Writing Java handlers

Basic class structure

Java implementation of an event handler must include:

• A no-arg public constructor; the default constructor is fine
• Method implemented with signature of the form: public void on<eventName>()

For example, an excerpt of zero.core.fileserver.FileServer shows handler logic for GET, POST, PUT, and DELETE events:

public class FileServer {
    public void onGET() {
        String path = zget("/event/path");
        File f = new File(path);
        try {
            returnFile(f);
        } catch (IOException e) {
            // FileServer is invoked only after confirming that f exists, so 
            // an IOException indicates another problem
            throw new RuntimeException(e);
        }
    }

    public void onPOST() {
        Map<String, Object> headers = new HashMap<String, Object>();
        headers.put("Allow", "GET");
        ErrorPage.send(HttpURLConnection.HTTP_BAD_METHOD, headers, null);
    }

    public void onPUT() {
        Map<String, Object> headers = new HashMap<String, Object>();
        headers.put("Allow", "GET");
        ErrorPage.send(HttpURLConnection.HTTP_BAD_METHOD, headers, null);
    }

    public void onDELETE() {
        Map<String, Object> headers = new HashMap<String, Object>();
        headers.put("Allow", "GET");
        ErrorPage.send(HttpURLConnection.HTTP_BAD_METHOD, headers, null);
    }
}

Exceptions

Handlers can throw an Exception to indicate an internal error. Details about exception handling are described in the Event processing section.