How to Fix java.net.ConnectException Connection refused connect in Java

java.net.ConnectException: Connection refused:

When software in a client-server architecture requests to establish a TCP connection from the client to the server, the most common type of networking exception in Java is called "connection refused: connect." To solve the communication issue, we must handle the exception with caution. Let's first examine the potential causes of the java.net problem.ConnectException: Refused connection.

Both the client and the server should be connected to a network, such as the internet or a local area network. It will raise an exception on the client side if it is absent.
If the server is not in use. Ports like 8080 (for Tomcat), 3000 or 4200 (for React/Angular), 3306 (for MySQL), and 27017 (for MongoDB) are typically either completely down, indicating the instance hasn't started, or other agents occupy them.
Occasionally, a server may be up and running, but it may not be listening on a port due to overridden settings, etc.
Generally, the Firewall is present for security purposes and will block connectivity if necessary.
Erroneously, the incorrect port or random port generation number is mentioned in the port.
Incorrect connection string information.

Implementation:

FileName: JavaNetConnectionException.java

import java.net.*;
import java.io.*;
import java.util.*;
public class JavaNetConnectionException
{
    public static void main(String[] args) 
    {
      // Since the hostname has been defined, 
      //The localhost value, which is simply 127.0.0.1, is arbitrary.
      String hostname = "127.0.0.1";
      //Here port is defined
      int port = 13;
      // Try block to check for exceptions
      try (Socket socket = new Socket(hostname, port)) {
         // InputStream to read data from socket
         InputStream inputStream = socket.getInputStream();
         InputStreamReader inputStreamReader = new InputStreamReader(inputStream);
         int data;
         StringBuilder outputString = new StringBuilder();
         // Data read from an input stream
         while ((data = inputStreamReader.read()) != -1) {
            outputString.append((char) data);
         }
      } catch (IOException ex) 
      {
         // If the hostname and port number provided are incorrect,
        // communication cannot be created, and an error is raised. 
        // An exception will occur when the socket cannot be reached.
        System.out.println("Connection Refused Exception: " + ex);
      }
   }
}

Output:

How to Fix java.net.ConnectException Connection refused connect in Java

This exception indicates that any service is not using the IP address or port we are attempting to connect to:

The IP, host, or port you are attempting to connect to needs to be corrected.
The server we are using has yet to be started.
There is no one connecting to the server we are using.
The listen delay queue is full on Windows servers.

Let us now examine the methods for resolving the issues with Java.net.ConnectException.

Here, the client and server programs are written.

Implementation:

FileName: TCPClient.java

// TCP Client Side Program
import java.io.*;
import java.net.*;
import java.util.*;
class TCPClient 
{
    public static void main(String argv[]) throws Exception 
    {
        String FromServer;
        String ToServer;
        // establishing the connection of client with the socket
        Socket clientSocket = new Socket("localhost", 8008);
        BufferedReader inFromUser = new BufferedReader(new InputStreamReader( System.in));
        // printing the output using PrinterWriter
        PrintWriter outToServer = new PrintWriter( clientSocket.getOutputStream(), true);
        BufferedReader inFromServer = new BufferedReader(new InputStreamReader( clientSocket.getInputStream()));
	// Closing the connection from the client side
        while (true) 
	{
            FromServer = inFromServer.readLine();
            if (FromServer.equals("c") || FromServer.equals("C")) 
	    {
                clientSocket.close();
                break;
            } 
	    else 
            {
	    // received a message from the client
                System.out.println("Recieved message :" + FromServer);
                System.out.println("Send message (Type C or c to Quit):");
                ToServer = inFromUser.readLine();
                if (ToServer.equals("C") || ToServer.equals("c")) 
		{
                    outToServer.println(ToServer);
                    clientSocket.close();
                    break;
                } 
	    else 
	    {
                    outToServer.println(ToServer);
                }
            }
        }
    }
}

Implementation:

FileName: TCPServer.java

// TCP Server side Program
import java.util.*;
import java.io.*;
import java.net.*;
class   
{
    public static void main(String argv[]) throws Exception 
    {
        String fromclient;
        String toclient;
        // establishing the connection of the server with the socket 
        ServerSocket Server = new ServerSocket(8008);
        System.out.println("TCPServer Waiting for client on port 8008");
        while (true) 
        {
	//Accepting the server connection 
            Socket connected = Server.accept();
            System.out.println(" The Client" + " " + connected.getInetAddress() + ":" + connected.getPort() + " is Connected ");
	// getting the input from the user
            BufferedReader inFromUser = new BufferedReader( new InputStreamReader(System.in));
            BufferedReader inFromClient = new BufferedReader( new InputStreamReader(connected.getInputStream()));
	//Using printer writer gives output
            PrintWriter outToClient = new PrintWriter( connected.getOutputStream(), true);
            while (true) 
            {
                System.out.println("Send Message(Type C or c in order to Quit):");
                toclient = inFromUser.readLine();
	     // to close the connection established with the client
                if (toclient.equals("c") || toclient.equals("C")) 
                {
                    outToClient.println(toclient);
                    connected.close();
                    break;
                } 
	     else 
	     {
                    outToClient.println(toclient);
                }
                fromclient = inFromClient.readLine();
                if (fromclient.equals("c") || fromclient.equals("C")) 
	    {
                    connected.close();
                    break;
                } 
	     else 
	     {
                    System.out.println("Recieved message:" + fromclient);
                }
            }
        }
    }
}

Output: