Java URLConnection Class

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URLConnection is an abstract class in the java.net package that represents a communication link between a Java application and a URL. It allows both reading from and writing to a resource referenced by a URL.

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It's useful for downloading data, uploading files, or interacting with HTTP APIs (via HttpURLConnection subclass).

Syntax

 URL url = new URL("https://www.example.com");
URLConnection connection = url.openConnection();
connection.connect();
Method Description
connect() Opens a communication link
getInputStream() Reads from the URL
getOutputStream() Writes to the URL
setDoOutput(true) Enables writing (POST or PUT)
getContentType() Returns MIME type
getContentLength() Returns content size
getHeaderField(String name) Retrieves a header field
setRequestProperty(String key, String value) Adds custom request headers

Example

Reading Content from a URL

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

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

            BufferedReader reader = new BufferedReader(new InputStreamReader(conn.getInputStream()));
            String line;
            while ((line = reader.readLine()) != null) {
                System.out.println(line);
            }
            reader.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

Example

Setting Request Properties

URL url = new URL("https://www.example.com");
URLConnection conn = url.openConnection();
conn.setRequestProperty("User-Agent", "Mozilla/5.0");

Example

POST Request with HttpURLConnection

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

public class PostExample {
    public static void main(String[] args) throws IOException {
        URL url = new URL("https://httpbin.org/post");
        HttpURLConnection conn = (HttpURLConnection) url.openConnection();

        conn.setRequestMethod("POST");
        conn.setDoOutput(true);
        conn.setRequestProperty("Content-Type", "application/x-www-form-urlencoded");

        String data = "username=javauser&password=1234";

        try (OutputStream os = conn.getOutputStream()) {
            os.write(data.getBytes());
            os.flush();
        }

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