TCP/IP Client Sockets, URL, and URLConnection

TCP/IP Client Sockets

A socket is one endpoint of a two-way network connection. Java's Socket class implements a client socket — it connects to a server listening on a host and port.

import java.io.*;
import java.net.Socket;

public class TCPClient {
    public static void main(String[] args) throws IOException {
        // Connect to server at host:port
        Socket socket = new Socket("localhost", 5000);

        // Output stream: send data to the server
        PrintWriter out = new PrintWriter(socket.getOutputStream(), true);
        out.println("Hello Server!");

        // Input stream: read the server's reply
        BufferedReader in = new BufferedReader(
            new InputStreamReader(socket.getInputStream()));
        System.out.println("Server says: " + in.readLine());

        socket.close();
    }
}

Key Socket Methods

The URL Class

A URL (Uniform Resource Locator) identifies a resource on the Internet. Java's URL class parses and lets you fetch the resource it points to.

import java.net.URL;

public class URLDemo {
    public static void main(String[] args) throws Exception {
        URL url = new URL("https://www.example.com:443/path/page.html?id=1");

        System.out.println("Protocol: " + url.getProtocol()); // https
        System.out.println("Host:     " + url.getHost());     // www.example.com
        System.out.println("Port:     " + url.getPort());     // 443
        System.out.println("File:     " + url.getFile());     // /path/page.html?id=1
        System.out.println("Path:     " + url.getPath());     // /path/page.html
        System.out.println("Query:    " + url.getQuery());    // id=1
    }
}

Reading a URL Directly

import java.io.*;
import java.net.URL;

public class URLReader {
    public static void main(String[] args) throws Exception {
        URL url = new URL("https://www.example.com");
        BufferedReader in = new BufferedReader(
            new InputStreamReader(url.openStream()));

        String line;
        while ((line = in.readLine()) != null) {
            System.out.println(line);
        }
        in.close();
    }
}

The URLConnection Class

URLConnection gives finer control than openStream() — request headers, response headers, and both reading and writing.

import java.io.*;
import java.net.URL;
import java.net.URLConnection;

public class URLConnectionDemo {
    public static void main(String[] args) throws Exception {
        URL url = new URL("https://www.example.com");
        URLConnection conn = url.openConnection();

        conn.setRequestProperty("User-Agent", "JavaClient/1.0");
        conn.setDoOutput(false);
        conn.connect();

        System.out.println("Content-Type:   " + conn.getContentType());
        System.out.println("Content-Length: " + conn.getContentLength());
        System.out.println("Last Modified:  " + conn.getLastModified());

        BufferedReader in = new BufferedReader(
            new InputStreamReader(conn.getInputStream()));
        String line;
        while ((line = in.readLine()) != null) {
            System.out.println(line);
        }
        in.close();
    }
}

URL vs URLConnection

Key Takeaway: Socket gives a raw TCP client connection for custom protocols. URL and URLConnection build on sockets to fetch web resources — use URL.openStream() for quick reads and URLConnection when you need headers or to send data.