Java HttpURLConnection

HttpURLConnection is the HTTP-aware subclass of URLConnection. When you openConnection() on an http: or https: URL, the object you get back is an HttpURLConnection — cast it to reach the HTTP-specific features: setting the request method, reading the status code, and getting the separate error stream. It is the JDK's original HTTP client, available since Java 1.1.

Modern note: for new code, prefer java.net.http.HttpClient (next chapter) — it is cleaner, supports HTTP/2, and handles async. HttpURLConnection is still everywhere in older codebases, so it is worth knowing well.

Casting and choosing a method

URL url = URI.create("http://example.com/api").toURL();
HttpURLConnection conn = (HttpURLConnection) url.openConnection();
conn.setRequestMethod("POST");                       // GET is the default
conn.setRequestProperty("Content-Type", "application/json");
conn.setConnectTimeout(2000);
conn.setReadTimeout(2000);

setRequestMethod takes "GET", "POST", "PUT", "DELETE", etc. Add request headers with setRequestProperty.

Sending a request body

To send a body you must opt in with setDoOutput(true) (which also makes the method default to POST), then write to the output stream:

conn.setDoOutput(true);
try (OutputStream os = conn.getOutputStream()) {
    os.write(payload.getBytes(StandardCharsets.UTF_8));
}

Status code and the two streams

The defining HTTP feature is the status code, and the trap that follows from it:

int code = conn.getResponseCode();          // triggers the request
InputStream body = (code >= 200 && code < 400)
        ? conn.getInputStream()             // success body
        : conn.getErrorStream();            // error body (4xx/5xx)

getInputStream() throws IOException on a 4xx/5xx response. The error body lives on a different stream, getErrorStream(). Forgetting this is the classic HttpURLConnection bug: an error response blows up instead of letting you read the server's explanation. Always branch on getResponseCode() first. Finish with conn.disconnect().

A worked example: a POST round trip

This program runs a loopback server that echoes the request body back with a 201 Created, then performs a POST through HttpURLConnection: setting the method, writing a body, reading the status, and choosing the right stream.

java— editable, runs on the server

import com.sun.net.httpserver.HttpServer;
import java.io.BufferedReader;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.OutputStream;
import java.net.HttpURLConnection;
import java.net.InetSocketAddress;
import java.net.URI;
import java.net.URL;
import java.nio.charset.StandardCharsets;

public class HttpUrlConnectionDemo {
  public static void main(String[] args) throws Exception {
    HttpServer server = HttpServer.create(new InetSocketAddress("127.0.0.1", 0), 0);
    server.createContext("/echo", ex -> {
      String received = new String(
          ex.getRequestBody().readAllBytes(), StandardCharsets.UTF_8);
      byte[] body = ("you sent: " + received).getBytes(StandardCharsets.UTF_8);
      ex.sendResponseHeaders(201, body.length);   // 201 Created
      try (OutputStream os = ex.getResponseBody()) { os.write(body); }
    });
    server.start();
    int port = server.getAddress().getPort();

URL url = URI.create("http://127.0.0.1:" + port + "/echo").toURL();
    HttpURLConnection conn = (HttpURLConnection) url.openConnection();
    conn.setRequestMethod("POST");
    conn.setRequestProperty("Content-Type", "text/plain");
    conn.setConnectTimeout(2000);
    conn.setReadTimeout(2000);
    conn.setDoOutput(true);                         // we will send a body

try (OutputStream os = conn.getOutputStream()) {
      os.write("hello server".getBytes(StandardCharsets.UTF_8));
    }

int code = conn.getResponseCode();
    System.out.println("status: " + code + " " + conn.getResponseMessage());

// Success bodies and error bodies are on DIFFERENT streams
    InputStream stream = (code >= 200 && code < 400)
        ? conn.getInputStream()
        : conn.getErrorStream();
    try (BufferedReader r = new BufferedReader(
            new InputStreamReader(stream, StandardCharsets.UTF_8))) {
      r.lines().forEach(l -> System.out.println("body  : " + l));
    }

conn.disconnect();
    server.stop(0);
  }
}

What to take from the run:

Casting the URLConnection to HttpURLConnection unlocked the HTTP layer: setRequestMethod("POST"), getResponseCode(), and getResponseMessage() exist only on the subclass. For an http: URL the object really is an HttpURLConnection, so the cast always succeeds.
Sending a body required setDoOutput(true) before writing to getOutputStream(). Without that call the connection stays read-only and writing fails — opting into output is the switch that turns a GET into a body-carrying request.
getResponseCode() returned 201 and is what actually fired the request to the server. The status code is the first thing to read, because the next decision — which stream to read — depends on it.
The example branched between getInputStream() and getErrorStream(). Here 201 is success so the input stream carried the echoed body, but on a 4xx/5xx response getInputStream() would have thrown and the server's message would only be reachable through getErrorStream(). That split is the API's sharpest edge.
The flow took five-plus configuration calls for one POST, and disconnect() to clean up. That verbosity — manual streams, the error-stream trap, no built-in async — is exactly why the next chapter's HttpClient exists.

Practice

A client does a PUT with 'HttpURLConnection'. When the server returns '400 Bad Request', the code crashes with an 'IOException' on 'conn.getInputStream()' instead of logging the server's error message. What is the correct fix?

Check 'getResponseCode()' first; for 4xx/5xx responses read the body from 'getErrorStream()' rather than 'getInputStream()', which throws on error statuses
Wrap 'getInputStream()' in a retry loop, since a 400 is a transient network error that usually succeeds on the second try
Call 'setDoInput(false)' so that error responses are ignored and no exception is thrown
Switch the request method to GET, because 'HttpURLConnection' cannot read response bodies for PUT requests