Java并发编程实例--18.修改锁的公平性

ReentrantLock和ReentrantReadWriteLock类的构造函数可接受一个布尔类型参数fair，表示你可以控制这2个类的行为。

其默认值为false，代表non-fair（不公平）模式。

这种模式下，如果有多个线程等待获得锁并且只能有一个线程获得，上面2个锁类会无条件选择一个。（也就是随机抽一个）

值为true的话就表示fair模式，即公平模式。

意思是有个先来后到，就跟咱们买东西要排队一样。锁会分给那个等了最长时间的线程。





以上逻辑仅适用于lock() and unlock()方法。因为例如tryLock()方法压根不会让线程睡眠。

本例中，我们将修改一下之前使用锁机制同步代码块的程序，加入这个参数来对比一下fair和non-fair两种模式的差别。

PrintQueue.java

package com.dylan.thread.ch2.c06.task;

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

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

	/**
	 * Creates a lock to control the access to the queue.
	 * With the boolean attribute, we control the fairness of
	 * the Lock
	 */
	private final Lock queueLock=new ReentrantLock(false);

	/**
	 * Method that prints the Job. The printing is divided in two phase two
	 * show how the fairness attribute affects the election of the thread who
	 * has the control of the lock
	 * @param document The 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();
		}

		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.c06.task;

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

	/**
	 * The queue to send the documents
	 */
	private PrintQueue printQueue;

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

	/**
	 * Core method of the Job. Sends the document to the queue
	 */
	@Override
	public void run() {
		System.out.printf("%s: Going to print a job\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.c06.core;

import com.dylan.thread.ch2.c06.task.Job;
import com.dylan.thread.ch2.c06.task.PrintQueue;

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

	/**
	 * Main method of the example
	 * @param args
	 */
	public static void main (String args[]){
		// Creates the print queue
		PrintQueue printQueue=new PrintQueue();

		// Cretes ten jobs and the Threads to run them
		Thread thread[]=new Thread[10];
		for (int i=0; i<10; i++){
			thread[i]=new Thread(new Job(printQueue),"Thread "+i);
		}

		// Launch a thread ever 0.1 seconds
		for (int i=0; i<10; i++){
			thread[i].start();
			try {
				Thread.sleep(100);
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
		}
	}

}

运行结果：

1.不公平模式：

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

2.公平模式：

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