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Introduction to Text Indexing with Apache Jakarta Lucene

by Otis Gospodnetic
01/15/2003

What Lucene Is

Lucene is a Java library that adds text indexing and searching capabilities to an application. It is not a complete application that one can just download, install, and run. It offers a simple, yet powerful core API. To start using it, one needs to know only a few Lucene classes and methods.

Lucene offers two main services: text indexing and text searching. These two activities are relatively independent of each other, although indexing naturally affects searching. In this article I will focus on text indexing, and we will look at some of the core Lucene classes that provide text indexing capabilities.

Lucene Background

Lucene was originally written by Doug Cutting and was available for download from SourceForge. It joined the Apache Software Foundation's Jakarta family of open source server-side Java products in September of 2001. With each release since then, the project has enjoyed more visibility, attracting more users and developers. As of November 2002, Lucene version 1.2 has been released, with version 1.3 in the works. In addition to those organizations mentioned on the "Powered by Lucene" page, I have heard of FedEx, Overture, Mayo Clinic, Hewlett Packard, New Scientist magazine, Epiphany, and others using, or at least evaluating, Lucene.

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Installing Lucene

Like most other Jakarta projects, Lucene is distributed as pre-compiled binaries or in source form. You can download the latest official release from Lucene's release page. There are also nightly builds, if you'd like to use the newest features. To demonstrate Lucene usage, I will assume that you will use the pre-compiled distribution. Simply download the Lucene .jar file and add its path to your CLASSPATH environment variable. If you choose to get the source distribution and build it yourself, you will need Jakarta Ant and JavaCC, which is available as a free download. Although the company that created JavaCC no longer exists, you can still get JavaCC from the URL listed in the References section of this article.

Indexing with Lucene

Before we jump into code, let's look at some of the fundamental Lucene classes for indexing text. They are IndexWriter, Analyzer, Document, and Field.

IndexWriter is used to create a new index and to add Documents to an existing index.

Before text is indexed, it is passed through an Analyzer. Analyzers are in charge of extracting indexable tokens out of text to be indexed, and eliminating the rest. Lucene comes with a few different Analyzer implementations. Some of them deal with skipping stop words (frequently-used words that don't help distinguish one document from the other, such as "a," "an," "the," "in," "on," etc.), some deal with converting all tokens to lowercase letters, so that searches are not case-sensitive, and so on.

An index consists of a set of Documents, and each Document consists of one or more Fields. Each Field has a name and a value. Think of a Document as a row in a RDBMS, and Fields as columns in that row.

Now, let's consider the simplest scenario, where you have a piece of text to index, stored in an instance of String. Here is how you could do it, using the classes described above:

import org.apache.lucene.index.IndexWriter;
import org.apache.lucene.analysis.Analyzer;
import org.apache.lucene.analysis.standard.StandardAnalyzer;
import org.apache.lucene.document.Document;
import org.apache.lucene.document.Field;

/**
 * LuceneIndexExample class provides a simple
 * example of indexing with Lucene.  It creates a fresh
 * index called "index-1" in a temporary directory every
 * time it is invoked and adds a single document with a
 * single field to it.
 */
public class LuceneIndexExample
{
    public static void main(String args[]) throws Exception
    {
        String text = "This is the text to index with Lucene";

        String indexDir =
            System.getProperty("java.io.tmpdir", "tmp") +
            System.getProperty("file.separator") + "index-1";
        Analyzer analyzer = new StandardAnalyzer();
        boolean createFlag = true;

        IndexWriter writer =
            new IndexWriter(indexDir, analyzer, createFlag);
        Document document  = new Document();
        document.add(Field.Text("fieldname", text));
        writer.addDocument(document);
        writer.close();
    }
}

Let's step through the code. Lucene stores its indices in directories on the file system. Each index is contained within a single directory, and multiple indices should not share a directory. The first parameter in IndexWriter's constructor specifies the directory where the index should be stored. The second parameter provides the implementation of Analyzer that should be used for pre-processing the text before it is indexed. This particular implementation of Analyzer eliminates stop words, converts tokens to lower case, and performs a few other small input modifications, such as eliminating periods from acronyms. The last parameter is a boolean flag that, when true, tells IndexWriter to create a new index in the specified directory, or overwrite an index in that directory, if it already exists. A value of false instructs IndexWriter to instead add Documents to an existing index. We then create a blank Document, and add a Field called fieldname to it, with a value of the String that we want to index. Once the Document is populated, we add it to the index via the instance of IndexWriter. Finally, we close the index. This is important, as it ensures that all index changes are flushed to the disk.

