Semaphores in Java Example

Semaphores in java handles permits to execute the next lines of instructions. They are often use to restrict the number of threads to access a resource.

Before getting a resource the thread must acquire a permit form a semaphore and release the resource after using the shared resource which can be memory, storage or data.

Constructors

Semaphore ( i n t   permits )
Creates a Semaphore  with  the given  number of  permits and no fair fairness setting .
Semaphore ( i n t   permits ,   boolean   fair )
Creates a Semaphore  with  the given  number of  permits and the given  fairness setting .

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Now for a sample semaphore program that counts until 20 using two threads that uses semaphores for data consistency.

Counter.java

import java.util.concurrent.Semaphore; import javax.swing.JTextArea; public class Counter extends Thread{ private static int count=0; private String ID; private JTextArea TBox; private Semaphore sem; public ServiceQueue(String name,JTextArea jArea, Semaphore s){ ID=name; TBox=jArea; sem=s; } private void nonCrit(int i){ System.out.println(ID+" is not in critical: "+i); } private synchronized void doCrit(){ count++; TBox.append(ID+" gave count: "+count+"\n"); } public void run(){ for(int ctr=0;ctr<10;ctr++){ nonCrit(ctr); try{ sem.acquire(); } catch (InterruptedException e){ } doCrit(); sem.release(); } } }

Executioner.java

import javax.swing.*; import java.util.concurrent.Semaphore; import java.awt.*; public class Executioner extends JFrame{ public Executioner(){ super("Hello World"); JTextArea TA=new JTextArea(20,15); Semaphore sem = new Semaphore(3, true); setLayout(new FlowLayout()); add(TA); Counter Q1=new ServiceQueue("1st Class",TA,sem); Counter Q2=new ServiceQueue("3rd Class",TA,sem); setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); setVisible(true); C1.start(); C2.start(); pack(); } public static void main(String args[]){ new Executioner(); } }