# Lucene 10.3.2 core API

Apache Lucene is a high-performance, full-featured search engine library. It supports structured search, full-text search, faceting, nearest-neighbor search across high-dimensionality vectors, spell correction or query suggestions. Here's a simple example how to use Lucene for indexing and searching (using JUnit to check if the results are what we expect):

        Analyzer analyzer = new StandardAnalyzer();

        Path indexPath = Files.createTempDirectory("tempIndex");
        try (Directory directory = FSDirectory.open(indexPath)) {
          IndexWriterConfig config = new IndexWriterConfig(analyzer);
          try (IndexWriter iwriter = new IndexWriter(directory, config)) {
            Document doc = new Document();
            String text = "This is the text to be indexed.";
            doc.add(new Field("fieldname", text, TextField.TYPE_STORED));
            iwriter.addDocument(doc);
          }

          // Now search the index:
          try (DirectoryReader ireader = DirectoryReader.open(directory)) {
            IndexSearcher isearcher = new IndexSearcher(ireader);
            // Parse a simple query that searches for "text":
            QueryParser parser = new QueryParser("fieldname", analyzer);
            Query query = parser.parse("text");
            ScoreDoc[] hits = isearcher.search(query, 10).scoreDocs;
            assertEquals(1, hits.length);
            // Iterate through the results:
            StoredFields storedFields = isearcher.storedFields();
            for (int i = 0; i < hits.length; i++) {
              Document hitDoc = storedFields.document(hits[i].doc);
              assertEquals("This is the text to be indexed.", hitDoc.get("fieldname"));
            }
          }
        } finally {
          IOUtils.rm(indexPath);
        }

The Lucene API is divided into several packages:

- `org.apache.lucene.analysis` defines an abstract `Analyzer` API for converting text from a `String` or `Reader` into a `TokenStream`, an enumeration of token `Attribute`s. A TokenStream can be composed by applying `TokenFilter`s to the output of a `Tokenizer`. Tokenizers and TokenFilters are strung together and applied with an `Analyzer`. analysis-common provides a number of Analyzer implementations, including StopAnalyzer and the grammar-based StandardAnalyzer.
- `org.apache.lucene.codecs` provides an abstraction over the encoding and decoding of the inverted index structure, as well as different implementations that can be chosen depending upon application needs.
- `org.apache.lucene.document` provides a simple `Document` class. A Document is simply a set of named `Field`s, whose values may be numbers, strings or instances of `Reader`.
- `org.apache.lucene.index` provides two primary classes: `IndexWriter`, which creates and adds documents to indices; and `IndexReader`, which accesses the data in the index.
- `org.apache.lucene.search` provides data structures to represent queries (ie `TermQuery` for individual words, `PhraseQuery` for phrases, and `BooleanQuery` for boolean combinations of queries) and the `IndexSearcher` which turns queries into `TopDocs`. A number of QueryParsers are provided for producing query structures from strings or xml.
- `org.apache.lucene.store` defines an abstract class for storing persistent data, the `Directory`, which is a collection of named files written by an `IndexOutput` and read by an `IndexInput`. Multiple implementations are provided, but `FSDirectory` is generally recommended as it tries to use operating system disk buffer caches efficiently.
- `org.apache.lucene.util` contains a few handy data structures and util classes, ie `FixedBitSet` and `PriorityQueue`.

To use Lucene, an application should:

1. Create `Document`s by adding `Field`s;
2. Create an `IndexWriter` and add documents to it with `addDocument()`;
3. Call QueryParser.parse() to build a query from a string; and
4. Create an `IndexSearcher` and pass the query to its `search()` method.

Some simple examples of code which does this are:

- IndexFiles.java creates an index for all the files contained in a directory.
- SearchFiles.java prompts for queries and searches an index.

