Basic help to write a Java Servlet

Development Add-ons
1

First of all, I am (unfortunately) not a professional programmer, that’s why I am asking for some basic help as startup.
I want to write a server side script in Java to receive URL-Requests from IoT devices and perform a post request to OH rest API to send commands to an item.
Those IoT devices do not support http/post request.
For this I have written a javascript (see here) but then I found out, that some IoT devices do not support javascript - that’s why I want to write a servlet.
Basically a small replacement of the BasicUI CMD servlet.

My questions now are where and how to start.
Ideally I just write the java code and embed it so that the code runs when a specific website is opened without the requirement to install a couple of professional programmer’s development tools.

Does anybody have a good starting point or experiences you’d like to share?

Many thanks in advance

2

Seems you found your solution else where. Just in case if you are curious how shortest way looks a like see code below. Address to call is /cmd?item1=ON&item2=OFF&item3=10.3, so it allows to produce batch requests.

package foo;

import java.io.IOException;
import java.util.Enumeration;
import java.util.Hashtable;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import org.openhab.core.events.EventPublisher;
import org.openhab.core.items.GenericItem;
import org.openhab.core.items.Item;
import org.openhab.core.items.ItemRegistry;
import org.openhab.core.items.events.ItemCommandEvent;
import org.openhab.core.items.events.ItemEventFactory;
import org.openhab.core.types.Command;
import org.openhab.core.types.TypeParser;
import org.osgi.service.component.annotations.Activate;
import org.osgi.service.component.annotations.Component;
import org.osgi.service.component.annotations.Deactivate;
import org.osgi.service.component.annotations.Reference;
import org.osgi.service.http.HttpService;

@Component
public class CommandServlet extends HttpServlet {

  private final HttpService httpService;
  private final ItemRegistry registry;
  private final EventPublisher eventPublisher;

  @Activate
  public CommandServlet(@Reference HttpService httpService, @Reference ItemRegistry registry, @Reference EventPublisher eventPublisher) throws Exception {
    this.httpService = httpService;
    this.registry = registry;
    this.eventPublisher = eventPublisher;

    httpService.registerServlet("/cmd", this, new Hashtable<>(), httpService.createDefaultHttpContext());
  }

  @Deactivate
  void deactivate() {
    httpService.unregister("/cmd");
  }

  @Override
  protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    Enumeration<String> params = req.getParameterNames();
    while (params.hasMoreElements()) {
      String item = params.nextElement();
      String cmd = req.getParameter(item);

      Item itemObj = registry.get(item);
      if (itemObj instanceof GenericItem) {
        Command command = TypeParser.parseCommand(itemObj.getAcceptedCommandTypes(), cmd);
        ItemCommandEvent event = ItemEventFactory.createCommandEvent(item, command);
        eventPublisher.post(event);
      }
    }

    resp.setStatus(HttpServletResponse.SC_OK);
  }

}
3

Hi Lukasz,
yes Piotr has developed a nice http listener which is a good solution.
However, I wanted to start coding something on my own in java and - who knows - maybe one day a binding (most probably not within the next five years :slight_smile: ). So, I wanted to pursue this definitly further.

First of all, many thanks for your code.
On a high level, what are the next steps (compiling) and where do I have to copy the file?

Thank you very much in advance

4

You need to follow developer guide and install necessary dev tools (Java, IDE or text editor + maven). Once your tools are installed properly you can run binding skeleton script you can find in openhab addons repository. There you will get everything needed to attempt first compilation.

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