Java - Socket connect() Method

Description

The Java Socket connect() method connects this socket to the server.

Declaration

Following is the declaration for java.net.Socket.connect() method.

public void connect(SocketAddress endpoint) throws IOException

Parameters

endpoint − the SocketAddress

Return Value

NA

Exception

IOException − if an error occurs during the connection.

IllegalBlockingModeException − if this socket has an associated channel, and the channel is in non-blocking mode.

IllegalArgumentException − if endpoint is null or is a SocketAddress subclass not supported by this socket.

Example 1

The following example shows the usage of Java Socket connect() methd to connect the socket to localhost at port 6066. As first step, we've created a Socket instance using no argument constructor. Then in order to create a SocketAddress object, we've initialized an InetAddress instance of localhost address. Using InetSocketAddress object, we've created a SocketAddress object and then using connect() method, we're connecting the address to the socket. Once done, we're printing the local port and inetaddress as shown. In the end, we closed the socket using close() method. As localhost server is not up, it throws error.

package com.tutorialspoint;

import java.io.IOException;
import java.net.InetAddress;
import java.net.InetSocketAddress;
import java.net.Socket;
import java.net.SocketAddress;

public class SocketDemo {
   public static void main(String[] args) throws IOException {
      Socket socket = new Socket();  
      InetAddress inetAddress=InetAddress.getByName("localhost");  
      SocketAddress socketAddress=new InetSocketAddress(inetAddress, 6066);  
      socket.connect(socketAddress); 

      System.out.println("Port number: "+socket.getLocalPort()); 
      System.out.println("Inet Address: "+socket.getInetAddress()); 
      socket.close();
   }
}

Output

Let us compile and run the above program, this will produce the following result −

Exception in thread "main" java.net.ConnectException: Connection refused: connect
	at java.base/java.net.PlainSocketImpl.connect0(Native Method)
	at java.base/java.net.PlainSocketImpl.socketConnect(PlainSocketImpl.java:101)
	at java.base/java.net.AbstractPlainSocketImpl.doConnect(AbstractPlainSocketImpl.java:412)
	at java.base/java.net.AbstractPlainSocketImpl.connectToAddress(AbstractPlainSocketImpl.java:255)
	at java.base/java.net.AbstractPlainSocketImpl.connect(AbstractPlainSocketImpl.java:237)
	at java.base/java.net.SocksSocketImpl.connect(SocksSocketImpl.java:392)
	at java.base/java.net.Socket.connect(Socket.java:608)
	at java.base/java.net.Socket.connect(Socket.java:557)
	at com.tutorialspoint.SocketDemo.main(SocketDemo.java:14)

Example 2

The following example shows the usage of Java Socket connect() methd to bind the socket to localhost at port 6066. As first step, we've created a Socket instance using no argument constructor. Now using connect() method, we're trying to connect to a null address to the socket. Once done, we're printing the local port being assigned to the socket as default and inetaddress as shown. In the end, we closed the socket using close() method. As address is null, connect method throws error.

package com.tutorialspoint;

import java.io.IOException;
import java.net.Socket;

public class SocketDemo {
   public static void main(String[] args) throws IOException {
      Socket socket = new Socket();  
      socket.connect(null); 

      System.out.println("Port number: "+socket.getLocalPort()); 
      System.out.println("Inet Address: "+socket.getInetAddress()); 
      socket.close();
   }
}

Output

Let us compile and run the above program, this will produce the following result −

Exception in thread "main" java.lang.IllegalArgumentException: connect: The address can't be null
	at java.base/java.net.Socket.connect(Socket.java:579)
	at java.base/java.net.Socket.connect(Socket.java:557)
	at com.tutorialspoint.SocketDemo.main(SocketDemo.java:9)

Example 3

The following example shows the usage of Java Socket connect() methd to bind the socket to localhost at port 6066 if socket is already closed. As first step, we've created a Socket instance using no argument constructor. Then in order to create a SocketAddress object, we've initialized an InetAddress instance of localhost address. Using InetSocketAddress object, we've created a SocketAddress object. Using socket.close(), we're closing the socket and then using connect() method, we connect the address to the socket as shown below −

package com.tutorialspoint;

import java.io.IOException;
import java.net.InetAddress;
import java.net.InetSocketAddress;
import java.net.Socket;
import java.net.SocketAddress;

public class SocketDemo {
   public static void main(String[] args) throws IOException {
      Socket socket = new Socket();  
      InetAddress inetAddress=InetAddress.getByName("localhost");  
      SocketAddress socketAddress=new InetSocketAddress(inetAddress, 6066);
      socket.close();	  
      socket.connect(socketAddress); 

      System.out.println("Port number: "+socket.getLocalPort()); 
      System.out.println("Inet Address: "+socket.getInetAddress()); 
      socket.close();
   }
}

Output

Let us compile and run the above program, this will produce the following result −

Exception in thread "main" java.net.SocketException: Socket is closed
	at java.base/java.net.Socket.connect(Socket.java:585)
	at java.base/java.net.Socket.connect(Socket.java:557)
	at com.tutorialspoint.SocketDemo.main(SocketDemo.java:15)