Understanding Java URL and URLConnection

Java's URL class represents a resource identified by a uniform resource locator, e.g., http://baidu.com. It can be used to retrieve the content of a web page.

Example code shows how to create a URL object from the first command‑line argument, open an input stream, and print the page content.

import java.net.*;
import java.io.*;
public class GetINt {
    public static void main(String[] args) throws Exception {
        URL url = new URL(args[0]);
        InputStream html = url.openStream();
        int c;
        do {
            c = html.read();
            if (c != -1) {
                System.out.print((char) c);
            }
        } while (c != -1);
    }
}

For more control over HTTP transactions, URL.openConnection() returns a URLConnection object. When the protocol is HTTP, the returned object is an instance of HttpURLConnection, which allows setting request method, headers, and handling input/output streams.

import java.net.*;
import java.io.*;
public class Handin {
    public static void main(String[] args) throws Exception {
        URL url = new URL("http://example.com");
        HttpURLConnection con = (HttpURLConnection) url.openConnection();
        con.setDoInput(true);
        con.setDoOutput(true);
        con.setRequestMethod("POST");
        con.setRequestProperty("Content-type", "application/X-WWW-form-urlencoded");
        con.connect();
        PrintWriter pri = new PrintWriter(new OutputStreamWriter(con.getOutputStream(), "UTF-8"));
        pri.print("data=" + URLEncoder.encode(args[0], "UTF-8"));
        pri.flush();
        System.out.println(con.getResponseMessage());
        InputStream in = con.getInputStream();
        int c;
        while ((c = in.read()) != -1) {
            System.out.print((char) c);
        }
    }
}

The URL and URLConnection classes delegate the actual work to URLStreamHandler and URLConnection subclasses. A URLStreamHandlerFactory creates protocol‑specific handlers, which in turn produce appropriate URLConnection objects.

import java.net.*;
import java.io.*;
public class TimeURLConnection extends URLConnection {
    private Socket con;
    public static final int DEFAULT_PORT = 13;
    public void connect() throws IOException {
        if (!connected) {
            int port = url.getPort();
            if (port < 0) port = DEFAULT_PORT;
            con = new Socket(url.getHost(), port);
            connected = true;
        }
    }
    public InputStream getInputStream() throws IOException {
        connect();
        return con.getInputStream();
    }
}

In summary, the URL API provides a simple way to access web resources, while URLConnection and its subclasses give fine‑grained control over HTTP communication.