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Apache Lucene - Basic Demo Sources Walkthrough
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<h1>
Apache Lucene - Basic Demo Sources Walkthrough
</h1>
<div id="minitoc-area">
<ul class="minitoc">
<li>
<a href="#About the Code">About the Code</a>
</li>
<li>
<a href="#Location of the source (developers/deployers)">Location of the source (developers/deployers)</a>
</li>
<li>
<a href="#index.jsp (developers/deployers)">index.jsp (developers/deployers)</a>
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<a href="#header.jsp (developers/deployers)">header.jsp (developers/deployers)</a>
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<a href="#results.jsp (developers)">results.jsp (developers)</a>
</li>
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<a href="#More sources (developers)">More sources (developers)</a>
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<li>
<a href="#Where to go from here? (everyone!)">Where to go from here? (everyone!)</a>
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<li>
<a href="#When to contact the Author">When to contact the Author</a>
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</ul>
</div>
<a name="N10013"></a><a name="About the Code"></a>
<h2 class="boxed">About the Code</h2>
<div class="section">
<p>
In this section we walk through the sources behind the basic Lucene Web Application demo: where to
find them, their parts and their function. This section is intended for Java developers wishing to
understand how to use Lucene in their applications or for those involved in deploying web
applications based on Lucene.
</p>
</div>
<a name="N1001C"></a><a name="Location of the source (developers/deployers)"></a>
<h2 class="boxed">Location of the source (developers/deployers)</h2>
<div class="section">
<p>
Relative to the directory created when you extracted Lucene or retrieved it from Subversion, you
should see a directory called <span class="codefrag">src</span> which in turn contains a directory called
<span class="codefrag">jsp</span>. This is the root for all of the Lucene web demo.
</p>
<p>
Within this directory you should see <span class="codefrag">index.jsp</span>. Bring this up in vi or your editor of
choice.
</p>
</div>
<a name="N10031"></a><a name="index.jsp (developers/deployers)"></a>
<h2 class="boxed">index.jsp (developers/deployers)</h2>
<div class="section">
<p>
This jsp page is pretty boring by itself. All it does is include a header, display a form and
include a footer. If you look at the form, it has two fields: <span class="codefrag">query</span> (where you enter
your search criteria) and <span class="codefrag">maxresults</span> where you specify the number of results per page.
By the structure of this JSP it should be easy to customize it without even editing this particular
file. You could simply change the header and footer. Let's look at the <span class="codefrag">header.jsp</span>
(located in the same directory) next.
</p>
</div>
<a name="N10043"></a><a name="header.jsp (developers/deployers)"></a>
<h2 class="boxed">header.jsp (developers/deployers)</h2>
<div class="section">
<p>
The header is also very simple by itself. The only thing it does is include the
<span class="codefrag">configuration.jsp</span> (which you looked at in the last section of this guide) and set the
title and a brief header. This would be a good place to put your own custom HTML to "pretty" things
up a bit. We won't cover the footer because all it does is display the footer and close your tags.
Let's look at the <span class="codefrag">results.jsp</span>, the meat of this application, next.
</p>
</div>
<a name="N10052"></a><a name="results.jsp (developers)"></a>
<h2 class="boxed">results.jsp (developers)</h2>
<div class="section">
<p>
Most of the functionality lies in <span class="codefrag">results.jsp</span>. Much of it is for paging the search
results, which we'll not cover here as it's commented well enough. The first thing in this page is
the actual imports for the Lucene classes and Lucene demo classes. These classes are loaded from
the jars included in the <span class="codefrag">WEB-INF/lib</span> directory in the <span class="codefrag">luceneweb.war</span> file.
</p>
<p>
You'll notice that this file includes the same header and footer as <span class="codefrag">index.jsp</span>. From
there it constructs an <span class="codefrag">IndexSearcher</span> with the
<span class="codefrag">indexLocation</span> that was specified in <span class="codefrag">configuration.jsp</span>. If there is an
error of any kind in opening the index, it is displayed to the user and the boolean flag
<span class="codefrag">error</span> is set to tell the rest of the sections of the jsp not to continue.
