18.3.2. TCP Client Example

import java.net.*;
import java.io.*;
public class TCPClient {
public static void main(String[] args) throws IOException {
Socket socket = new Socket("localhost", 5000);
PrintWriter out = new PrintWriter(socket.getOutputStream(), true);
BufferedReader in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
out.println("Hello Server!");
System.out.println("Server response: " + in.readLine());
in.close();
out.close();
socket.close();
}
}

This Java code demonstrates a simple TCP client. The client does the following:

1. It establishes a connection to a server running on the same machine at port 5000 using the Socket class.
2. It creates a PrintWriter object to send messages to the server.
3. It creates a BufferedReader object to read responses from the server.
4. The client sends the message 'Hello Server!' to the server.
5. It waits for a response from the server and prints it to the console.
6. Finally, it closes the input and output streams, as well as the socket connection, ensuring no resources are left hanging.

You can think of this TCP client as a customer at a restaurant. The customer (client) sends an order (message) to the waiter (server) and waits for the food (response). Just like the customer gives the order and then patiently waits for their meal, the TCP client sends a message and waits for a reply from the server.

The line of code Socket socket = new Socket("localhost", 5000); establishes a connection to the server. Here, 'localhost' refers to the local machine and '5000' is the port number on which the server is listening.

In this snippet, localhost is a special hostname that refers to the local machine where the client code is running. Port 5000 must match the port that the server is set to listen on. When the client runs this line, it attempts to create a socket connection to the specified server. If the server is not active or not listening on that port, an error will occur.

Imagine trying to call a specific person on a phone (the server); if you dial the wrong number (port), you won't reach the intended person. Similarly, this line of code makes sure to contact the right server on the correct line (port).

The line out.println("Hello Server!"); sends a message to the server using the PrintWriter object.

The PrintWriter object named 'out' allows the client to send text data to the server. Calling println sends the string 'Hello Server!' to the server along with a newline character. This method is blocking, meaning the client will wait until the data is sent before proceeding to the next line of code.

Think of it like sending a postcard. Once you write your message on the postcard (sending the 'Hello Server!' message), you put it in the mailbox and wait for the postal service to handle the delivery.

The line System.out.println("Server response: " + in.readLine()); waits for and displays the response from the server.

This part of the code uses the BufferedReader object named 'in' to read a line of text sent back by the server. The readLine method will block until a message is received from the server, and this message will then be output to the console prefixed by 'Server response: '.

This is similar to waiting for the waiter to come back with your food order. You can't move to the next step (enjoying your meal) until the waiter returns (the server sends a response).

The lines in.close();, out.close();, and socket.close(); are used to properly close the input and output streams and the socket connection.

Closing these resources is critical in network programming because it frees up the system resources associated with the socket connection. Failing to close these connections can lead to memory leaks or exhaustion of available sockets on the machine. By calling these close methods, it ensures that the connection is properly terminated.

Just like you would leave a restaurant after your meal, properly closing the connections ensures that all staff (system resources) are released and prepared for the next customer (client request), keeping everything running smoothly.

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