vaadin-framework/documentation/articles/CreatingAServlet3.0Application.asciidoc

---
title: Creating A Servlet 3.0 Application
order: 3
layout: page
---

[[creating-a-servlet-3.0-application]]
= Creating a servlet 3.0 application

Servlet 3.0 introduces a `@WebServlet` annotation which can be used to
replace the traditional web.xml. The straightforward approach to create
a Vaadin application using servlet 3.0 annotations is to simply move
whatever is in web.xml to a custom servlet class (extends `VaadinServlet`)
and annotate it using `@WebServlet` and add `@WebInitParams` as needed. You
will end up with something like

[source,java]
....
@WebServlet(value = "/*", asyncSupported = true, initParams = {
  @WebInitParam(name = "ui", value = "com.example.MyUI"),
  @WebInitParam(name = "productionMode", value = "false")
})
public class MyServlet extends VaadinServlet {
}
....

The problem you will face sooner or later with both this approach as
well as using web.xml is that you will misspell some parameter name or
class name. Maybe you change the UI class name and the init parameter is
not automatically updated - and the head scratching and debugging
starts.

Vaadin 7.1 introduces two features which makes this a lot easier,
`@VaadinServletConfiguration` and automatic UI finding.

`@VaadinServletConfiguration` is a type safe, Vaadin version of
`@WebInitParam` which provides you with the option to select UI by
referring the UI class directly, toggle `productionMode` using a boolean
and more. The above example rewritten using `@VaadinServletConfiguration`
looks like:

[source,java]
....
@WebServlet(value = "/*", asyncSupported = true)
@VaadinServletConfiguration(productionMode = false, ui = MYUI.class)
public class MyServlet extends VaadinServlet {
}
....

Automatic UI finding takes this even one step further and allows you to
leave out `@VaadinServletConfiguration` completely if you define your
servlet class as a static inner class to your UI class. The above
example could therefore also be written as

[source,java]
....
public class MYUI extends UI {
    @WebServlet(value = "/*", asyncSupported = true)
    public static class Servlet extends VaadinServlet {
    }
....

For clarity the variant with `@VaadinServletConfiguration` is likely the
better option. Please do note that `@VaadinServletConfiguration` comes
with defaults for some parameters, most importantly
`legacyPropertyToStringMode`, which might be important if you are
migrating an older application.