Catalina Functional Specifications - Invoker Servlet

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# Catalina Functional Specifications ## Invoker Servlet | print-friendly version
Overview
> Introduction
> The purpose of the Invoker Servlet is to allow a web application to dynamically register new servlet definitions that correspond with a `<servlet>` element in the `/WEB-INF/web.xml` deployment descriptor, and execute requests utilizing the new servlet definitions. From the perspective of the newly registered servlets, all servlet lifecycle requirements of the Servlet Specification (such as calling `init()` and `destroy()` at the correct times) will be respected.
> External Specifications
> I do not know of any formal specification of the behavior of an invoker servlet that is publicly available. Anyone know of one?
> Implementation Requirements
> The implementation of this functionality shall conform to the following requirements: Implemented as a servlet. Exist in the `org.apache.catalina.servlets` package so that it can be loaded by the Catalina class loader. Implement the `org.apache.catalina.ContainerServlet` interface, so that it gains knowledge of the `Wrapper` that is responsible for itself and, therefore, access to other internal Catalina components. Support a configurable debugging detail level. Log debugging and operational messages (suitably internationalized) via the `getServletContext().log()` method.
Dependencies
> Environmental Dependencies
> The following environmental dependencies must be met in order for the Invoker servlet to operate correctly: The invoker servlet must be registered in the application deployment descriptor (or the default deployment descriptor in file `$CATALINA_HOME/conf/web.xml`) using a "path mapped" servlet mapping. The historical default mapping is to URL pattern "`/servlet/*`", although the invoker servlet must operate correctly with an arbitrary mapping.
> Container Dependencies
> Correct operation of the invoker servlet depends on the following specific features of the surrounding container: Correct support for the `ContainerServlet` interface, including calling `setWrapper()` before the `init()` method of the invoker servlet is called. The web application class loader must be stored as the context class loader of the request processing thread.
Functionality
> Initialization Functionality
> The following processing must be performed when the `init()` method of the invoker servlet is called: Ensure that the container has called `setWrapper()`. If not, throw a permanent `UnavailableException`. Look up and cache the `Context` that corresponds to our `Wrapper`. This is the component with which new servlet definitions and mappings will be registered.
> Per-Request Functionality
> On each request, the following processing shall be performed: Calculate the `{ServletPath}` for this request, either from request attribute `javax.servlet.include.servlet_path` or by calling `request.getServletPath()`. Calculate the `{PathInfo}` for this request, either from request attribute `javax.servlet.include.path_info` or by calling `request.getPathInfo()`. If the calculated `{PathInfo}` is null, return HTTP status 400 (bad request). Parse the calculated `{PathInfo}` value as follows: Ignore the leading slash character. Accumulate characters up to the next '/' (if any) as the `{ServletSelector}`. If a '/' was encountered, accumulate all characters from that slash (inclusive) to the end of the string as `{PathRemainder}`. If no slash was encountered, set `{PathRemainder}` to a zero-length string. Determine whether `{ServletSelector}` is the name of an existing servlet definition, and process it as follows: Ask our associated `Context` to find and return a child `Wrapper` named `{ServletSelector}`. If there is no such child, skip to the next major step. Register a new servlet mapping for this `Wrapper`, using a URL pattern calculated as follows: `{ServletPath}` + "/" + `{ServletSelector}` + "/*" Create a request dispatcher using a path calculated as follows: `{ServletPath}` + "/" + `{ServletSelector}` + `{PathRemainder}` Forward this request to the created request dispatcher, and exit from this request. Assume that `{ServletSelector}` is the fully qualified name of a Java class that implements `javax.servlet.Servlet` and process it as follows: Synthesize a new `{ServletName}` for the servlet definition that will be created. If there is already a child `Wrapper` associated with this name, return HTTP status 500 (internal server error), because a mapping should have already been created for this servlet. Attempt to load a class named `{ServletSelector}` from the web application class loader (i.e. the context class loader for our current thread). If this fails, return HTTP status 404 (not found). Instantiate an instance of this class. If an error occurs, return HTTP status 404 (not found). If this class does not implement the `javax.servlet.Servlet` interface, return HTTP status 404 (not found). Create and register a new `Wrapper` child with our `Context`, under name `{ServletName}`. Register a new servlet mapping for this `Wrapper`, using a URL pattern calculated as follows: `{ServletPath}` + "/" + `{ServletSelector}` + "/*" Create a request dispatcher using a path calculated as follows: `{ServletPath}` + "/" + `{ServletSelector}` + `{PathRemainder}` Forward this request to the created request dispatcher, and exit from this request.
> Finalization Functionality
> No specific processing is required when the `destroy()` method is called:
Testable Assertions
In addition the the assertions implied by the functionality requirements listed above, the following additional assertions shall be tested to validate the behavior of the invoker servlet: It is possible to access an existing servlet definition by name through the invoker. The existing servlet definition can include either a `<servlet-class>` or `<jsp-file>` subelement. When an existing servlet definition is accessed by name, the request will be ultimately processed by the same servlet instance that would have processed it had a mapping to that servlet definition been used on the request directly. It is possible to access an anonymous servlet by class name through the invoker. When an anonymous servlet is accessed, the servlet instance is processed according to the lifecycle requirements of the Servlet Specification. When an anonymous servlet is accessed, the servlet instance receives a `ServletConfig` instance with no servlet initialization parameters. It is possible to utilize the invoker servlet via a direct request. It is possible to utilize the invoker servlet via a call to `RequestDispatcher.forward()`, or the corresponding `<jsp:forward>` tag in a JSP page. It is possible to utilize the invoker servlet via a call to `RequestDispatcher.include()`, or the corresponding `<jsp:include>` tag in a JSP page. It is possible to use any HTTP method (including GET and POST) that is supported by the Servlet class that is ultimately executed. The invoker servlet should never be asked to process a second or subsequent request for the same `{ServletSelector}` (because it will have registered an appropriate servlet mapping.
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