Question

我在这里遇到了一个小问题。我正在用Java做一个并发程序。问题是：有4个人（学生）试图访问打印机，打印5个文档。但是当时只有一个人可以打印（有点明显）5个文件。完成后，他们通知其他人他们完成了，其他线程访问资源。我有一个Main类，学生类和Monitor（激光打印机），Document类，它包含有关文档的信息（如页数，名称用户ID等）+少数打印机接口。我已成功运行线程，但它们不同步（互斥）
所以问题是如何实现互斥（当时只有一个人可以打印他的文件数量）感谢您的关注，时间和提示:)

主要课程

    String S1Name = "bob";
    String S2Name = "klara";
    String S3Name = "John";
    String S4Name = "Iga";

    String T1Name = "Man";
    String T2Name = "Woman";


    final int NoOfDocs = 5;
    ServicePrinter sp = new LaserPrinter();

    ThreadGroup groupA = new ThreadGroup("Group A"); 
    ThreadGroup groupB = new ThreadGroup("Group B");

    Student student1 = new Student(sp,NoOfDocs,S1Name, groupA);
    Student student2 = new Student(sp,NoOfDocs,S2Name, groupA);
    Student student3 = new Student(sp,NoOfDocs,S3Name, groupA);
    Student student4 = new Student(sp,NoOfDocs,S4Name, groupA);

    TonerTechnician TT = new TonerTechnician(groupB);
    PaperTechnician PT = new PaperTechnician(groupB);

    /*
     * Start Student Threads
     */
    student1.start();
    student2.start();
    student3.start();
    student4.start();

    /*
     * Start Technician threads
     */
    TT.start();
    PT.start();

学生班

 private final ServicePrinter serviceprinter;
    private final int NoOfDocs;
    private final String Name;
    private final ThreadGroup threadgroup;

    public Student(ServicePrinter serviceprinter, int NoOfDocs, String Name, ThreadGroup threadgroup)
    {
        this.serviceprinter = serviceprinter;
        this.NoOfDocs = NoOfDocs; 
        this.Name = Name;
        this.threadgroup = threadgroup;
    }
    @Override
    public void run()
    {
        /*
         * each students prints 5 documents (different name and length)
         */
        final LaserPrinter lp = new LaserPrinter();
        //sleep from 1 to 5 sec random time
        final Random random = new Random();       
        char[] chars = "abcdefghijklmnopqrstuvwxyz".toCharArray();
        StringBuilder sb = new StringBuilder();
        /*
         * Create random document name 10 characters long
         */
        for (int i = 0; i < 10; i++) 
        {
            char c = chars[random.nextInt(chars.length)];
            sb.append(c);
        }
        String docName  = sb.toString();
        /*
         * print 5 documents (random sleep time between printing)
         */
        for(int i = 0; i < NoOfDocs; i++)
        {
            try 
            {
                Document coursework = new Document(Name,docName,random.nextInt(90)+10);
                lp.printDocument(coursework);
                Thread.sleep(random.nextInt(1000)+4000);                
            } 
            catch (InterruptedException ex) 
            {
                Logger.getLogger(Student.class.getName()).log(Level.SEVERE, null, ex);
            }            
        }
        System.out.println("User: " + Name+ " completed printing");

监控班级

    int tonerLevel = 500;
    int paperLevel = 250;

    private final String PrinterName = "HP";
    private final String PrinterID = "LX-440";
    private int CurrentPaperLevel;
    private int CurrentTonerLevel;
    private int NoOfDocsPrinted;

    @Override
    public synchronized void printDocument(Document document) {
        System.out.println(document);

    }

Answer 1

这是一个简单的互斥实现，但您应该使用java.util.concurrent包进行同步

编辑：将互斥锁更改为信号量（更有意义）

一个简单的互斥实现：

public class Mutex {

    private int semaphore;

    public synchronized void aquire() throws InterruptedException {

        if(semaphore < 0) {
            wait();
        }

        semaphore--;

    }

    public synchronized void release() {

        if(semaphore < 0) {
            semaphore++;
            notify();            
        }
    }
}

Answer 2

您似乎在run方法中创建了本地打印机对象，而不是使用传递给Student类的共享对象。尝试使用您传递的共享打印机，看看你得到了什么。我们还需要了解您在printDocument中如何使用ServicePrinter。这是因为您在ServicePrinter课程中使用的是Student对象，printDocument中ServicePrinter的实施可能不正确（即如果您确实实施了该{{1}}在超类中）

如何在此应用程序中使用synchronized方法实现互斥

2 个答案: