Develop Reference

How to run two Java files in VSCODE?

I'm trying to make a basic java echo client server app and the textbook I'm reading says I should run the Server.java file first and then the Client.java second. But for some reason VSCode doesn't seem to be doing that. I run my Server.java file and get this which is what I'm expecting:
Simple Echo Server
Waiting for connection.....
And then I go to my Client.java file and run that, but nothing happens there are no errors, it stays at the two lines shown above, I can CTRL+C to terminate the batch job.
I'm expecting it to say this:
Simple Echo Server
Waiting for connection.....
Connected to client
But that's not happening - I am getting no errors though. I don't think it's a problem with my code since it's identical to the textbook's but I'll post it here.
Server.java
import java.io.*;
import java.net.ServerSocket;
import java.net.Socket;
public class Server {
public static void main(String[] args) throws Exception {
System.out.println("Simple Echo Server");
try (ServerSocket serverSocket = new ServerSocket(6000)) {
System.out.println("Waiting for connection.....");
Socket clientSocket = serverSocket.accept();
System.out.println("Connected to client");
} catch (IOException ex) {
}
}
}
Client.java
import java.io.*;
import java.io.BufferedReader;
import java.io.PrintWriter;
import java.net.InetAddress;
import java.net.Socket;
public class Client {
public static void main(String[] args) throws Exception {
try {
System.out.println("Waiting for connection.....");
InetAddress localAddress = InetAddress.getLocalHost();
try (Socket clientSocket = new Socket(localAddress, 6000);
PrintWriter out = new PrintWriter(clientSocket.getOutputStream(), true);
BufferedReader br = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()))) {
}
}
catch (IOException ex) {
} // Handle exceptions
}
}
Is it possible that VS code can't run two java files at one time?
EDIT Tried the dual configuration below, but the result is the same, nothing is changing.

It's achievable in VS Code.
Click to create launch.json, keep the default configurations which should be similar to mine in the following picture then add compounds in it:
Turn to the selection box and choose compounds to run by clicking the left green triangle button, you'll get your wanted result:

Running a live Java Server on Amazon AWS

I am developing a Client-Server application with several other programmers, in Java. At this point in time I do not want to be running the code locally. I want to be able to connect to the Server from any machine.
I wrote a test server and test client, just to make sure that things are working properly. But they are not. I am using Amazon AWS EC2 Linux that comes with Java. I am able to compile and run my Server after I SSH into the EC2, but the Client on my local disk is just not connecting. Here is the code.
// Code found online (https://cs.lmu.edu/~ray/notes/javanetexamples/)
import java.io.IOException;
import java.io.PrintWriter;
import java.net.ServerSocket;
import java.net.Socket;
import java.util.Scanner;
import java.util.concurrent.Executors;
import java.util.concurrent.ExecutorService;
public class TestServer {
public static void main(String[] args) throws Exception {
try (ServerSocket listener = new ServerSocket(50000)) {
System.out.println("The capitalization server is running...");
System.out.println(listener.getInetAddress());
ExecutorService pool = Executors.newFixedThreadPool(20);
while (true) {
pool.execute(new Capitalizer(listener.accept()));
}
}
}
private static class Capitalizer implements Runnable {
private Socket socket;
Capitalizer(Socket socket) {
this.socket = socket;
}
#Override
public void run() {
System.out.println("Connected: " + socket);
try {
Scanner in = new Scanner(socket.getInputStream());
PrintWriter out = new PrintWriter(socket.getOutputStream(), true);
while (in.hasNextLine()) {
out.println(in.nextLine().toUpperCase());
}
} catch (Exception e) {
System.out.println("Error:" + socket);
} finally {
try { socket.close(); } catch (IOException e) {}
System.out.println("Closed: " + socket);
}
}
}
}
// Code found online (https://cs.lmu.edu/~ray/notes/javanetexamples/)
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.net.Socket;
import java.util.Scanner;
public class TestClient {
public static void main(String[] args) throws Exception {
try (Socket socket = new Socket("ADDRESS HERE", 50000)) {
System.out.println("Enter lines of text then Ctrl+D or Ctrl+C to quit");
Scanner scanner = new Scanner(System.in);
Scanner in = new Scanner(socket.getInputStream());
PrintWriter out = new PrintWriter(socket.getOutputStream(), true);
while (scanner.hasNextLine()) {
out.println(scanner.nextLine());
System.out.println(in.nextLine());
}
}
}
}
In place of "ADDRESS HERE" in the Client, I have tried the private IP and public IP of my Amazon EC2 instance. I have also tried the public DNS name. Nothing seems to work. There is just no connection from the Client to the Server. In fact, "Enter lines of text then Ctrl+D or Ctrl+C to quit" never prints.
All help is appreciated. Thank you.

Allow your IP address to send request to the EC2. For this, you need to go to your Security Group and add your IP there. Follow these steps-
GO to your AWS console.
Click on EC2, then under Resources you will find Security Groups.
Select your security group.
Follow the steps in the given image.

Since you're able to connect to EC2 instance via SSH, your Security Group allows this.
Now you need to allow requests from the client in this Security Group. You will either need to provide a concrete IP, IP range or allow all IPs (not recommended) in the group.
You can find how to do this here.

Redirect Client in Java-Run Server

I am creating a Java HTTP server that checks to make sure a client is not banned before redirecting to the main server. I have already created everything for the server that is needed, I just don't know how to redirect to another port that is running the main server. Here is my code:
package netlyaccesscontrol;
import java.io.BufferedReader;
import java.io.FileNotFoundException;
import java.io.FileReader;
import java.io.IOException;
import java.net.ServerSocket;
import java.net.Socket;
public class AllowedCheck {
public static void main(String[] args) {
String line = null;
try {
FileReader reader = new FileReader("Banned.txt");
BufferedReader buffer = new BufferedReader(reader);
ServerSocket s = new ServerSocket(80);
Socket c = s.accept();
String clientIP = c.getInetAddress().toString();
while ((line = buffer.readLine()) != null) {
if (clientIP == line) {
s.close();
} else {
// redirect to main server here
}
}
} catch (FileNotFoundException ex) {
System.out.println("The banned IP address file does not exist.");
} catch (IOException ex) {
ex.printStackTrace();
}
}
}

The redirection that you are thinking of is something supported by HTTP and the browsers. There's a specific HTTP response code that tells the caller to redirect and a way to specify it.
Raw sockets are a low-level network protocol that is not going to support redirection as you expect. The most you might be able to do is have this program be a proxy and, upon success, push all incoming data/outgoing responses to/from the ultimate server. But what you have here is by no means going to cut it.

Java socket test returning null value

I'm trying to make a simple server and client program in Eclipse using java but whenever I run the program both consoles output null. Im not sure why this is happening. I read that a common problem is that I've already created an instance of the server and am trying to create another instance but I'm sure thats not the problem. I also read that I might not have root access and need to use a port that is higher than 1024, so I did and I still have the same error.
Server code:
import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.net.ServerSocket;
import java.net.Socket;
public class Socket_Server_Side {
public static void main(String [] args) throws Exception {
Socket_Server_Side server = new Socket_Server_Side();
server.run();
}
public void run() throws Exception {
ServerSocket SvrSocket = new ServerSocket(1025);
SvrSocket.setReuseAddress(true);
Socket socket = SvrSocket.accept();
InputStreamReader Ir = new InputStreamReader(socket.getInputStream());
BufferedReader Br = new BufferedReader(Ir);
String message = Br.readLine();
System.out.println(message);
if (message != null) {
System.out.println("Message succesfully sent and recieved!");
}
}
}
Client Code:
import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.PrintStream;
import java.net.Socket;
public class Socket_Client_Side {
public static void main(String [] args) throws Exception{
Socket_Client_Side client = new Socket_Client_Side();
client.run();
}
public void run() throws Exception {
Socket socket2 = new Socket("localhost", 1025);
PrintStream Ps = new PrintStream(socket2.getOutputStream());
Ps.println("Hello Server");
InputStreamReader Ir = new InputStreamReader(socket2.getInputStream());
BufferedReader Br = new BufferedReader(Ir);
String message = Br.readLine();
System.out.println(message);
}
}

From what I can tell, your code works fine. The reason that your client is printing null is due to it not receiving any input itself.
Let's describe the scenario in which you're communicating at a high level.
You have two programs; the server and the client. The server is actively listening for any input to it, and the client sends input to the server. This process is (so far) one-way, and the way we have the code structured, we are only ever writing anything to the server.
Now, you have two channels of communication with a socket: input and output. Recall that we are sending data across to the server, and the server isn't sending any data to the client. This means:
The server should only care about input, and not anything to do with output.
The client should only care about output, and not anything to do with input.
That said, this code in your client is superfluous:
InputStreamReader Ir = new InputStreamReader(socket2.getInputStream());
BufferedReader Br = new BufferedReader(Ir);
String message = Br.readLine();
System.out.println(message);
The reason for this is that your client does not have any input coming in to it, so any messages that did suddenly show up would be a strange thing indeed.

Java Socket Server for Game Login/Authentication

I am currently working on a Java Application using the classes under java.net.* for a MMORPG I am creating. It'll be connected with a Unity3D Game (using .NET Sockets for TCP). I am unsure how to handle the login/authentication of players.
Suggestions? I was thinking about handling authentication this via a secure form over https, creating and storing a temporary login key in the database, and then sending the key back to the client, allotting them a minute to connect to a game server using the randomly generated key. Is this a secure and reliable solution, or is there something better I can do?
Server Class
import java.io.IOException;
import java.net.InetAddress;
import java.net.ServerSocket;
import java.net.Socket;
import java.util.ArrayList;
public class Server extends Thread {
public static final int MAX_CLIENTS = 500;
private ServerSocket listener;
protected ArrayList<Client> clients;
public Server(int listenPort) throws IOException {
listener = new ServerSocket(
listenPort, MAX_CLIENTS,
InetAddress.getLocalHost()
);
clients = new ArrayList<Client>();
}
#Override
public void run() {
while (true) {
try {
Socket socket = listener.accept();
Client client = new Client(socket, this);
clients.add(client);
new Thread(client).start();
} catch (IOException e) {}
}
}
public static void main(String[] args) throws IOException {
new Server(4428).start();
}
}
Client Class
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.net.Socket;
public class Client implements Runnable {
private Socket socket;
private Server server;
private BufferedReader in;
private PrintWriter out;
public Client(Socket sock, Server serv) {
socket = sock;
server = serv;
in = null;
out = null;
}
#Override
public void run() {
try {
in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
out = new PrintWriter(socket.getOutputStream(), true);
while (true) {
// Read Data with read()
}
} catch (IOException e) {}
}
public void read() throws IOException {
}
public void send(String data) {
}
}

I would recommend using HTTPS for login and for important account actions (buy/sell/transfer items). When the client logins to the server over HTTPS the server should generate a long (64-128 bytes) random session id that is stored in the database. Also generate a temporary one-time authentication token (long random value) for the client's TCP connection.
The reason for using HTTPS is to help stop MITM attacks from stealing the users in game assets.
If the traffic between client & server is low you could use a TLS socket instead of a normal TCP socket. Just make sure to have the client validate(Signed by a trusted CA and that the domain matches) the server's certificate.
Even if you use HTTPS for important actions a MITM attack could still mess with the users actions (moving, attacking, chat) or change what the user sees (hide monsters or other users).