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<!DOCTYPE document [
  <!ENTITY project SYSTEM "project.xml">
]>
<document url="fs-invoker.html">

  &project;

  <properties>
    <author email="craigmcc@apache.org">Craig McClanahan</author>
    <title>Invoker Servlet</title>
    <revision>$Id: fs-invoker.xml 918479 2010-03-03 14:08:02Z kkolinko $</revision>
  </properties>

<body>

<section name="Table of Contents">
<toc/>
</section>

<section name="Overview">


  <subsection name="Introduction">

    <p>The purpose of the <strong>Invoker Servlet</strong> is to allow a
    web application to dynamically register new <em>servlet definitions</em>
    that correspond with a <code>&lt;servlet&gt;</code> element in the
    <code>/WEB-INF/web.xml</code> 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 <code>init()</code> and
    <code>destroy()</code> at the correct times) will be respected.</p>

  </subsection>


  <subsection name="External Specifications">

    <p>I do not know of any formal specification of the behavior of an
    invoker servlet that is publicly available.  Anyone know of one?</p>

  </subsection>


  <subsection name="Implementation Requirements">

    <p>The implementation of this functionality shall conform to the
    following requirements:</p>
    <ul>
    <li>Implemented as a servlet.</li>
    <li>Exist in the <code>org.apache.catalina.servlets</code> package
        so that it can be loaded by the Catalina class loader.</li>
    <li>Implement the <code>org.apache.catalina.ContainerServlet</code>
        interface, so that it gains knowledge of the <code>Wrapper</code>
        that is responsible for itself and, therefore, access to other
        internal Catalina components.</li>
    <li>Support a configurable debugging detail level.</li>
    <li>Log debugging and operational messages (suitably internationalized)
        via the <code>getServletContext().log()</code> method.</li>
    </ul>

  </subsection>


</section>


<section name="Dependencies">


  <subsection name="Environmental Dependencies">

    <p>The following environmental dependencies must be met in order for
    the Invoker servlet to operate correctly:</p>
    <ul>
    <li>The invoker servlet must be registered in the application deployment
        descriptor (or the default deployment descriptor in file
        <code>$CATALINA_BASE/conf/web.xml</code>) using a "path mapped"
        servlet mapping.  The historical default mapping is to URL pattern
        "<code>/servlet/*</code>", although the invoker servlet must operate
        correctly with an arbitrary mapping.</li>
    </ul>

  </subsection>


  <subsection name="Container Dependencies">

    <p>Correct operation of the invoker servlet depends on the following
    specific features of the surrounding container:</p>
    <ul>
    <li>Correct support for the <code>ContainerServlet</code> interface,
        including calling <code>setWrapper()</code> <strong>before</strong>
        the <code>init()</code> method of the invoker servlet is called.</li>
    <li>The web application class loader must be stored as the context
        class loader of the request processing thread.</li>
    </ul>

  </subsection>


</section>


<section name="Functionality">


  <subsection name="Initialization Functionality">

    <p>The following processing must be performed when the <code>init()</code>
    method of the invoker servlet is called:</p>
    <ul>
    <li>Ensure that the container has called <code>setWrapper()</code>.  If
        not, throw a permanent <code>UnavailableException</code>.</li>
    <li>Look up and cache the <code>Context</code> that corresponds to our
        <code>Wrapper</code>.  This is the component with which new servlet
        definitions and mappings will be registered.</li>
    </ul>

