LangChain4j RAG Implementation Patterns

When to Use This Skill

Use this skill when:

Building knowledge-based AI applications requiring external document access
Implementing question-answering systems over large document collections
Creating AI assistants with access to company knowledge bases
Building semantic search capabilities for document repositories
Implementing chat systems that reference specific information sources
Creating AI applications requiring source attribution
Building domain-specific AI systems with curated knowledge
Implementing hybrid search combining vector similarity with traditional search
Creating AI applications requiring real-time document updates
Building multi-modal RAG systems with text, images, and other content types

Overview

Implement complete Retrieval-Augmented Generation (RAG) systems with LangChain4j. RAG enhances language models by providing relevant context from external knowledge sources, improving accuracy and reducing hallucinations.

Instructions

Initialize RAG Project

Create a new Spring Boot project with required dependencies:

pom.xml:

xml

<dependency>
    <groupId>dev.langchain4j</groupId>
    <artifactId>langchain4j-spring-boot-starter</artifactId>
    <version>1.8.0</version>
</dependency>
<dependency>
    <groupId>dev.langchain4j</groupId>
    <artifactId>langchain4j-open-ai</artifactId>
    <version>1.8.0</version>
</dependency>

Setup Document Ingestion

Configure document loading and processing:

java

@Configuration
public class RAGConfiguration {

    @Bean
    public EmbeddingModel embeddingModel() {
        return OpenAiEmbeddingModel.builder()
            .apiKey(System.getenv("OPENAI_API_KEY"))
            .modelName("text-embedding-3-small")
            .build();
    }

    @Bean
    public EmbeddingStore<TextSegment> embeddingStore() {
        return new InMemoryEmbeddingStore<>();
    }
}

Create document ingestion service:

java

@Service
@RequiredArgsConstructor
public class DocumentIngestionService {

    private final EmbeddingModel embeddingModel;
    private final EmbeddingStore<TextSegment> embeddingStore;

    public void ingestDocument(String filePath, Map<String, Object> metadata) {
        Document document = FileSystemDocumentLoader.loadDocument(filePath);
        document.metadata().putAll(metadata);

        DocumentSplitter splitter = DocumentSplitters.recursive(
            500, 50, new OpenAiTokenCountEstimator("text-embedding-3-small")
        );

        List<TextSegment> segments = splitter.split(document);
        List<Embedding> embeddings = embeddingModel.embedAll(segments).content();
        embeddingStore.addAll(embeddings, segments);
    }
}

Configure Content Retrieval

Setup content retrieval with filtering:

java

@Configuration
public class ContentRetrieverConfiguration {

    @Bean
    public ContentRetriever contentRetriever(
            EmbeddingStore<TextSegment> embeddingStore,
            EmbeddingModel embeddingModel) {

        return EmbeddingStoreContentRetriever.builder()
            .embeddingStore(embeddingStore)
            .embeddingModel(embeddingModel)
            .maxResults(5)
            .minScore(0.7)
            .build();
    }
}

Create RAG-Enabled AI Service

Define AI service with context retrieval:

java

interface KnowledgeAssistant {
    @SystemMessage("""
        You are a knowledgeable assistant with access to a comprehensive knowledge base.

        When answering questions:
        1. Use the provided context from the knowledge base
        2. If information is not in the context, clearly state this
        3. Provide accurate, helpful responses
        4. When possible, reference specific sources
        5. If the context is insufficient, ask for clarification
        """)
    String answerQuestion(String question);
}

@Service
@RequiredArgsConstructor
public class KnowledgeService {

    private final KnowledgeAssistant assistant;

    public KnowledgeService(ChatModel chatModel, ContentRetriever contentRetriever) {
        this.assistant = AiServices.builder(KnowledgeAssistant.class)
            .chatModel(chatModel)
            .contentRetriever(contentRetriever)
            .build();
    }

    public String answerQuestion(String question) {
        return assistant.answerQuestion(question);
    }
}

Examples

Basic Document Processing

java

public class BasicRAGExample {
    public static void main(String[] args) {
        var embeddingStore = new InMemoryEmbeddingStore<TextSegment>();

        var embeddingModel = OpenAiEmbeddingModel.builder()
            .apiKey(System.getenv("OPENAI_API_KEY"))
            .modelName("text-embedding-3-small")
            .build();

        var ingestor = EmbeddingStoreIngestor.builder()
            .embeddingModel(embeddingModel)
            .embeddingStore(embeddingStore)
            .build();

        ingestor.ingest(Document.from("Spring Boot is a framework for building Java applications with minimal configuration."));

        var retriever = EmbeddingStoreContentRetriever.builder()
            .embeddingStore(embeddingStore)
            .embeddingModel(embeddingModel)
            .build();
    }
}

Multi-Domain Assistant

java

interface MultiDomainAssistant {
    @SystemMessage("""
        You are an expert assistant with access to multiple knowledge domains:
        - Technical documentation
        - Company policies
        - Product information
        - Customer support guides

