MailGun Webhook Handler

MailgunWebhook.java

public class MailgunWebhook extends HttpServlet {
	
	private static final long serialVersionUID = 1L;
	
	public void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
		
		try {
			resp.setContentType("application/json");
			
			String token = null;
			String timestamp = null;
			String signature = null;
			String event = null;
			String customVar = null;
			
			String contentType = req.getHeader("Content-Type");
			if (contentType != null && contentType.contains("multipart/form")) {
				
				DiskFileItemFactory factory = new DiskFileItemFactory();
				ServletContext servletContext = this.getServletConfig().getServletContext();
				File repository = (File) servletContext.getAttribute("javax.servlet.context.tempdir");
				factory.setRepository(repository);
	
				// Create a new file upload handler
				ServletFileUpload upload = new ServletFileUpload(factory);
	
				// Parse the request
				List<FileItem> items = upload.parseRequest(req);
				if (items != null) {
					for (FileItem item : items) {
						if (item.getFieldName() != null) {
							if (item.getFieldName().equals("signature")) {
								signature = item.getString();
							}
							if (item.getFieldName().equals("token")) {
								token = item.getString();
							}
							if (item.getFieldName().equals("timestamp")) {
								timestamp = item.getString();
							}
							if (item.getFieldName().equals("event")) {
								event = item.getString();
							}
							if (item.getFieldName().equals("custom_var")) {
								customVar = item.getString();
							}
						}
					}
				}
				
			} else {
				
				signature = req.getParameter("signature");
				token = req.getParameter("token");
				timestamp = req.getParameter("timestamp");
				event = req.getParameter("event");
				customVar = req.getParameter("custom_var");
			}
			
			if (token != null && timestamp != null && signature != null) {
				boolean isValid = MailGun.isSignatureValid(token, Long.valueOf(timestamp), signature);
				if (!isValid) {
					sendErrorResponse(resp,"Invalid signature");
					return;
				}
				
				handleEvent(event, customVar);
			}
			
		} catch (Exception e) {
			sendErrorResponse(resp, e.getMessage());
			return;
		}
	}

	/**
	 * Handling webhook events
	 * 
	 * @param event
	 * @param customVar
	 * @throws Exception
	 */
	private void handleEvent(String event, String customVar) throws Exception {
		// track delivery for campaign
		if (event != null && customVar != null) {
			// code to handle the event
		}
	}

	/**
	 * Constructing error reponse
	 * @param resp
	 * @param message
	 * @throws IOException
	 */
	private void sendErrorResponse(HttpServletResponse resp, String message) throws IOException {
		resp.setStatus(Status.BAD_REQUEST.getStatusCode());
		resp.getWriter().write(new JSONObject().put("error", message).toString());
	}
}