Thread lock example

Client.java

/*
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 * and open the template in the editor.
 */
package myservice;

/**
 *
 * @author vcamaral
 */
public class Client extends Thread {
    
    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        // TODO code application logic here
        for(int i = 0; i < 12; i += 3) {
            ConnectionThread ct = new ConnectionThread(i);
            ct.start();
        }
    }
}

ConnectionThread.java

/*
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 * and open the template in the editor.
 */
package myservice;

import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.IOException;
import java.net.Socket;
import java.net.UnknownHostException;
import java.util.logging.Level;
import java.util.logging.Logger;

/**
 *
 * @author vcamaral
 */
public class ConnectionThread extends Thread {
    private int currentThread;
    
    public ConnectionThread(int i) {
        this.currentThread = i;
    }
    
    @Override
    public void run() {
        for(int i = this.currentThread; i < this.currentThread + 3; i++) {
            String str = "";

            try {
                Socket s = new Socket("127.0.0.1", 9876);
                DataOutputStream dos = new DataOutputStream(s.getOutputStream());
                DataInputStream dis = new DataInputStream(s.getInputStream());
                dos.writeUTF("Hello from client!\n");
                str = dis.readUTF();
                System.out.println(str);
            } catch (UnknownHostException ex) {
                Logger.getLogger(Client.class.getName()).log(Level.SEVERE, null, ex);
            } catch (IOException ex) {
                Logger.getLogger(Client.class.getName()).log(Level.SEVERE, null, ex);
            }
        }
    }
}

MyService.java

/*
 * To change this template, choose Tools | Templates
 * and open the template in the editor.
 */
package myservice;

import java.io.*;
import java.net.ServerSocket;
import java.net.Socket;

/**
 *
 * @author vcamaral
 */
public class MyService {
    private LockObject lockObj = new LockObject();

    public static String readFile(String fileName) throws FileNotFoundException, IOException {
        File f = new File(fileName);
        FileInputStream fis = new FileInputStream(f);
        int tam = (int) f.length();
        byte b[] = new byte[tam];
        fis.read(b);
        String str = new String(b);

        return str;
    }

    public static void writeFile(String data) {
        try {
            FileWriter fw = new FileWriter("test.txt", true);
            fw.write(data);
            fw.flush();
            fw.close();
        } catch (Exception e) {
            System.out.println(e.getMessage());
        }
    }

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        // TODO code application logic here
        String str = "";

        while (true) {
            try {
                ServerSocket ss = new ServerSocket(9876);
                System.out.println("Server running...");
                Socket s = ss.accept();
                DataOutputStream dos = new DataOutputStream((s.getOutputStream()));
                DataInputStream dis = new DataInputStream(s.getInputStream());
                str = dis.readUTF();
                synchronized(lockObj){
                   writeFile(str);
                   dos.writeUTF(readFile("test.txt"));
                }
            } catch (Exception e) {
                System.out.println(e.getMessage());
            }
        }
    }
}