Java中Semaphore的图书馆员资源分配问题
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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.维基百科上的Semaphore page给出了图书馆对信号量的类比.在第一部分中,我试图模拟这个问题.在我的情况下,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.
我在实施中遇到的问题是关于学生与主题专家的关联.您将如何在以下secnario中执行此操作？ SubjectMatterExpert需要做的就是打印学生ID(现在).
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)的学生班级：
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();
}
}
}
这是修改过的(编辑过的)图书馆员班级：
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Simulator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;
}
}
解决方法:
在您描述的原始图书管理员问题中,问题并不关心哪个学生在哪个房间,因此使用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.一种方法是在图书管理员类上使用2种方法,一种用于请求SME,另一种用于返回它.
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使用哪个SME,那么您需要一个不同的构造来管理资源,信号量已不再足够.一个例子可能是队列.
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);
}
}
}
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