HowToSliceWebApplication

Background

For genuinely modular web applications, you will want to modularise not only your application back end modules but also your web modules. Impala allows you to do this, effectively 'slicing' your application front end into different web modules, each of which can serve separate portions of your applications URL space.

In other wors, Impala allows you to allocate different URLs to different modules.

For full details see WebModuleBasedHandlerRegistration.

Setup

1) Set up web.xml

Simply add the following entry to an empty web.xml:

<filter>
    <filter-name>web</filter-name>
    <filter-class>org.impalaframework.web.spring.integration.ModuleProxyFilter</filter-class>
</filter>

<filter-mapping>
    <filter-name>web</filter-name>
    <url-pattern>/*</url-pattern>
</filter-mapping>

This registers a servlet filter which directs all requests to Impala's module selection mechanism.

2) Set up Impala's module-aware HttpServletRequest and ServletContext

Add the following entry to impala.properties

module.prefix.mapping.enabled=true
partitioned.servlet.context=true

The first of the entries is required, and allows Impala modules to 'subscribe' to incoming requests, as set up in 3) below. The second ensure that web modules are exposed a 'module-aware' ServletContext. With this setting enabled, both ServletContext and HttpSession attributes are partitioned, so that by default they are only visible within the module (unless prefixed with shared:). It also allows ServletContext.getResource() and related methods to return resources first from the module class path, and then from the usual web application root directory.

3) Set up modules to receive requests for particular URLs

Add the following entry into your web module's Spring configuration file.

<web:mapping>
    <web:to-module prefix = "/urlprefix" setServletPath="true"/> 
</web:mapping>

This entry, from the Impala web namespace, sets any URL with the prefix urlprefix to be directed to the the current module. It also modifies the value that will be returned from HttpServletRequest.getServletPath() to be /urlprefix (without the serServletPath entry, the servlet path will be determined purely from the ModuleProxyFilter match in 1).

4) Map requests within modules to servlets and filters

The Impala web namespace allows you to set up servlets and filters within the module which can receive requests within the module, as shown in the example below.

<web:mapping>
    <web:to-module prefix = "/urlprefix" setServletPath="true"/> 
    <web:to-handler extension = "htm" servletName="dispatcher" filterNames = "characterEncodingFilter"/>
    <web:to-handler extension = "css" servletName="resources"/>
</web:mapping>

<web:servlet id = "dispatcher" 
    servletClass = "org.impalaframework.web.spring.servlet.InternalModuleServlet"/>

<web:servlet id = "resources" 
    servletClass = "org.springframework.js.resource.ResourceServlet"
    initParameters = "cacheTimeout=10"/>

<web:filter id = "characterEncodingFilter" 
    filterClass = "org.springframework.web.filter.CharacterEncodingFilter" 
    initParameters = "forceEncoding=true,encoding=utf8">
</web:filter>

Above, we have a dispatcher servlet (to handler application requests), a resources handling servlet, and a filter. The web:to-handler entries map requests to the module with extension htm to the encoding filter and despatcher servlet, and CSS style sheets simply to the resource servlet.