How to Send HTTP Parameters Using POST Method in Java

Question

How can I modify my existing Java code to send HTTP parameters using the POST method instead of GET?

void sendRequest(String request, String httpMethod) {
    URL url = new URL("http://example.com/index.php");
    HttpURLConnection connection = (HttpURLConnection) url.openConnection();
    connection.setDoOutput(true);
    connection.setInstanceFollowRedirects(false);
    connection.setRequestMethod(httpMethod);
    connection.setRequestProperty("Content-Type", "application/x-www-form-urlencoded");
    // Adding parameters if the request method is POST
    if (httpMethod.equals("POST")) {
        String params = "param1=a&param2=b&param3=c"; // Your parameters here
        OutputStream os = connection.getOutputStream();
        os.write(params.getBytes("UTF-8"));
        os.close();
    }
    connection.connect();
}

Answer

When sending HTTP requests in Java, specifically when utilizing the `HttpURLConnection` class, it is critical to properly set up your connection to handle different HTTP methods such as GET and POST effectively. Here’s how to adapt your existing GET method to support sending parameters via POST as well.

void sendRequest(String request, String httpMethod) {
    URL url = new URL("http://example.com/index.php");
    HttpURLConnection connection = (HttpURLConnection) url.openConnection();
    connection.setDoOutput(true);
    connection.setInstanceFollowRedirects(false);
    connection.setRequestMethod(httpMethod);
    connection.setRequestProperty("Content-Type", "application/x-www-form-urlencoded");
    // Adding parameters if the request method is POST
    if (httpMethod.equals("POST")) {
        String params = "param1=a&param2=b&param3=c"; // Your parameters here
        OutputStream os = connection.getOutputStream();
        os.write(params.getBytes("UTF-8"));
        os.close();
    }
    connection.connect();
}

Causes

The initial setup used the GET method, which appends parameters in the URL.
Switching to POST requires sending parameters in the request body instead of the URL.

Solutions

Modify the header to set the `Content-Type` to `application/x-www-form-urlencoded` for POST requests.
Use an `OutputStream` to write the parameters in the request body when the method is POST.

Common Mistakes

Mistake: Forgetting to set `setDoOutput(true)` before sending a POST request.

Solution: Always set `setDoOutput(true)` for POST requests.

Mistake: Not properly encoding parameters when sending them via POST.

Solution: URL-encode the parameters using `URLEncoder.encode()` as needed.

Helpers

Java HTTP POST request
Java HttpURLConnection POST
send parameters via POST Java
Java POST method example