ActionServlet Overview

Here's a brief digest of how ActionServlet is designed (see Specification for detailed info):

ActionServlet acts as 'Controller' in Model-View-Controller (MVC) programming model. Views are usually HTML templates processed by a template engine (currently Velocity or WebMacro).
Web applications are built of (reusable) components. Component classes can be of any type (JavaBeans, EJBs, Corba objects etc.).
With ActionServlet, application flow is configured by a XML file (called ActionConfig) with a simple syntax, which provides a way to:
- select which method of which component to be invoked by HTTP request:
  - by matching action URL (using regular expressions) or
  - by matching HTTP parameters 'form' and 'action',
- pass parameters to a component method upon request - parameters for action method can be taken from:
  - Context object,
  - HTTP request (usual ?param=value style),
  - URL (like http://server/shop/addCart/banana),
  - other component (by <input-variable> element),
  - direct value (<input-variable> element),
- react to action method invocation, because every method:
  - returns a value (or void) - then <on-return> element is used,
  - throws an exception - then <on-exception> element is used.
  Example: Excerpt of ActionConfig:
```
<component name="Authenticator" class="my.package.Authenticator" persistence="session">
   <action name="/login" method="login(String userName, String password)">

      <input-variable name="password" value="defaultValue" if="!$password"/>
      <output-variable name="loginOK" 
                       component="Authenticator" value="isLoggedIn()"/>

      <on-return value="void" show-template="SuccessfulLogin.wm"/>
      <on-exception class="LoginException" show-template="Login.wm"/>

   </action>
</component>
```
  This config means that upon a request, where URL is formed like http://my.server.com/servlet/login:
  1. values of userName and password parameters are retrieved from HTTP (providing default password if necessary),
  2. authenticator's method login is invoked,
  3. $loginOK variable value is set to the return value of Authenticator.isLoggedIn(),
  4. upon successful action SuccessfulLogin.wm page/on login error Login.wm is shown.
ActionServlet can send to browser not only rendered HTML templates, but anything you like - downloaded file, generated picture etc. - or redirect to given URL. Just use 'show-value-of' or 'show-url' attributes in place of 'show-template'.
Actions can invoke sub-actions by <invoke> element, so HTTP parameters can be passed to more than one component or invoke methods upon exceptions/return values, default actions can be defined for each template and so on.
Type handlers are used to convert between types - this makes sending data between components easy. For example, you can take java.util.Vector returned by one action method and pass it to another component method as an array.