  </subsection>


  <subsection name="Per-Request Functionality">

    <p>On each request, the following processing shall be performed:</p>
    <ol>
    <li>Calculate the <code>{ServletPath}</code> for this request, either from
        request attribute <code>javax.servlet.include.servlet_path</code> or
        by calling <code>request.getServletPath()</code>.</li>
    <li>Calculate the <code>{PathInfo}</code> for this request, either from
        request attribute <code>javax.servlet.include.path_info</code> or
        by calling <code>request.getPathInfo()</code>.  If the calculated
        <code>{PathInfo}</code> is null, return HTTP status 400
        (bad request).</li>
    <li>Parse the calculated <code>{PathInfo}</code> value as follows:
        <ol>
        <li>Ignore the leading slash character.</li>
        <li>Accumulate characters up to the next '/' (if any) as the
            <code>{ServletSelector}</code>.</li>
        <li>If a '/' was encountered, accumulate all characters from that
            slash (inclusive) to the end of the string as
            <code>{PathRemainder}</code>.  If no slash was encountered,
            set <code>{PathRemainder}</code> to a zero-length string.</li>
        </ol></li>
    <li>Determine whether <code>{ServletSelector}</code> is the name of an
        existing servlet definition, and process it as follows:
        <ol>
        <li>Ask our associated <code>Context</code> to find and return a
            child <code>Wrapper</code> named <code>{ServletSelector}</code>.
            </li>
        <li>If there is no such child, skip to the next major step.</li>
        <li>Register a new servlet mapping for this <code>Wrapper</code>,
            using a URL pattern calculated as follows:
            <code>{ServletPath}</code> + "/" + <code>{ServletSelector}</code>
            + "/*"</li>
        <li>Create a request dispatcher using a path calculated as follows:
            <code>{ServletPath}</code> + "/" + <code>{ServletSelector}</code>
            + <code>{PathRemainder}</code></li>
        <li>Forward this request to the created request dispatcher, and
            exit from this request.</li>
        </ol></li>
    <li>Assume that <code>{ServletSelector}</code> is the fully qualified
        name of a Java class that implements <code>javax.servlet.Servlet</code>
        and process it as follows:
        <ol>
        <li>Synthesize a new <code>{ServletName}</code> for the servlet
            definition that will be created.</li>
        <li>If there is already a child <code>Wrapper</code> associated with
            this name, return HTTP status 500 (internal server error), because
            a mapping should have already been created for this servlet.</li>
        <li>Attempt to load a class named <code>{ServletSelector}</code> 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).</li>
        <li>Instantiate an instance of this class.  If an error occurs,
            return HTTP status 404 (not found).</li>
        <li>If this class does not implement the
            <code>javax.servlet.Servlet</code> interface, return HTTP status
            404 (not found).</li>
        <li>Create and register a new <code>Wrapper</code> child with our
            <code>Context</code>, under name <code>{ServletName}</code>.</li>
        <li>Register a new servlet mapping for this <code>Wrapper</code>,
            using a URL pattern calculated as follows:
            <code>{ServletPath}</code> + "/" + <code>{ServletSelector}</code>
            + "/*"</li>
        <li>Create a request dispatcher using a path calculated as follows:
            <code>{ServletPath}</code> + "/" + <code>{ServletSelector}</code>
            + <code>{PathRemainder}</code></li>
        <li>Forward this request to the created request dispatcher, and
            exit from this request.</li>
        </ol></li>
    </ol>

  </subsection>


  <subsection name="Finalization Functionality">

    <p>No specific processing is required when the <code>destroy()</code>
    method is called:</p>

  </subsection>


</section>


<section name="Testable Assertions">

  <p>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:</p>
  <ul>
  <li>It is possible to access an existing servlet definition by name
      through the invoker.  The existing servlet definition can include
      either a <code>&lt;servlet-class&gt;</code> or
      <code>&lt;jsp-file&gt;</code> subelement.</li>
  <li>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.</li>
  <li>It is possible to access an anonymous servlet by class name
      through the invoker.</li>
  <li>When an anonymous servlet is accessed, the servlet instance is processed
      according to the lifecycle requirements of the Servlet Specification.
      </li>
  <li>When an anonymous servlet is accessed, the servlet instance receives
      a <code>ServletConfig</code> instance with no servlet initialization
      parameters.</li>
  <li>It is possible to utilize the invoker servlet via a direct request.</li>
  <li>It is possible to utilize the invoker servlet via a call to
      <code>RequestDispatcher.forward()</code>, or the corresponding
      <code>&lt;jsp:forward&gt;</code> tag in a JSP page.</li>
  <li>It is possible to utilize the invoker servlet via a call to
      <code>RequestDispatcher.include()</code>, or the corresponding
      <code>&lt;jsp:include&gt;</code> tag in a JSP page.</li>
  <li>It is possible to use any HTTP method (including GET and POST) that
      is supported by the Servlet class that is ultimately executed.</li>
  <li>The invoker servlet should never be asked to process a second or
      subsequent request for the same <code>{ServletSelector}</code> (because
      it will have registered an appropriate servlet mapping.</li>
  </ul>

</section>


</body>

</document>