        Tailor your response based on the type of question and available context.
        Always indicate which domain the information comes from.
        """)
    String answerQuestion(@MemoryId String userId, String question);
}

Hierarchical RAG

java

@Service
@RequiredArgsConstructor
public class HierarchicalRAGService {

    private final EmbeddingStore<TextSegment> chunkStore;
    private final EmbeddingStore<TextSegment> summaryStore;
    private final EmbeddingModel embeddingModel;

    public String performHierarchicalRetrieval(String query) {
        List<EmbeddingMatch<TextSegment>> summaryMatches = searchSummaries(query);
        List<TextSegment> relevantChunks = new ArrayList<>();

        for (EmbeddingMatch<TextSegment> summaryMatch : summaryMatches) {
            String documentId = summaryMatch.embedded().metadata().getString("documentId");
            List<EmbeddingMatch<TextSegment>> chunkMatches = searchChunksInDocument(query, documentId);
            chunkMatches.stream()
                .map(EmbeddingMatch::embedded)
                .forEach(relevantChunks::add);
        }

        return generateResponseWithChunks(query, relevantChunks);
    }
}

Best Practices

Document Segmentation

Use recursive splitting with 500-1000 token chunks for most applications
Maintain 20-50 token overlap between chunks for context preservation
Consider document structure (headings, paragraphs) when splitting
Use token-aware splitters for optimal embedding generation

Metadata Strategy

Include rich metadata for filtering and attribution:
- User and tenant identifiers for multi-tenancy
- Document type and category classification
- Creation and modification timestamps
- Version and author information
- Confidentiality and access level tags

Query Processing

Implement query preprocessing and cleaning
Consider query expansion for better recall
Apply dynamic filtering based on user context
Use re-ranking for improved result quality

Performance Optimization

Cache embeddings for repeated queries
Use batch embedding generation for bulk operations
Implement pagination for large result sets
Consider asynchronous processing for long operations

Common Patterns

Simple RAG Pipeline

java

@RequiredArgsConstructor
@Service
public class SimpleRAGPipeline {

    private final EmbeddingModel embeddingModel;
    private final EmbeddingStore<TextSegment> embeddingStore;
    private final ChatModel chatModel;

    public String answerQuestion(String question) {
        Embedding queryEmbedding = embeddingModel.embed(question).content();
        EmbeddingSearchRequest request = EmbeddingSearchRequest.builder()
            .queryEmbedding(queryEmbedding)
            .maxResults(3)
            .build();

        List<TextSegment> segments = embeddingStore.search(request).matches().stream()
            .map(EmbeddingMatch::embedded)
            .collect(Collectors.toList());

        String context = segments.stream()
            .map(TextSegment::text)
            .collect(Collectors.joining("\n\n"));

        return chatModel.generate(context + "\n\nQuestion: " + question + "\nAnswer:");
    }
}

Hybrid Search (Vector + Keyword)

java

@Service
@RequiredArgsConstructor
public class HybridSearchService {

    private final EmbeddingStore<TextSegment> vectorStore;
    private final FullTextSearchEngine keywordEngine;
    private final EmbeddingModel embeddingModel;

    public List<Content> hybridSearch(String query, int maxResults) {
        // Vector search
        List<Content> vectorResults = performVectorSearch(query, maxResults);

        // Keyword search
        List<Content> keywordResults = performKeywordSearch(query, maxResults);

        // Combine and re-rank using RRF algorithm
        return combineResults(vectorResults, keywordResults, maxResults);
    }
}

Troubleshooting

Common Issues

Poor Retrieval Results

Check document chunk size and overlap settings
Verify embedding model compatibility
Ensure metadata filters are not too restrictive
Consider adding re-ranking step

Slow Performance

Use cached embeddings for frequent queries
Optimize database indexing for vector stores
Implement pagination for large datasets
Consider async processing for bulk operations

High Memory Usage

Use disk-based embedding stores for large datasets
Implement proper pagination and filtering
Clean up unused embeddings periodically
Monitor and optimize chunk sizes

References

API Reference - Complete API documentation and interfaces
Examples - Production-ready examples and patterns
Official LangChain4j Documentation

langchain4j-rag-implementation-patterns

NPX Install

Tags

SKILL.md Content

LangChain4j RAG Implementation Patterns

When to Use This Skill

Overview

Instructions

Initialize RAG Project

Setup Document Ingestion

Configure Content Retrieval

Create RAG-Enabled AI Service

Examples

Basic Document Processing

Multi-Domain Assistant

Hierarchical RAG

Best Practices

Document Segmentation

Metadata Strategy

Query Processing

Performance Optimization

Common Patterns

Simple RAG Pipeline

Hybrid Search (Vector + Keyword)

Troubleshooting

Common Issues

References