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 document
Thread9: Going to print a document
Thread8: Going to print a document
Thread7: Going to print a document
Thread6: Going to print a document
Thread5: Going to print a document
Thread4: Going to print a document
Thread0: Going to print a document
Thread3: Going to print a document
Thread2: Going to print a document
Thread1: PrintQueue: Printing a Job during 9 seconds
Thread1: The document has been printed
Thread9: PrintQueue: Printing a Job during 8 seconds
Thread9: The document has been printed
Thread8: PrintQueue: Printing a Job during 5 seconds
Thread8: The document has been printed
Thread7: PrintQueue: Printing a Job during 2 seconds
Thread7: The document has been printed
Thread6: PrintQueue: Printing a Job during 0 seconds
Thread6: The document has been printed
Thread5: PrintQueue: Printing a Job during 5 seconds
Thread5: The document has been printed
Thread4: PrintQueue: Printing a Job during 2 seconds
Thread4: The document has been printed
Thread0: PrintQueue: Printing a Job during 7 seconds
Thread0: The document has been printed
Thread3: PrintQueue: Printing a Job during 2 seconds
Thread3: The document has been printed
Thread2: PrintQueue: Printing a Job during 7 seconds
Thread 2: The document has been printed


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Java并发编程之线程管理(基本线程同步3)

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