如何在此应用程序中使用同步方法实现互斥

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。但当时只有一个可以打印（很明显）5 个文件。当他们完成时，他们通知其他人他们已经完成并且其他线程访问该资源。我有一个主类，学生类和监视器（激光打印机），文档类，其中包含有关文档的信息，例如（页数，名称用户 ID 等）+ 一些打印机接口。我已成功运行线程，但它们不同步（互斥）
所以问题是我如何实现互斥（当时只有一个人可以打印他的文档数量）
感谢您的关注、时间和提示 :)

主类

    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");

监控类

    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

您似乎在 run 方法中创建了一个本地打印机对象，而不是使用您传递给 Student 类的共享打印机对象。尝试使用您传递的共享打印机，看看您会得到什么。我们还需要看看你如何在ServicePrinter 中使用printDocument。这是因为您在Student 类中使用了ServicePrinter 对象，而printDocument 在ServicePrinter 中的实现可能不正确（也就是说，如果您实际上在超类中实现了它）

Answer 2

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

编辑：将互斥量更改为信号量（更有意义）

一个简单的互斥锁实现：

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();            
        }
    }
}