Analyzers

As I already mentioned, Analyzers are components that pre-process input text. They are also used when searching. Because the search string has to be processed the same way that the indexed text was processed, it is crucial to use the same Analyzer for both indexing and searching. Not using the same Analyzer will result in invalid search results.

The Analyzer class is an abstract class, but Lucene comes with a few concrete Analyzers that pre-process their input in different ways. Should you need to pre-process input text and queries in a way that is not provided by any of Lucene's Analyzers, you will need to implement a custom Analyzer. If you are indexing text with non-Latin characters, for instance, you will most definitely need to do this.

In this example of a custom Analyzer, we will assume we are indexing text in English. Our PorterStemAnalyzer will perform Porter stemming on its input. As stated by its creator, the Porter stemming algorithm (or "Porter stemmer") is a process for removing the more common morphological and inflexional endings from words in English. Its main function is to be part of a term normalization process that is usually done when setting up Information Retrieval systems.

This Analyzer will use an implementation of the Porter stemming algorithm provided by Lucene's PorterStemFilter class.

import org.apache.lucene.analysis.Analyzer;
import org.apache.lucene.analysis.TokenStream;
import org.apache.lucene.analysis.StopFilter;
import org.apache.lucene.analysis.LowerCaseTokenizer;
import org.apache.lucene.analysis.PorterStemFilter;

import java.io.Reader;
import java.util.Hashtable;

/**
 * PorterStemAnalyzer processes input
 * text by stemming English words to their roots.
 * This Analyzer also converts the input to lower case
 * and removes stop words.  A small set of default stop
 * words is defined in the STOP_WORDS
 * array, but a caller can specify an alternative set
 * of stop words by calling non-default constructor.
 */
public class PorterStemAnalyzer extends Analyzer
{
    private static Hashtable _stopTable;

    /**
     * An array containing some common English words
     * that are usually not useful for searching.
     */
    public static final String[] STOP_WORDS =
    {
        "0", "1", "2", "3", "4", "5", "6", "7", "8",
        "9", "000", "$",
        "about", "after", "all", "also", "an", "and",
        "another", "any", "are", "as", "at", "be",
        "because", "been", "before", "being", "between",
        "both", "but", "by", "came", "can", "come",
        "could", "did", "do", "does", "each", "else",
        "for", "from", "get", "got", "has", "had",
        "he", "have", "her", "here", "him", "himself",
        "his", "how","if", "in", "into", "is", "it",
        "its", "just", "like", "make", "many", "me",
        "might", "more", "most", "much", "must", "my",
        "never", "now", "of", "on", "only", "or",
        "other", "our", "out", "over", "re", "said",
        "same", "see", "should", "since", "so", "some",
        "still", "such", "take", "than", "that", "the",
        "their", "them", "then", "there", "these",
        "they", "this", "those", "through", "to", "too",
        "under", "up", "use", "very", "want", "was",
        "way", "we", "well", "were", "what", "when",
        "where", "which", "while", "who", "will",
        "with", "would", "you", "your",
        "a", "b", "c", "d", "e", "f", "g", "h", "i",
        "j", "k", "l", "m", "n", "o", "p", "q", "r",
        "s", "t", "u", "v", "w", "x", "y", "z"
    };

    /**
     * Builds an analyzer.
     */
    public PorterStemAnalyzer()
    {
        this(STOP_WORDS);
    }

    /**
     * Builds an analyzer with the given stop words.
     *
     * @param stopWords a String array of stop words
     */
    public PorterStemAnalyzer(String[] stopWords)
    {
        _stopTable = StopFilter.makeStopTable(stopWords);
    }

