Apache Lucene: Hello World Example

Apache Lucen is a full text-search library for java which helps
you add search capability to your application/website.
Note that Lucene is specifically an API, not an application.

Apache Lucene is a power full search library on which the
widely used search engines like Apache Solr,ElasticSearch are based

You can add contents to Lucene from various sources,like a SQL/NoSQL database, a filesystem, or even from websites.

Some Terminologies

Index

Lucene uses index called an inverted index, because it inverts
a page-centric data structure (page->words) to a keyword-centric
data structure (word->pages).

Document

An index consists of one or more Documents.
Indexing involves adding Documents to an IndexWriter,
and searching involves retrieving Documents from an index via an IndexSearcher.

Fields

A Document consists of one or more Fields. A Field is simply a name-value pair.

Let's get started:

For quickly getting started I am going to create an In-Memory Index.

Step-1

Create an empty Maven project using the following console command:

$ mvn archetype:create -DgroupId=demo.lucene -DartifactId=lucene-demo

Step-2

Add Lucene dependencies to pom.xml file as defined below

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd">
  <modelVersion>4.0.0</modelVersion>
  <groupId>demo.lucene</groupId>
  <artifactId>lucene-demo</artifactId>
  <version>1.0</version>
  <name>Lucene-HelloWorld</name>
  <description>Hello World Lucene Search Example</description>
  <dependencies>
    <dependency>
      <groupId>org.apache.lucene</groupId>
      <artifactId>lucene-core</artifactId>
      <version>2.4.1</version>
    </dependency>
    <dependency>
      <groupId>lucene</groupId>
      <artifactId>lucene</artifactId>
      <version>1.4.3</version>
    </dependency>
  </dependencies>
</project>

Step-3

Create a Java file HelloWorldLucene.java class

import org.apache.lucene.analysis.standard.StandardAnalyzer;
import org.apache.lucene.document.Document;
import org.apache.lucene.document.Field;
import org.apache.lucene.document.StringField;
import org.apache.lucene.document.TextField;
import org.apache.lucene.index.DirectoryReader;
import org.apache.lucene.index.IndexReader;
import org.apache.lucene.index.IndexWriter;
import org.apache.lucene.index.IndexWriterConfig;
import org.apache.lucene.queryparser.classic.ParseException;
import org.apache.lucene.queryparser.classic.QueryParser;
import org.apache.lucene.search.IndexSearcher;
import org.apache.lucene.search.Query;
import org.apache.lucene.search.ScoreDoc;
import org.apache.lucene.search.TopScoreDocCollector;
import org.apache.lucene.store.Directory;
import org.apache.lucene.store.RAMDirectory;
import org.apache.lucene.util.Version;

import java.io.IOException;

public class HelloLucene {
  public static void main(String[] args) throws IOException, ParseException {
    // Specify the analyzer for tokenizing text.
    // The same analyzer should be used for indexing and searching
    StandardAnalyzer analyzer = new StandardAnalyzer(Version.LUCENE_40);

    // Create the index
    Directory index = new RAMDirectory();

    IndexWriterConfig config = new IndexWriterConfig(Version.LUCENE_40, analyzer);

    IndexWriter w = new IndexWriter(index, config);
    addDoc(w, "Lucene in Action", "193398817");
    addDoc(w, "Lucene for Dummies", "55320055Z");
    addDoc(w, "Managing Gigabytes", "55063554A");
    addDoc(w, "The Art of Computer Science", "9900333X");
    w.close();

    //Query
    String querystr = args.length > 0 ? args[0] : "lucene";

    // the "title" arg specifies the default field to use
    // when no field is explicitly specified in the query.
    Query q = new QueryParser(Version.LUCENE_40, "title", analyzer).parse(querystr);

    // Search
    int hitsPerPage = 10;
    IndexReader reader = DirectoryReader.open(index);
    IndexSearcher searcher = new IndexSearcher(reader);
    TopScoreDocCollector collector = TopScoreDocCollector.create(hitsPerPage, true);
    searcher.search(q, collector);
    ScoreDoc[] hits = collector.topDocs().scoreDocs;

    // Display results
    System.out.println("Found " + hits.length + " hits.");
    for(int i=0;i<hits.length;++i) {
      int docId = hits[i].doc;
      Document d = searcher.doc(docId);
      System.out.println((i + 1) + ". " + d.get("isbn") + "\t" + d.get("title"));
    }

    // reader can only be closed when there
    // is no need to access the documents any more.
    reader.close();
  }

  private static void addDoc(IndexWriter w, String title, String isbn) throws IOException {
    Document doc = new Document();
    doc.add(new TextField("title", title, Field.Store.YES));

    // Use a string field for isbn because we don't want it tokenized
    doc.add(new StringField("isbn", isbn, Field.Store.YES));
    w.addDocument(doc);
  }
}

Step-4

Compile project

$ mvn clean package

Step-5

Run Project

$ java -jar <jar_name> <Main-Class_name> <query_string>

In our case run command:

$ java -jar lucene-demo-1.0.jar HelloWorldLucen "lucene"

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