Java并发编程实例--16.使用ReentrantLock实现线程同步

Java提供另一机制去同步代码块。它比synchronized关键字更强大且易用。

它是基于Lock接口和其实现类例如：ReentrantLock。

这一机制对比synchronized关键字的优势在于：

1.可以构建更加灵活的同步代码块；

2.Lock接口提供了一些额外功能，例如tryLock()方法，它会返回一个布尔值以表示当前资源是否被锁。

3.Lock接口可以实现读写分离，多个读者但只有一个修改者。

4.Lock接口提供更好的性能。

本例中，我们将通过模拟打印队列去学习如何使用锁去同步代码块。

PrintQueue.java

package com.dylan.thread.ch2.c04.task;

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

/**
 * This class simulates a print queue
 *
 */
public class PrintQueue {

	/**
	 * Lock to control the access to the queue.
	 */
	private final Lock queueLock=new ReentrantLock();

	/**
	 * Method that prints a document
	 * @param document document to print
	 */
	public void printJob(Object document){
		queueLock.lock();

		try {
			Long duration=(long)(Math.random()*10000);
			System.out.printf("%s: PrintQueue: Printing a Job during %d seconds\n",Thread.currentThread().getName(),(duration/1000));
			Thread.sleep(duration);
		} catch (InterruptedException e) {
			e.printStackTrace();
		} finally {
			queueLock.unlock();
		}
	}
}

Job.java

package com.dylan.thread.ch2.c04.task;

/**
 * This class simulates a job that send a document to print.
 *
 */
public class Job implements Runnable {

	/**
	 * Queue to print the documents
	 */
	private PrintQueue printQueue;

	/**
	 * Constructor of the class. Initializes the queue
	 * @param printQueue
	 */
	public Job(PrintQueue printQueue){
		this.printQueue=printQueue;
	}

	/**
	 * Core method of the Job. Sends the document to the print queue and waits
	 *  for its finalization
	 */
	@Override
	public void run() {
		System.out.printf("%s: Going to print a document\n",Thread.currentThread().getName());
		printQueue.printJob(new Object());
		System.out.printf("%s: The document has been printed\n",Thread.currentThread().getName());
	}
}

Main.java

package com.dylan.thread.ch2.c04.core;

import com.dylan.thread.ch2.c04.task.Job;
import com.dylan.thread.ch2.c04.task.PrintQueue;

/**
 * Main class of the example.
 *
 */
public class Main {

	/**
	 * Main method of the class. Run ten jobs in parallel that
	 * send documents to the print queue at the same time.
	 */
	public static void main (String args[]){

		// Creates the print queue
		PrintQueue printQueue=new PrintQueue();

		// Creates ten Threads
		Thread thread[]=new Thread[10];
		for (int i=0; i<10; i++){
			thread[i]=new Thread(new Job(printQueue),"Thread "+i);
		}

		// Starts the Threads
		for (int i=0; i<10; i++){
			thread[i].start();
		}
	}

}

运行结果：

Thread 0: Going to print a document
Thread 5: Going to print a document
Thread 0: PrintQueue: Printing a Job during 0 seconds
Thread 8: Going to print a document
Thread 7: Going to print a document
Thread 3: Going to print a document
Thread 1: Going to print a document
Thread 4: Going to print a document
Thread 2: Going to print a document
Thread 9: Going to print a document
Thread 6: Going to print a document
Thread 0: The document has been printed
Thread 5: PrintQueue: Printing a Job during 7 seconds
Thread 5: The document has been printed
Thread 8: PrintQueue: Printing a Job during 0 seconds
Thread 7: PrintQueue: Printing a Job during 5 seconds
Thread 8: The document has been printed
Thread 7: The document has been printed
Thread 3: PrintQueue: Printing a Job during 7 seconds
Thread 3: The document has been printed
Thread 1: PrintQueue: Printing a Job during 3 seconds
Thread 1: The document has been printed
Thread 4: PrintQueue: Printing a Job during 7 seconds
Thread 4: The document has been printed
Thread 2: PrintQueue: Printing a Job during 5 seconds
Thread 9: PrintQueue: Printing a Job during 8 seconds
Thread 2: The document has been printed
Thread 9: The document has been printed
Thread 6: PrintQueue: Printing a Job during 4 seconds
Thread 6: The document has been printed