    /**
     * Processes the input by first converting it to
     * lower case, then by eliminating stop words, and
     * finally by performing Porter stemming on it.
     *
     * @param reader the Reader that
     *               provides access to the input text
     * @return an instance of TokenStream
     */
    public final TokenStream tokenStream(Reader reader)
    {
        return new PorterStemFilter(
            new StopFilter(new LowerCaseTokenizer(reader),
                _stopTable));
    }
}

The tokenStream(Reader) method is the core of the PorterStemAnalyzer. It lower-cases input, eliminates stop words, and uses the PorterStemFilter to remove common morphological and inflexional endings. This class includes only a small set of stop words for English. When using Lucene in a production system for indexing and searching text in English, I suggest that you use a more complete list of stop words, such as this one.

To use our new PorterStemAnalyzer class, we need to modify a single line of our LuceneIndexExample class shown above, to instantiate PorterStemAnalyzer instead of StandardAnalyzer:

Old line:

Analyzer analyzer = new StandardAnalyzer();

New line:

Analyzer analyzer = new PorterStemAnalyzer();

The rest of the code remains unchanged. Anything indexed after this change will pass through the Porter stemmer. The process of text indexing with PorterStemAnalyzer is depicted in Figure 1.


Figure 1: The indexing process with PorterStemAnalyzer.

Because different Analyzers process their text input differently, note again that changing the Analyzer for an existing index is dangerous. It will result in erroneous search results later, in the same way that using a different Analyzer for both indexing and searching will produce invalid results.

Field Types

Lucene offers four different types of fields from which a developer can choose: Keyword, UnIndexed, UnStored, and Text. Which field type you should use depends on how you want to use that field and its values.

Keyword fields are not tokenized, but are indexed and stored in the index verbatim. This field is suitable for fields whose original value should be preserved in its entirety, such as URLs, dates, personal names, Social Security numbers, telephone numbers, etc.

UnIndexed fields are neither tokenized nor indexed, but their value is stored in the index word for word. This field is suitable for fields that you need to display with search results, but whose values you will never search directly. Because this type of field is not indexed, searches against it are slow. Since the original value of a field of this type is stored in the index, this type is not suitable for storing fields with very large values, if index size is an issue.

UnStored fields are the opposite of UnIndexed fields. Fields of this type are tokenized and indexed, but are not stored in the index. This field is suitable for indexing large amounts of text that does not need to be retrieved in its original form, such as the bodies of Web pages, or any other type of text document.

Text fields are tokenized, indexed, and stored in the index. This implies that fields of this type can be searched, but be cautious about the size of the field stored as Text field.

If you look back at the LuceneIndexExample class, you will see that I used a Text field:

document.add(Field.Text("fieldname", text));

If we wanted to change the type of field "fieldname," we would call one of the other methods of class Field:

document.add(Field.Keyword("fieldname", text));

or

document.add(Field.UnIndexed("fieldname", text));

or

document.add(Field.UnStored("fieldname", text));

Although the Field.Text, Field.Keyword, Field.UnIndexed, and Field.UnStored calls may at first look like calls to constructors, they are really just calls to different Field class methods. Table 1 summarizes the different field types.

Table 1: An overview of different field types.

Field method/typeTokenizedIndexedStored
Field.Keyword(String, String)NoYesYes
Field.UnIndexed(String, String)NoNoYes
Field.UnStored(String, String)YesYesNo
Field.Text(String, String)YesYesYes
Field.Text(String, Reader)YesYesNo

Conclusion

In this article, we have learned about adding basic text indexing capabilities to your applications using IndexWriter and its associated classes. We have also developed a custom Analyzer that can perform Porter stemming on its input. Finally, we have looked at different field types and learned what each of them can be used for. In the next article of this Lucene series, we shall look at indexing in more depth, and address issues such as performance and multi-threading.

References

Otis Gospodnetic is an active Apache Jakarta member, a member of Apache Jakarta Project Management Committee, a developer of Lucene and maintainer of the jGuru's Lucene FAQ.


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