Lucene

System Requirements

Apache Lucene runs of Java 6 or greater. When using Java 7, be sure to install at least Update 1! With all Java versions it is strongly recommended to not use experimental -XX JVM options. It is also recommended to always use the latest update version of your Java VM, because bugs may affect Lucene. An overview of known JVM bugs can be found on http://wiki.apache.org/lucene-java/SunJavaBugs.
CPU, disk and memory requirements are based on the many choices made in implementing Lucene (document size, number of documents, and number of hits retrieved to name a few). The benchmarks page has some information related to performance on particular platforms.
To build Apache Lucene from source, refer to the BUILD.txt file in the distribution directory.

Setting your CLASSPATH

First, you should download the latest Lucene distribution and then extract it to a working directory.
You need four JARs: the Lucene JAR, the queryparser JAR, the common analysis JAR, and the Lucene demo JAR. You should see the Lucene JAR file in the core/ directory you created when you extracted the archive -- it should be named something like lucene-core-{version}.jar. You should also see files called lucene-queryparser-{version}.jar, lucene-analyzers-common-{version}.jar and lucene-demo-{version}.jar under queryparser, analysis/common/ and demo/, respectively.
Put all four of these files in your Java CLASSPATH.

Indexing Files

Once you've gotten this far you're probably itching to go. Let's build an index! Assuming you've set your CLASSPATH correctly, just type:

    java org.apache.lucene.demo.IndexFiles -docs {path-to-lucene}/src

This will produce a subdirectory called index which will contain an index of all of the Lucene source code. To search the index type:

    java org.apache.lucene.demo.SearchFiles

You'll be prompted for a query. Type in a swear word and press the enter key. You'll see that the Lucene developers are very well mannered and get no results. Now try entering the word "string". That should return a whole bunch of documents. The results will page at every tenth result and ask you whether you want more results.

About the code

In this section we walk through the sources behind the command-line Lucene 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.

Location of the source

The files discussed here are linked into this documentation directly:

• IndexFiles.java: code to create a Lucene index.
• SearchFiles.java: code to search a Lucene index.

IndexFiles

As we discussed in the previous walk-through, the IndexFiles class creates a Lucene Index. Let's take a look at how it does this.
The main() method parses the command-line parameters, then in preparation for instantiating IndexWriter, opens a Directory, and instantiates StandardAnalyzer and IndexWriterConfig.
The value of the -index command-line parameter is the name of the filesystem directory where all index information should be stored. If IndexFiles is invoked with a relative path given in the -index command-line parameter, or if the -index command-line parameter is not given, causing the default relative index path "index" to be used, the index path will be created as a subdirectory of the current working directory (if it does not already exist). On some platforms, the index path may be created in a different directory (such as the user's home directory).
The -docs command-line parameter value is the location of the directory containing files to be indexed.
The -update command-line parameter tells IndexFiles not to delete the index if it already exists. When -update is not given, IndexFiles will first wipe the slate clean before indexing any documents.
Lucene Directorys are used by the IndexWriter to store information in the index. In addition to the FSDirectory implementation we are using, there are several other Directory subclasses that can write to RAM, to databases, etc.
Lucene Analyzers are processing pipelines that break up text into indexed tokens, a.k.a. terms, and optionally perform other operations on these tokens, e.g. downcasing, synonym insertion, filtering out unwanted tokens, etc. The Analyzer we are using is StandardAnalyzer, which creates tokens using the Word Break rules from the Unicode Text Segmentation algorithm specified in Unicode Standard Annex #29; converts tokens to lowercase; and then filters out stopwords. Stopwords are common language words such as articles (a, an, the, etc.) and other tokens that may have less value for searching. It should be noted that there are different rules for every language, and you should use the proper analyzer for each. Lucene currently provides Analyzers for a number of different languages (see the javadocs under lucene/analysis/common/src/java/org/apache/lucene/analysis).
The IndexWriterConfig instance holds all configuration for IndexWriter. For example, we set the OpenMode to use here based on the value of the -update command-line parameter.
Looking further down in the file, after IndexWriter is instantiated, you should see the indexDocs() code. This recursive function crawls the directories and creates Document objects. The Document is simply a data object to represent the text content from the file as well as its creation time and location. These instances are added to the IndexWriter. If the -update command-line parameter is given, the IndexWriterConfig OpenMode will be set to OpenMode.CREATE_OR_APPEND, and rather than adding documents to the index, the IndexWriter will update them in the index by attempting to find an already-indexed document with the same identifier (in our case, the file path serves as the identifier); deleting it from the index if it exists; and then adding the new document to the index.

Searching Files

The SearchFiles class is quite simple. It primarily collaborates with an IndexSearcher, StandardAnalyzer, (which is used in the IndexFiles class as well) and a QueryParser. The query parser is constructed with an analyzer used to interpret your query text in the same way the documents are interpreted: finding word boundaries, downcasing, and removing useless words like 'a', 'an' and 'the'. The Query object contains the results from the QueryParser which is passed to the searcher. Note that it's also possible to programmatically construct a rich Query object without using the query parser. The query parser just enables decoding the Lucene query syntax into the corresponding Query object.
SearchFiles uses the IndexSearcher.search(query,n) method that returns TopDocs with max n hits. The results are printed in pages, sorted by score (i.e. relevance).