Java并发编程之线程管理（基本线程同步3）

               (上一节的继续)

2.2 使用Lock机制

        Java提供了另外一种机制来同步代码块。它是比synchrozied关键字更为强大且弹性的机制。它是基于锁接口和实现了这个接口的类（如ReetrantLock类）。这个机制表现出一些优势，如下所示：

1.       以一种更加弹性的方式允许同步块结构，而使用synchrozied关键字，你必须以一个机构化的方式来获取和释放对于同步块代码的控制。实用Lock接口允许你获取更为复杂的结构去实现你的临界部分。

2.       Lock接口提供除synchronized外额外的功能，新功能之一就是它实现了tryLock()方法。这个方法尝试着去获取锁的控制，如果它不能够获取（已经被其它的线程占有），它返回这个锁。Synchronized关键字修饰的方法中，当线程A尝试着去执行synchronized代码块时，而这个synchronized代码块已经被线程B所执行，这时，线程A不得不挂起直到线程B完成它的同步块代码的执行。在Lock实现中，有可以执行tryLock方法，它返回一个布尔值来显示是否那儿有另外的线程运行这个受Lock保护的代码。

3.       Lock接口允许读写操作分离，有多个读者和唯一的写者。

4.       Lock接口比synchronized关键字提供了更好的性能。

看下面这个例子，比较好阐明这Lock的使用。

定义打印机队列任务，打印服务PrintQueue类。

 

public class PrintQueue {      /**     * Lock to control the access to the queue.     */    private finalLock queueLock=new ReentrantLock();       /**     * Method that prints a document     * @param document document to print     */    public voidprintJob(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();        }    } }

定义打印调用类Task，调用对应的打印任务。

 

public class Task implements Runnable {    /**     * Queue to print the documents     */    private PrintQueue printQueue;       /**     * Constructor of the class. Initializes thequeue     * @param printQueue     */    public Task(PrintQueue printQueue){        this.printQueue=printQueue;    }       /**     * Core method of the Job. Sends the documentto the print queue and waits     *  forits finalization     */    @Override    public voidrun() {        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());         }    /**     * @param args     */    public staticvoidmain(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 Task(printQueue),"Thread "+i);        }               // Starts the Threads        for (int i=0; i<10; i++){            thread[i].start();        }     } }

运行结构如下：

Thread1: Going to print a documentThread9: Going to print a documentThread8: Going to print a documentThread7: Going to print a documentThread6: Going to print a documentThread5: Going to print a documentThread4: Going to print a documentThread0: Going to print a documentThread3: Going to print a documentThread2: Going to print a documentThread1: PrintQueue: Printing a Job during 9 secondsThread1: The document has been printedThread9: PrintQueue: Printing a Job during 8 secondsThread9: The document has been printedThread8: PrintQueue: Printing a Job during 5 secondsThread8: The document has been printedThread7: PrintQueue: Printing a Job during 2 secondsThread7: The document has been printedThread6: PrintQueue: Printing a Job during 0 secondsThread6: The document has been printedThread5: PrintQueue: Printing a Job during 5 secondsThread5: The document has been printedThread4: PrintQueue: Printing a Job during 2 secondsThread4: The document has been printedThread0: PrintQueue: Printing a Job during 7 secondsThread0: The document has been printedThread3: PrintQueue: Printing a Job during 2 secondsThread3: The document has been printedThread2: PrintQueue: Printing a Job during 7 secondsThread 2: The document has been printed