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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 $</revision>
</properties>
<body>
<section name="Table of Contents">
<toc/>
</section>
<section name="Overview">
<subsection name="Introduction">
<p>The purpose of the <strong>Invoker Servlet</strong> is to allow a
web application to dynamically register new <em>servlet definitions</em>
that correspond with a <code><servlet></code> element in the
<code>/WEB-INF/web.xml</code> deployment descriptor, and execute
requests utilizing the new servlet definitions. From the perspective
of the newly registered servlets, all servlet lifecycle requirements
of the Servlet Specification (such as calling <code>init()</code> and
<code>destroy()</code> at the correct times) will be respected.</p>
</subsection>
<subsection name="External Specifications">
<p>I do not know of any formal specification of the behavior of an
invoker servlet that is publicly available. Anyone know of one?</p>
</subsection>
<subsection name="Implementation Requirements">
<p>The implementation of this functionality shall conform to the
following requirements:</p>
<ul>
<li>Implemented as a servlet.</li>
<li>Exist in the <code>org.apache.catalina.servlets</code> package
so that it can be loaded by the Catalina class loader.</li>
<li>Implement the <code>org.apache.catalina.ContainerServlet</code>
interface, so that it gains knowledge of the <code>Wrapper</code>
that is responsible for itself and, therefore, access to other
internal Catalina components.</li>
<li>Support a configurable debugging detail level.</li>
<li>Log debugging and operational messages (suitably internationalized)
via the <code>getServletContext().log()</code> method.</li>
</ul>
</subsection>
</section>
<section name="Dependencies">
<subsection name="Environmental Dependencies">
<p>The following environmental dependencies must be met in order for
the Invoker servlet to operate correctly:</p>
<ul>
<li>The invoker servlet must be registered in the application deployment
descriptor (or the default deployment descriptor in file
<code>$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><servlet-class></code> or
<code><jsp-file></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><jsp:forward></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><jsp:include></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>