Client

import java.io.IOException;
import java.net.URI;
import java.net.http.HttpClient;
import java.net.http.HttpRequest;
import java.net.http.HttpResponse;

public class Main {
    public static void main(String[] args) throws IOException, InterruptedException {
        HttpClient client = HttpClient.newHttpClient();
        HttpRequest request = HttpRequest.newBuilder(URI.create("http://localhost:8080/hello"))
                .GET()
                .header("accept", "text/plain") // MIME тип
                .build();
        HttpResponse<String> response = client.send(request, HttpResponse.BodyHandlers.ofString());
        System.out.println(response.headers());
        System.out.println(response.body());

        request = HttpRequest.newBuilder(URI.create("http://localhost:8080/hello"))
                .setHeader("Content-type", "text/plain")
                .POST(HttpRequest.BodyPublishers.ofString("qweqwe"))
                .build();
        response = client.send(request, HttpResponse.BodyHandlers.ofString());
        System.out.println(response.headers());
        System.out.println(response.body());
    }
}