Understanding URLs in Java: Communicating with Web Resources

Learn how to work with URLs in Java, a crucial part of web communication. This guide covers URL structure, syntax, and examples for accessing online resources, with explanations on using Java classes for handling and parsing URLs effectively.

Understanding URLs in Java

A URL (Uniform Resource Locator) represents a resource on the World Wide Web, such as a web page or an FTP directory. This section explains how to write Java programs that communicate with a URL. A URL can be divided into parts:

URL Structure

protocol://host:port/path?query#ref

Examples of protocols include HTTP, HTTPS, FTP, and File. The path is often referred to as the filename, while the host is also known as the authority.

Example URL

Here is an example of a URL with the HTTP protocol:

https://www.example.com/index.htm?language=en#section1

Note that this URL does not specify a port. In such cases, the default port for the protocol is used. For HTTP, the default port is 80.

Constructors of the URL Class

The java.net.URL class represents a URL and provides various methods to manipulate URLs in Java. Below are some of its constructors:

Methods of the URL Class

The URL class contains several methods that help in accessing different parts of a URL. Below are some of the key methods:

Example: URLDemo Program

The following URLDemo program demonstrates the various parts of a URL. The user inputs a URL from the command line, and the program outputs each part of the given URL.

import java.io.IOException;
import java.net.URL;

public class URLDemo {
public static void main(String [] args) {
try {
    URL url = new URL("https://www.example.com/");
    
    System.out.println("URL is " + url.toString());
    System.out.println("protocol is " + url.getProtocol());
    System.out.println("authority is " + url.getAuthority());
    System.out.println("file name is " + url.getFile());
    System.out.println("host is " + url.getHost());
    System.out.println("path is " + url.getPath());
    System.out.println("port is " + url.getPort());
    System.out.println("default port is " + url.getDefaultPort());
    System.out.println("query is " + url.getQuery());
    System.out.println("ref is " + url.getRef());
} catch (IOException e) {
    e.printStackTrace();
}
}
}

URL is https://www.example.com/
protocol is https
authority is www.example.com
file name is /index.htm
host is www.example.com
path is /index.htm
port is -1
default port is 443
query is null
ref is null

URLConnections Class Methods

The openConnection() method returns a java.net.URLConnection, an abstract class whose subclasses represent different types of URL connections. For example:

• If the protocol is HTTP, openConnection() returns an HttpURLConnection object.
• If the URL represents a JAR file, openConnection() returns a JarURLConnection object.

Methods in URLConnection

The URLConnection class has numerous methods that allow for the manipulation and retrieval of data related to the URL connection. Below is a list of essential methods:

Example: URLConnectionDemo Program

The following URLConnectionDemo program connects to a URL entered from the command line. If the URL represents an HTTP resource, the connection is cast to HttpURLConnection, and the data in the resource is read line by line.

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.net.HttpURLConnection;
import java.net.URL;

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

connection.setRequestMethod("GET");
BufferedReader in = new BufferedReader(new InputStreamReader(connection.getInputStream()));
String inputLine;
StringBuffer response = new StringBuffer();

while ((inputLine = in.readLine()) != null) {
    response.append(inputLine);
}
in.close();

System.out.println("Response: " + response.toString());
}
}

Response: <!doctype html>
<html>
...
</html>

Conclusion

In this tutorial, we covered the basics of using URLs in Java, including the structure of URLs, constructors and methods of the URL and URLConnection classes, and provided examples to illustrate their usage. Understanding how to work with URLs is essential for network programming in Java.