In this source code example, we will write a Java program that uses HTTP Client API to submit form data (application/x-www-form-URL-encoded).

Java 11 HttpClient didn’t provide API for the form data, we have to construct it manually.

Java HttpClient POST Example - Send Form Data 

import java.io.IOException;
import java.net.URI;
import java.net.URLEncoder;
import java.net.http.HttpClient;
import java.net.http.HttpRequest;
import java.net.http.HttpResponse;
import java.nio.charset.StandardCharsets;
import java.util.HashMap;
import java.util.Map;

public class Main {

    public static void main(String[] args) throws IOException, InterruptedException {

        Map<Object, Object> data = new HashMap<>();
        data.put("fistname", "admin");
        data.put("lastname", "admin");
        data.put("age", 25);

        HttpClient client = HttpClient.newHttpClient();

        HttpRequest request = HttpRequest.newBuilder()
                .header("Content-Type", "application/x-www-form-urlencoded")
                .uri(URI.create("https://httpbin.org/post"))
                .POST(ofForm(data))
                .build();

        HttpResponse<String> response = client.send(request, HttpResponse.BodyHandlers.ofString());

        System.out.println("Status code: " + response.statusCode());
        System.out.println("\n Body: " + response.body());
    }

    public static HttpRequest.BodyPublisher ofForm(Map<Object, Object> data) {

        StringBuilder body = new StringBuilder();

        for (Object dataKey : data.keySet()) {

            if (body.length() > 0) {
                body.append("&");
            }

            body.append(encode(dataKey))
                    .append("=")
                    .append(encode(data.get(dataKey)));
        }

        return HttpRequest.BodyPublishers.ofString(body.toString());
    }

    private static String encode(Object obj) {
        return URLEncoder.encode(obj.toString(), StandardCharsets.UTF_8);
    }
}

Output:

Status code: 200

 Body: {
  "args": {}, 
  "data": "", 
  "files": {}, 
  "form": {
    "age": "25", 
    "fistname": "admin", 
    "lastname": "admin"
  }, 
  "headers": {
    "Content-Length": "36", 
    "Content-Type": "application/x-www-form-urlencoded", 
    "Host": "httpbin.org", 
    "User-Agent": "Java-http-client/17.0.2", 
    "X-Amzn-Trace-Id": "Root=1-63070225-277b1f41012b39780e96a216"
  }, 
  "json": null, 
  "origin": "103.59.74.90", 
  "url": "https://httpbin.org/post"
}