semaphore_task_.idea_artifacts_semaphore_task_jar.xml

<component name="ArtifactManager">
  <artifact type="jar" name="semaphore_task:jar">
    <output-path>$PROJECT_DIR$/out/artifacts/semaphore_task_jar</output-path>
    <root id="archive" name="semaphore_task.jar">
      <element id="module-output" name="semaphore_task" />
    </root>
  </artifact>
</component>

semaphore_task_.idea_misc.xml

<?xml version="1.0" encoding="UTF-8"?>
<project version="4">
  <component name="ProjectRootManager" version="2" languageLevel="JDK_1_8" default="true" project-jdk-name="1.8" project-jdk-type="JavaSDK">
    <output url="file://$PROJECT_DIR$/out" />
  </component>
</project>

semaphore_task_.idea_modules.xml

<?xml version="1.0" encoding="UTF-8"?>
<project version="4">
  <component name="ProjectModuleManager">
    <modules>
      <module fileurl="file://$PROJECT_DIR$/semaphore_task.iml" filepath="$PROJECT_DIR$/semaphore_task.iml" />
    </modules>
  </component>
</project>

semaphore_task_semaphore_task.iml

<?xml version="1.0" encoding="UTF-8"?>
<module type="JAVA_MODULE" version="4">
  <component name="NewModuleRootManager" inherit-compiler-output="true">
    <exclude-output />
    <content url="file://$MODULE_DIR$">
      <sourceFolder url="file://$MODULE_DIR$/src" isTestSource="false" />
    </content>
    <orderEntry type="inheritedJdk" />
    <orderEntry type="sourceFolder" forTests="false" />
  </component>
</module>

semaphore_task_src_Book.java

public class Book {

    private String title;

    public Book(String title) {
        this.title = title;
    }

    public String getTitle() {
        return title;
    }

    public void setTitle(String title) {
        this.title = title;
    }
}

semaphore_task_src_LibraryClient.java

import java.util.ArrayList;
import java.util.EmptyStackException;
import java.util.Stack;
import java.util.concurrent.Semaphore;

public class LibraryClient {

    private static ArrayList<Reader> readers = new ArrayList<>();
    private static ArrayList<Thread> threads = new ArrayList<>();
    private static Stack<Book> books = new Stack<>();
    private static final int CLASS_COUNT = 150;

    public static void main(String[] args) {

        Semaphore semaphore = new Semaphore(4);

        books.push(new Book("The Hound of the Baskervilles"));
        books.push(new Book("The Da Vinci Code"));
        books.push(new Book("The Alchemist"));
        books.push(new Book("Harry Potters and The Philosopher's Stone"));

        for (int i = 1; i < CLASS_COUNT; i++) {
            readers.add(new Reader(i, semaphore));
        }

        try {
            for (Reader reader : readers) {
                Thread thread = new Thread(reader);
                threads.add(thread);
                thread.start();
                Thread.sleep(250);
            }

            for (Thread thread : threads)
                thread.join();

        } catch (InterruptedException exception) {
            exception.printStackTrace();
        }
    }

    public static Book popBookFromLibrary() {

        Book book = null;

        try {
            book = books.pop();
        } catch (EmptyStackException exception) {
            System.out.println("There's no book left in the library!");
        }

        return book;
    }

    public static void pushBookToLibrary(Book book) {

        books.push(book);
    }
}

semaphore_task_src_META-INF_MANIFEST.MF

Manifest-Version: 1.0
Main-Class: LibraryClient

semaphore_task_src_Reader.java

import java.util.concurrent.Semaphore;

public class Reader implements Runnable {

    int id;
    Semaphore semaphore;

    public Reader(int id, Semaphore semaphore) {
        this.id = id;
        this.semaphore = semaphore;
    }

    @Override
    public void run() {

        Book book = null;

        try {
            System.out.println("Student #" + id + " is waiting to enter the library...");
            semaphore.acquire();

            book = LibraryClient.popBookFromLibrary();
            System.out.println("Student #" + id + " has started reading " + book.getTitle() + ".");

            for (int i = 0; i < 3; i++) {
                Thread.sleep(1000);
                System.out.println("Student #" + id + " is reading " + book.getTitle() + " at the moment...");
            }

            System.out.println("Student #" + id + " has finished reading.");

        } catch (InterruptedException exception) {
            exception.printStackTrace();
        }

        System.out.println("Student #" + id + " is leaving...");
        LibraryClient.pushBookToLibrary(book);

        semaphore.release();
    }
}