount= "+writerCount.get());
						wsm.acquire(); // 信號量獲取允許
						writerCount.getAndIncrement();
					} catch (InterruptedException ex) {
						Logger.getLogger(ThreadSync.class.getName()).log(
								Level.SEVERE, null, ex);
					}
					String new_sentence = new String("\tnew line in Book");
					busy();
					ts.write(new_sentence);
					wsm.release(); // 信號量釋放
					writerCount.getAndDecrement();

					busy();

				}
			} // of while
		}
	}

	// 讀者
	private class Reader implements Runnable {

		ThreadSync ts;

		Reader(ThreadSync ts) {
			super();
			this.ts = ts;

		}

		public void run() {
			while (true) {
				if (writerCount.get() == 0) { // 沒有寫者時 纔可以讀
					try {
						//System.out.println("Read---readerCount= "+readerCount.get());
						//System.out.println("Read---writerCount= "+writerCount.get());
						sm.acquire(); // 讀者獲取允許
						readerCount.getAndIncrement();
					} catch (InterruptedException ex) {
						Logger.getLogger(ThreadSync.class.getName()).log(
								Level.SEVERE, null, ex);
					}
					// System.out.print(t);

					ts.read();
					busy();
					System.out.println(Thread.currentThread().getName()
							+ " end of read");
					sm.release(); // 釋放允許
					readerCount.getAndDecrement();
					busy();
				}
			} // of while
		}
	}

	// 創建兩個讀者 一個寫者
	public void startThreads() {
		ThreadSync ts = new ThreadSync();
		t1 = new Thread(new Writer(ts), "Writer # 1");
		t2 = new Thread(new Writer(ts), "Writer # 2");
		t3 = new Thread(new Reader(ts), "Reader # 1");
		t4 = new Thread(new Reader(ts), "Reader # 2");
		// t5 = new Thread(new Reader(ts), "Reader # 3");
		t1.start();
		t2.start();
		t3.start();
		t4.start();
		// t5.start();
	}

	public static void main(String[] args) {
		thrdsync = new ThreadSync();
		System.out.println("Lets begin...\n");
		thrdsync.startThreads();
	}
}

輸出結果:

Lets begin...

Writer # 1 started to WRITE
Beginning of the Book
new line in Book
End of Book

Writer # 1 finished WRITING

Reader # 1 started to READ
Writer # 2 started to WRITE
Beginning of the Book
new line in Book
new line in Book
End of Book

Writer # 2 finished WRITING

Reader # 2 started to READ
Reader # 1 end of read
Reader # 2 end of read
Writer # 2 started to WRITE
Beginning of the Book
new line in Book
new line in Book
new line in Book
End of Book

Writer # 2 finished WRITING
Writer # 1 started to WRITE
Beginning of the Book
new line in Book
new line in Book
new line in Book
new line in Book
End of Book

Writer # 1 finished WRITING

Reader # 1 started to READ

Reader # 2 started to READ
Reader # 1 end of read
Reader # 2 end of read