To demonstrate these, try something like:

> `> java -cp lucene-core.jar:lucene-demo.jar:lucene-analysis-common.jar org.apache.lucene.demo.IndexFiles -index index -docs rec.food.recipes/soups`
> `adding rec.food.recipes/soups/abalone-chowder`
> `` [ ... ]

> `> java -cp lucene-core.jar:lucene-demo.jar:lucene-queryparser.jar:lucene-analysis-common.jar org.apache.lucene.demo.SearchFiles`
> `Query: chowder`
> `Searching for: chowder`
> `34 total matching documents`
> `1. rec.food.recipes/soups/spam-chowder`
> `` [ ... thirty-four documents contain the word "chowder" ... ]

> `Query: "clam chowder" AND Manhattan`
> `Searching for: +"clam chowder" +manhattan`
> `2 total matching documents`
> `1. rec.food.recipes/soups/clam-chowder`
> `` [ ... two documents contain the phrase "clam chowder" and the word "manhattan" ... ]
> [ Note: "+" and "-" are canonical, but "AND", "OR" and "NOT" may be used. ]

Packages
Package
Description
org.apache.lucene.analysis
Text analysis.
org.apache.lucene.analysis.standard
Fast, general-purpose grammar-based tokenizer `StandardTokenizer` implements the Word Break rules from the Unicode Text Segmentation algorithm, as specified in Unicode Standard Annex #29.
org.apache.lucene.analysis.tokenattributes
General-purpose attributes for text analysis.
org.apache.lucene.codecs
Codecs API: API for customization of the encoding and structure of the index.
org.apache.lucene.codecs.compressing
Compressing helper classes.
org.apache.lucene.codecs.hnsw
HNSW vector helper classes.
org.apache.lucene.codecs.lucene102
Lucene 10.2 file format.
org.apache.lucene.codecs.lucene103
Lucene 10.3 file format.
org.apache.lucene.codecs.lucene103.blocktree
BlockTree terms dictionary.
org.apache.lucene.codecs.lucene90
Lucene 9.0 file format.
org.apache.lucene.codecs.lucene90.compressing
Lucene 9.0 compressing format.
org.apache.lucene.codecs.lucene94
Lucene 9.4 file format.
org.apache.lucene.codecs.lucene95
Lucene 9.5 file format.
org.apache.lucene.codecs.lucene99
Lucene 9.9 file format.
org.apache.lucene.codecs.perfield
Postings format that can delegate to different formats per-field.
org.apache.lucene.document
The logical representation of a `Document` for indexing and searching.
org.apache.lucene.geo
Core geo-spatial types and utilities for handling geographical data and spatial operations in Lucene.
org.apache.lucene.index
Code to maintain and access indices.
org.apache.lucene.internal.hppc
Internal copy of a subset of classes from the HPPC library.
org.apache.lucene.internal.tests
Internal bridges to package-private internals, for use by the lucene test framework only.
org.apache.lucene.internal.vectorization
Internal implementations to support SIMD vectorization.
org.apache.lucene.search
Code to search indices.
org.apache.lucene.search.comparators
Comparators, used to compare hits so as to determine their sort order when collecting the top results with `TopFieldCollector`.
org.apache.lucene.search.knn
Classes related to vector search: knn and vector fields.
org.apache.lucene.search.similarities
This package contains the various ranking models that can be used in Lucene.
org.apache.lucene.store
Binary i/o API, used for all index data.
org.apache.lucene.util
Some utility classes.
org.apache.lucene.util.automaton
Finite-state automaton for regular expressions.
org.apache.lucene.util.bkd
Block KD-tree, implementing the generic spatial data structure described in this paper.
org.apache.lucene.util.compress
Compression utilities.
org.apache.lucene.util.fst
Finite state transducers
org.apache.lucene.util.graph
Utility classes for working with token streams as graphs.
org.apache.lucene.util.hnsw
Navigable Small-World graph, nominally Hierarchical but currently only has a single layer.
org.apache.lucene.util.mutable
Comparable object wrappers
org.apache.lucene.util.packed
Packed integer arrays and streams.
org.apache.lucene.util.quantization
Provides quantization methods for scaling vector values to smaller data types and possibly fewer dimensions