Registering a servlet

Create a servlet class

To create a servlet class, extend javax.servlet.http.HttpServlet. Most servlets extend this class.

HttpServlet provides an abstract class to be subclassed to create an HTTP servlet suitable for a website. A subclass of HttpServlet must override at least one method, usually one of these:

  • doGet for HTTP GET requests

  • doPost for HTTP POST requests

  • doPut for HTTP PUT requests

  • doDelete for HTTP DELETE requests

  • init and destroy to manage resources that are held for the life of the servlet

  • getServletInfo for the servlet to provide information about itself

There’s almost no reason to override the service method, which handles standard HTTP requests by dispatching them to the handler methods for each HTTP request type (the do methods listed above). Likewise, there’s almost no reason to override the doOptions and doTrace methods.

Servlets typically run on multithreaded servers, so be aware that a servlet must handle concurrent requests and be careful to synchronize access to shared resources. Shared resources include in-memory data such as instance or class variables and external objects such as files, database connections and network connections.

Additionally, implement SelfMappingServlet. This is an optional interface that servlets wrapped by ServletDispatchingFilter can implement to provide their own mapping. When implementing this method, the servlet can get the mapping from any arbitrary component. This means the mapping can be configured centrally and reused by other components, typically to generate links. See Dynamic servlet mapping for more information.

Define the servlet in your module descriptor

Define the servlet in your module descriptor. Include any mappings and init parameters the servlet class supports.

Example

The Magnolia Core module descriptor (core.xml) registers the ClasspathSpool servlet. The purpose of this servlet is to serve resource files such as CSS and JavaScript from the mgnl-resources folder of any module when requested with the path /.resources/<module name>.

core.xml
<servlets>
    <servlet>
        <name>ClasspathSpoolServlet</name>
        <class>info.magnolia.cms.servlets.ClasspathSpool</class>
        <comment>Used to serve resources from the classpath.</comment>
        <mappings>
            <mapping>/.resources/*</mapping>
        </mappings>
    </servlet>
</servlets>

Initialization parameters:

  • name: name of the servlet

  • class: fully qualified name of the servlet class

  • comment: description of what the servlet does

  • mappings: URLs and paths that direct requests to the servlet

You don’t need to add the servlet to your web.xml.

Configure the servlet in the filter chain

Configure the servlet in the filter chain in Configuration > /server/filters/servlets. The configuration is created automatically when the module is installed. The values are taken from the module descriptor.

Servlets are registered by RegisterModuleServletsTask at install time. This means that if the definition changes over time, the module developer should update the configuration by using ModuleVersionHandler.
Example

ClasspathSpoolServlet is configured in /server/filters/servlets/ClasspathSpoolServlet. The default /mgnl-resources is hard-coded but can be configured with the resourcesRoot init parameter. The servlet loads files from this folder and makes them available in the path defined in the pattern parameter.

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Properties:

  • <servlet name>: name of the servlet as defined in the servletName property

    • mappings: URLs and paths that direct requests to the servlet

      • <mapping name>

        • pattern: URL pattern that triggers the servlet

    • parameters: parameters supported by the servlet class

      • <parameter name>

    • class: filter class. All servlets use the info.magnolia.cms.filters.ServletDispatchingFilter class, which dispatches requests to a wrapped servlet. The filter initializes the servlet and its mappings. ServletConfig is wrapped to take init parameters into account.

    • comment: servlet description

    • enabled: enables and disables the servlet. The default value is true.

    • servletClass: fully qualified name of the servlet class

    • servletName: name of the servlet as defined in the module descriptor

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