java - Java中Semaphore的图书管理员资源分配问题

Question

请帮我解决这个由两部分组成的问题。这是第一部分:

(Part 2: I have updated the code since - requirements have changed a bit.)

我正在尝试用 Java 实现 Librarian 问题。 Semaphore page在维基百科上给出了信号量的图书馆类比。在第一部分，我试图模拟这个问题。就我而言，我使用 [主题专家] 而不是房间作为资源。

Suppose a library has 10 identical study rooms, intended to be used by one student at a time. To prevent disputes, students must request a room from the front counter if they wish to make use of a study room. When a student has finished using a room, the student must return to the counter and indicate that one room has become free. If no rooms are free, students wait at the counter until someone relinquishes a room.

Since the rooms are identical, the librarian at the front desk does not keep track of which room is occupied, only the number of free rooms available. When a student requests a room, the librarian decreases this number. When a student releases a room, the librarian increases this number. Once access to a room is granted, the room can be used for as long as desired, and so it is not possible to book rooms ahead of time.

我在实现过程中面临的问题是关于学生与主题专家的关联。在下面的场景中你会怎么做？ SubjectMatterExpert 需要做的就是打印学生 ID(目前)。

Part 2: New requirements:
- There are fixed number of Students, SMEs, and Book Closets
- Students have certain number of Books at the beginning (presently, books are just numbers)
- SMEs add or check out books from the Boook Closet at a Student's request
- Students specify add or check out action, number of books, and the Book Closet

这是修改(编辑)的学生类:

package librarysimulation;

public class Student extends Thread {

    String studentId = "";
    Librarian librarian = null;
    int bookCount = 0;

    public Student(String id, Librarian lib, int book) {
        studentId = id;
        librarian = lib;
        bookCount = book;
    }

    @Override
    public void run() {

        System.out.println("Student " + studentId + " is requesting SME...");
        librarian.requestSME();

        try {
            // Do something
            System.out.println("Student " + studentId + " has access to an SME.");
            //How do I ask the SME to add OR checkOut 'x' number of books
            //from a given BookCloset?
        } finally {
            librarian.releaseSME();
        }
    }
}

这是修改(编辑)的 Librarian 类:

package librarysimulation;

import java.util.concurrent.Semaphore;
import java.util.logging.Level;
import java.util.logging.Logger;

public class Librarian {

    public Semaphore sme;
    public int bookClosetCount = 0;

    public Librarian(int smeCount, int bookCloset) {
        sme = new Semaphore(smeCount, true);
        bookClosetCount = bookCloset;
        //openLibrary(smeCount);
    }

    //Receive SME request from the Student here
    public void requestSME() {
        try {
            sme.acquire();
            //assign student to SME
        } catch (InterruptedException ex) {
            Logger.getLogger(Librarian.class.getName()).log(Level.SEVERE, null, ex);
        }
    }

    //Release SME from the Student here
    public void releaseSME() {
        sme.release();//release SME
    }

    //Set the SME threads active (from constructor)
    //i.e., when the library opens, have the SMEs ready
    public final void openLibrary(int roomCount) {
        for (int i = 0; i < roomCount; i++) {
            SubjectMatterExpert s = new SubjectMatterExpert(String.valueOf(i));
            s.start();
        }
    }
}

这是修改(编辑)的主题专家类:

package librarysimulation;

public class SubjectMatterExpert extends Thread {
    String smeId = "";
    SubjectMatterExpert(String id) {
        smeId = id;
    }

    @Override
    public void run(){

        //Handle Student request
        //Students specify if they are checking out books or returning books
        //Students specify number of books
        //Students specify which closet

        //SME simply executes the method from the Book Closet instance
    }
}

这是修改(编辑)的模拟器类:

package librarysimulation;

public class Simulator extends Thread {

    public static final int STUDENT_COUNT = 50;
    public static final int SME_COUNT = 3;
    public static final int BOOKCLOSET_COUNT = 10;
    public static final int BOOK_PER_STUDENT_COUNT = 10;

    @Override
    public void run() {
        //Instantiate Library//New library with 3 SMEs
        Librarian lib = new Librarian(SME_COUNT, BOOKCLOSET_COUNT);
        //Create students
        int i = 0;
        while (i < STUDENT_COUNT) {
            Student s = new Student(String.valueOf(i), lib, BOOK_PER_STUDENT_COUNT);
            s.start();
            i++;
        }
    }

    public static void main(String[] args) {
        Simulator s = new Simulator();
        s.start();
    }
}

这是(新)Book Closet 类:

package librarysimulation;

public class BookCloset {

    int closetId;
    int bookCount = 0;

    public BookCloset(int id, int book) {
        closetId = id;
        bookCount = book;
    }

    public int addBook(int book){
        return bookCount + book;
    }

    public int checkOutBook(int book){
        int finalBookCount = bookCount - book;
        //Change book count iff it makes sense to do so
        if(finalBookCount >= 0)
            bookCount = finalBookCount;
        //If return value is -ve, handle accordingly
        return finalBookCount;
    }
}

Answer 1

在您描述的原始图书管理员问题中，该问题不关心哪个学生在哪个房间，因此使用一个简单的线程安全计数器(即信号量)来实现对资源的控制。根据对问题的描述，仍然需要更改您的实现。一种方法是图书馆员类上的 2 个方法，一个用于请求 SME，另一个用于返回它。

class Librarian {
    Semaphore sme = new Semaphore(NUMBER_OF_SMES);

    void requestSme() throws InterruptedException {
        sme.acquire();
    }

    void releaseSme() {
        sme.release();
    }
}

 class Student {
     Librarian librarian;

     public void run() {

         libarian.requestSme();
         try {
             // Do something
         finally {
             librarian.releaseSme();
         }
     }
}

但是，如果您确实需要知道哪个学生正在与哪个 SME 一起工作，那么您需要一个不同的结构来管理资源，信号量不再足够。一个示例可以是队列。

class Librarian {
    BlockingQueue<SubjectMatterExpert> q = 
        new ArrayBlockingQueue<SubjectMatterExpert>(NUMBER_OF_SMES);

    public Librarian() {
        for (int i = 0; i < NUMBER_OF_SMES; i++)
            q.put(new SubjectMatterExpert(String.valueOf(i));
    } 

    SubjectMatterExport requestSme() throws InterruptedException {
        q.take();
    }

    void releaseSme(SubjectMatterExpert toRelease) {
        q.put(toRelease);
    }
}

 class Student {
     Librarian librarian;

     public void run() {

         SubjectMatterExpert sme = libarian.requestSme();
         try {
             System.out.println("Student: " + this + ", SME: " sme);
         finally {
             if (sme != null)
                 librarian.releaseSme(sme);
         }
     }
}