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Apache Lucene - Basic Demo Sources Walk-through
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<h1>
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Apache Lucene - Basic Demo Sources Walk-through
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</h1>
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<div id="minitoc-area">
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<ul class="minitoc">
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<li>
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<a href="#About the Code">About the Code</a>
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<li>
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<a href="#Location of the source">Location of the source</a>
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<li>
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<a href="#IndexFiles">IndexFiles</a>
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<a href="#Searching Files">Searching Files</a>
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<li>
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<a href="#The Web example...">The Web example...</a>
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</li>
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</ul>
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<a name="N10013"></a><a name="About the Code"></a>
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<h2 class="boxed">About the Code</h2>
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<div class="section">
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<p>
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In this section we walk through the sources behind the command-line Lucene demo: where to find them,
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their parts and their function. This section is intended for Java developers wishing to understand
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how to use Lucene in their applications.
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</p>
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</div>
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<a name="N1001C"></a><a name="Location of the source"></a>
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<h2 class="boxed">Location of the source</h2>
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<div class="section">
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<p>
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Relative to the directory created when you extracted Lucene or retrieved it from Subversion, you
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should see a directory called <span class="codefrag">src</span> which in turn contains a directory called
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<span class="codefrag">demo</span>. This is the root for all of the Lucene demos. Under this directory is
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<span class="codefrag">org/apache/lucene/demo</span>. This is where all the Java sources for the demos live.
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</p>
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<p>
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Within this directory you should see the <span class="codefrag">IndexFiles.java</span> class we executed earlier.
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Bring it up in <span class="codefrag">vi</span> or your editor of choice and let's take a look at it.
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</p>
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</div>
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<a name="N10037"></a><a name="IndexFiles"></a>
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<h2 class="boxed">IndexFiles</h2>
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<div class="section">
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<p>
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As we discussed in the previous walk-through, the <span class="codefrag">IndexFiles</span> class creates a Lucene
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Index. Let's take a look at how it does this.
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</p>
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<p>
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The first substantial thing the <span class="codefrag">main</span> function does is instantiate <span class="codefrag">IndexWriter</span>. It passes the string
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"<span class="codefrag">index</span>" and a new instance of a class called <span class="codefrag">StandardAnalyzer</span>.
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The "<span class="codefrag">index</span>" string is the name of the filesystem directory where all index information
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should be stored. Because we're not passing a full path, this will be created as a subdirectory of
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the current working directory (if it does not already exist). On some platforms, it may be created
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in other directories (such as the user's home directory).
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</p>
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<p>
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The <span class="codefrag">IndexWriter</span> is the main
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class responsible for creating indices. To use it you must instantiate it with a path that it can
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write the index into. If this path does not exist it will first create it. Otherwise it will
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refresh the index at that path. You can also create an index using one of the subclasses of <span class="codefrag">Directory</span>. In any case, you must also pass an
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instance of <span class="codefrag">org.apache.lucene.analysis.Analyzer</span>.
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</p>
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<p>
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The particular <span class="codefrag">Analyzer</span> we
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are using, <span class="codefrag">StandardAnalyzer</span>, is
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little more than a standard Java Tokenizer, converting all strings to lowercase and filtering out
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useless words and characters from the index. By useless words and characters I mean common language
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words such as articles (a, an, the, etc.) and other strings that would be useless for searching
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(e.g. <b>'s</b>) . It should be noted that there are different rules for every language, and you
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should use the proper analyzer for each. Lucene currently provides Analyzers for a number of
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different languages (see the <span class="codefrag">*Analyzer.java</span> sources under <a href="http://svn.apache.org/repos/asf/lucene/java/trunk/contrib/analyzers/src/java/org/apache/lucene/analysis/">contrib/analyzers/src/java/org/apache/lucene/analysis</a>).
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</p>
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<p>
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Looking further down in the file, you should see the <span class="codefrag">indexDocs()</span> code. This recursive
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function simply crawls the directories and uses <span class="codefrag">FileDocument</span> to create <span class="codefrag">Document</span> objects. The <span class="codefrag">Document</span> is simply a data object to
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represent the content in the file as well as its creation time and location. These instances are
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added to the <span class="codefrag">indexWriter</span>. Take a look inside <span class="codefrag">FileDocument</span>. It's not particularly
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complicated. It just adds fields to the <span class="codefrag">Document</span>.
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</p>
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<p>
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As you can see there isn't much to creating an index. The devil is in the details. You may also
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wish to examine the other samples in this directory, particularly the <span class="codefrag">IndexHTML</span> class. It is a bit more
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complex but builds upon this example.
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</p>
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</div>
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<a name="N100AE"></a><a name="Searching Files"></a>
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<h2 class="boxed">Searching Files</h2>
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<div class="section">
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<p>
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The <span class="codefrag">SearchFiles</span> class is
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quite simple. It primarily collaborates with an <span class="codefrag">IndexSearcher</span>, <span class="codefrag">StandardAnalyzer</span>
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(which is used in the <span class="codefrag">IndexFiles</span> class as well) and a
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<span class="codefrag">QueryParser</span>. The
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query parser is constructed with an analyzer used to interpret your query text in the same way the
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documents are interpreted: finding the end of words and removing useless words like 'a', 'an' and
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'the'. The <span class="codefrag">Query</span> object contains
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the results from the <span class="codefrag">QueryParser</span> which is passed to
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the searcher. Note that it's also possible to programmatically construct a rich <span class="codefrag">Query</span> object without using the query
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parser. The query parser just enables decoding the <a href="queryparsersyntax.html">Lucene query
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syntax</a> into the corresponding <span class="codefrag">Query</span> object. Search can be executed in
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two different ways:
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<ul>
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<li>Streaming: A <span class="codefrag">HitCollector</span> subclass
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simply prints out the document ID and score for each matching document.</li>
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<li>Paging: Using a <span class="codefrag">TopDocCollector</span>
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the search results are printed in pages, sorted by score (i. e. relevance).</li>
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</ul>
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</p>
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</div>
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<a name="N100FB"></a><a name="The Web example..."></a>
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<h2 class="boxed">The Web example...</h2>
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<div class="section">
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<p>
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<a href="demo3.html">read on>>></a>
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</p>
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</div>
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Copyright ©
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2006 <a href="http://www.apache.org/licenses/">The Apache Software Foundation.</a>
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