How to perform full text retrieval and search in Java

How to perform full-text retrieval and search in Java

Full-text retrieval and search is a technique for finding specific keywords or phrases in large-scale text data. In applications that process large amounts of text data, such as search engines, email systems, and document management systems, full-text retrieval and search functions are very important.

As a widely used programming language, Java provides a wealth of libraries and tools that can help us implement full-text retrieval and search functions. This article will introduce how to use the Lucene library to implement full-text retrieval and search, and provide some specific code examples.

1. Introduce the Lucene library

First, we need to introduce the Lucene library into the project. The Lucene library can be introduced into the Maven project in the following ways:

<dependencies>
    <dependency>
        <groupId>org.apache.lucene</groupId>
        <artifactId>lucene-core</artifactId>
        <version>8.10.1</version>
    </dependency>
    <dependency>
        <groupId>org.apache.lucene</groupId>
        <artifactId>lucene-analyzers-common</artifactId>
        <version>8.10.1</version>
    </dependency>
</dependencies>

2. Create an index

Before performing full-text search, we need to create an index first. This index contains relevant information about the text data to be searched, so that we can perform subsequent search operations. The following is a simple example code for creating an index:

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;
import org.apache.lucene.index.IndexWriter;
import org.apache.lucene.index.IndexWriterConfig;
import org.apache.lucene.store.Directory;
import org.apache.lucene.store.FSDirectory;

import java.io.IOException;
import java.nio.file.Paths;

public class Indexer {
    private IndexWriter indexWriter;

    public Indexer(String indexDir) throws IOException {
        Directory dir = FSDirectory.open(Paths.get(indexDir));
        Analyzer analyzer = new StandardAnalyzer();
        IndexWriterConfig config = new IndexWriterConfig(analyzer);
        indexWriter = new IndexWriter(dir, config);
    }

    public void close() throws IOException {
        indexWriter.close();
    }

    public void addDocument(String content) throws IOException {
        Document doc = new Document();
        doc.add(new TextField("content", content, Field.Store.YES));
        indexWriter.addDocument(doc);
    }
}

In the above example code, we use IndexWriter to create the index and TextField to define the Indexed fields. When adding content to be indexed to the index, we need to first create a Document object, then add fields to the object, and finally call the addDocument method to add DocumentObject is added to the index.

3. Perform search

After creating the index, we can perform search operations. The following is a simple search sample code:

import org.apache.lucene.analysis.Analyzer;
import org.apache.lucene.analysis.standard.StandardAnalyzer;
import org.apache.lucene.document.Document;
import org.apache.lucene.index.DirectoryReader;
import org.apache.lucene.index.IndexReader;
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.TopDocs;
import org.apache.lucene.store.Directory;
import org.apache.lucene.store.FSDirectory;

import java.io.IOException;
import java.nio.file.Paths;

public class Searcher {
    private IndexSearcher indexSearcher;
    private QueryParser queryParser;

    public Searcher(String indexDir) throws IOException {
        Directory dir = FSDirectory.open(Paths.get(indexDir));
        Analyzer analyzer = new StandardAnalyzer();
        IndexReader indexReader = DirectoryReader.open(dir);
        indexSearcher = new IndexSearcher(indexReader);
        queryParser = new QueryParser("content", analyzer);
    }

    public ScoreDoc[] search(String queryString, int numResults) throws Exception {
        Query query = queryParser.parse(queryString);
        TopDocs topDocs = indexSearcher.search(query, numResults);
        return topDocs.scoreDocs;
    }

    public Document getDocument(int docID) throws IOException {
        return indexSearcher.doc(docID);
    }
}

In the above sample code, we use IndexSearcher to perform the search operation. Before performing a search, we need to create a Query object to represent the query to be searched, and use QueryParser to parse the query string into a Query object. We then use the search method of IndexSearcher to perform the search and return the ranking of the search results.

4. Usage example

The following is a sample code that uses the full-text retrieval and search function:

public class Main {
    public static void main(String[] args) {
        String indexDir = "/path/to/index/dir";
        
        try {
            Indexer indexer = new Indexer(indexDir);
            indexer.addDocument("Hello, world!");
            indexer.addDocument("Java is a programming language.");
            indexer.addDocument("Lucene is a full-text search engine.");
            indexer.close();

            Searcher searcher = new Searcher(indexDir);
            ScoreDoc[] results = searcher.search("Java", 10);
            for (ScoreDoc result : results) {
                Document doc = searcher.getDocument(result.doc);
                System.out.println(doc.getField("content").stringValue());
            }
        } catch (IOException e) {
            e.printStackTrace();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

In the above sample code, we first create a Indexer to create an index and add some text data. Then, we create a Searcher to perform the search and print out the text content of the search results.

Through the above sample code, we can use the Lucene library to easily implement full-text retrieval and search functions in Java. Using Lucene, we can efficiently find specific keywords or phrases in large-scale text data, thereby improving the efficiency and performance of text processing applications.

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