</p>
<p>
From there, this jsp attempts to get the search criteria, the start index (used for paging) and the
maximum number of results per page. If the maximum results per page is not set or not valid then it
and the start index are set to default values. If only the start index is invalid it is set to a
default value. If the criteria isn't provided then a servlet error is thrown (it is assumed that
this is the result of url tampering or some form of browser malfunction).
</p>
<p>
The jsp moves on to construct a <span class="codefrag">StandardAnalyzer</span> to
analyze the search text. This matches the analyzer used during indexing (<span class="codefrag">IndexHTML</span>), which is generally
recommended. This is passed to the <span class="codefrag">QueryParser</span> along with the
criteria to construct a <span class="codefrag">Query</span>
object. You'll also notice the string literal <span class="codefrag">"contents"</span> included. This specifies
that the search should cover the <span class="codefrag">contents</span> field and not the <span class="codefrag">title</span>,
<span class="codefrag">url</span> or some other field in the indexed documents. If there is any error in
constructing a <span class="codefrag">Query</span> object an
error is displayed to the user.
</p>
<p>
In the next section of the jsp the <span class="codefrag">IndexSearcher</span> is asked to search
given the query object. The results are returned in a collection called <span class="codefrag">hits</span>. If the
length property of the <span class="codefrag">hits</span> collection is 0 (meaning there were no results) then an
error is displayed to the user and the error flag is set.
</p>
<p>
Finally the jsp iterates through the <span class="codefrag">hits</span> collection, taking the current page into
account, and displays properties of the <span class="codefrag">Document</span> objects we talked about in
the first walkthrough. These objects contain "known" fields specific to their indexer (in this case
<span class="codefrag">IndexHTML</span> constructs a document
with "url", "title" and "contents").
</p>
<p>
Please note that in a real deployment of Lucene, it's best to instantiate <span class="codefrag">IndexSearcher</span> and <span class="codefrag">QueryParser</span> once, and then
share them across search requests, instead of re-instantiating per search request.
</p>
</div>
<a name="N100CE"></a><a name="More sources (developers)"></a>
<h2 class="boxed">More sources (developers)</h2>
<div class="section">
<p>
There are additional sources used by the web app that were not specifically covered by either
walkthrough. For example the HTML parser, the <span class="codefrag">IndexHTML</span> class and <span class="codefrag">HTMLDocument</span> class. These are very
similar to the classes covered in the first example, with properties specific to parsing and
indexing HTML. This is beyond our scope; however, by now you should feel like you're "getting
started" with Lucene.
</p>
</div>
<a name="N100E1"></a><a name="Where to go from here? (everyone!)"></a>
<h2 class="boxed">Where to go from here? (everyone!)</h2>
<div class="section">
<p>
There are a number of things this demo doesn't do or doesn't do quite right. For instance, you may
have noticed that documents in the root context are unreachable (unless you reconfigure Tomcat to
support that context or redirect to it), anywhere where the directory doesn't quite match the
context mapping, you'll have a broken link in your results. If you want to index non-local files or
have some other needs this isn't supported, plus there may be security issues with running the
indexing application from your webapps directory. There are a number of things left for you the
developer to do.
</p>
<p>
In time some of these things may be added to Lucene as features (if you've got a good idea we'd love
to hear it!), but for now: this is where you begin and the search engine/indexer ends. Lastly, one
would assume you'd want to follow the above advice and customize the application to look a little
more fancy than black on white with "Lucene Template" at the top. We'll see you on the Lucene
Users' or Developers' <a href="mailinglists.html">mailing lists</a>!
</p>
</div>
<a name="N100F1"></a><a name="When to contact the Author"></a>
<h2 class="boxed">When to contact the Author</h2>
<div class="section">
<p>
Please resist the urge to contact the authors of this document (without bribes of fame and fortune
attached). First contact the <a href="mailinglists.html">mailing lists</a>, taking care to <a href="http://www.catb.org/~esr/faqs/smart-questions.html">Ask Questions The Smart Way</a>.
Certainly you'll get the most help that way as well. That being said, feedback, and modifications
to this document and samples are ever so greatly appreciated. They are just best sent to the lists
or <a href="http://wiki.apache.org/lucene-java/HowToContribute">posted as patches</a>, so that
everyone can share in them. Thanks for understanding!
</p>
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