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。
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