How can I send simple messages from a webpage (HTML/Javascript) to a Java application?

It depends on protocol (adapter) your application is using. It can be a HTTP, websockets or something else.

I can assume that you are talking about HTTP, so the easiest way is to use form html element and POST method.

check this

Please slice your project into two(three, if there is a database management) parts:

1. Server side - core side. Here you are able to implement your java code. That means you must have running a server like tomcat. Create new web app project. Include tomcat or something else as server. Create a Servlet (do not ask how to create but search through internet). In that servlet override doPost method. Including this code inthat method gives you an assumption about server side:

response.getWriter().write("hello from server");

2. Client side. Client side is where you implement html, js and css code. Submit form to that servlet by post method.

If you have correctly configured web.xml then on submitting this form you should receive "hello from server" as a reponse. Use eclipse as IDE that is easier to configure.

You can use JSP / Servlet to communicate between client and server.

See here for some basic stuff

Question: Do you have maven? If not I would encourage you to download it. It's simple: https://maven.apache.org/

If yes, add these to your pom file:

    <parent>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-parent</artifactId>
    <version>1.3.0.RELEASE</version>
</parent>
<dependencies>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-web</artifactId>
    </dependency>
</dependencies>

Now in the class which has your main application add some annotations and a controller (method) to handle your POST request:

import org.springframework.boot.*;
import org.springframework.boot.autoconfigure.*;
import org.springframework.stereotype.*;
import org.springframework.web.bind.annotation.*;

import javax.servlet.http.HttpServletRequest;

@Controller
@EnableAutoConfiguration
public class SpringBootSampleApplication {

    @RequestMapping("/")
    @ResponseBody
    @CrossOrigin
    String home() {
        return "Hello World!";
    }

    /** Receives a POST request on path / with parameter name. Will return parameter
     * 
     * @param request
     * @return */
    @RequestMapping(value = "/", method = RequestMethod.POST)
    @ResponseBody
    @CrossOrigin // this will allow requests origination from any domain. see CORS
    String greet(HttpServletRequest request) {
        return "Greetings: " + request.getParameter("name");
    }

    /** starts the application
     * 
     * @param args
     * @throws Exception */
    public static void main(String[] args) throws Exception {
        SpringApplication.run(SpringBootSampleApplication.class, args);
    }
}

This will start a web server on port 8080.

• Access GET http://localhost:8080 to call the first method returning hello world;
• POST http://localhost:8080 to call the greet method returning Greetings: NameParam

Check out this code sample here: https://github.com/mikibrv/spring.boot.sample

I have also added a small index.html file in the resources as an example of AJAX.

It's a full project, generating 2 endpoints ( a GET and POST ) with CORS enabled and when you build it with Maven (mvn clean install) you get a fat JAR which you can call using : java -jar target/spring.boot.sample.jar

For more info: http://projects.spring.io/spring-boot/