How to Fix java.net.ConnectException Error in Java

Sheeraz Gul Feb 02, 2024

Java Java Error

How to Fix java.net.ConnectException Error in Java

This tutorial demonstrates the java.net.ConnectException error in Java.

the `java.net.ConnectException` Error in Java

The java.net.ConnectException is the most common exception while working with networking. It mostly occurs when making a TCP connection between the client, application, and server.

It is a checked application that can be handled in the code using the try-catch blocks. This exception is a sub-class of the IO exception.

Here is the hierarchy of this exception:

     java.lang.Object
        java.lang.Throwable
            java.lang.Exception
                java.io.IOException
                    java.net.SocketException
                        java.net.ConnectException

Here are the possible reasons for this exception:

When any client and server are not in the same network, both should be in the network like internet or LAN; otherwise, it will throw the ConnectException.
When the server is not running. Usually, the ports like 3306 (MySQL), 8080 (for Tomcat), 27017 (MongoDB), and 3000 or 4200 (for React/Angular) are completely down or occupied by some other entity, and it will throw the ConnectException.
When the security is disallowing communication, for example, in the case of a Firewall. In this case, it will throw the ConnectException.
When the settings are overridden, the server may be running but listing on ports, it will throw the ConnectException.
When the connection string is not correct, for example, Connection DemoConnection = DriverManager.getConnection("jdbc:mysql://localhost/:3306<databasename>?" + "user=<username>&password=<password>");. It will throw the ConnectException.
When the wrong port number is given by mistake, it will throw the ConnectException.

Let’s try an example for ConnectException:

package delftstack;

import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.net.*;
import java.util.*;

public class Example {
  public static void main(String[] args) throws IOException {
    String Host_Name = "127.0.0.1";
    int Port_Number = 808;

    Socket New_Socket = new Socket(Host_Name, Port_Number);

    InputStream Input_Stream = New_Socket.getInputStream();
    InputStreamReader Input_Stream_Reader = new InputStreamReader(Input_Stream);
    int Demo_Data;

    StringBuilder Output_String = new StringBuilder();
    while ((Demo_Data = Input_Stream_Reader.read()) != -1) {
      Output_String.append((char) Demo_Data);
    }
  }
}

The code above will throw the ConnectException because the port and IP are invalid. See output:

Exception in thread "main" java.net.ConnectException: Connection refused: connect
    at java.base/sun.nio.ch.Net.connect0(Native Method)
    at java.base/sun.nio.ch.Net.connect(Net.java:579)
    at java.base/sun.nio.ch.Net.connect(Net.java:568)
    at java.base/sun.nio.ch.NioSocketImpl.connect(NioSocketImpl.java:588)
    at java.base/java.net.SocksSocketImpl.connect(SocksSocketImpl.java:327)
    at java.base/java.net.Socket.connect(Socket.java:633)
    at java.base/java.net.Socket.connect(Socket.java:583)
    at java.base/java.net.Socket.<init>(Socket.java:507)
    at java.base/java.net.Socket.<init>(Socket.java:287)
    at delftstack.Example.main(Example.java:16)

Now, we can handle the exception by using the try-catch blocks. As it is a checked exception, it is necessary to add throws to the method.

Let’s see the solution:

package delftstack;

import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.net.*;
import java.util.*;

public class Example {
  public static void main(String[] args) throws IOException {
    String Host_Name = "127.0.0.1";
    int Port_Number = 808;

    try (Socket New_Socket = new Socket(Host_Name, Port_Number)) {
      InputStream Input_Stream = New_Socket.getInputStream();
      InputStreamReader Input_Stream_Reader = new InputStreamReader(Input_Stream);
      int Demo_Data;

      StringBuilder Output_String = new StringBuilder();
      while ((Demo_Data = Input_Stream_Reader.read()) != -1) {
        Output_String.append((char) Demo_Data);
      }
    }

    catch (ConnectException ex) {
      System.out.println(
          "The Connect Exception  occured becuase the given hostname and port are invalid : "
          + ex.getMessage());
    }
  }
}

Now the exception is handled, so it will not crash the code. See output:

The Connect Exception  occured becuase the given hostname and port are invalid : Connection refused: connect

Author: Sheeraz Gul

Sheeraz is a Doctorate fellow in Computer Science at Northwestern Polytechnical University, Xian, China. He has 7 years of Software Development experience in AI, Web, Database, and Desktop technologies. He writes tutorials in Java, PHP, Python, GoLang, R, etc., to help beginners learn the field of Computer Science.

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the `java.net.ConnectException` Error in Java