LangChain4j AI Services Patterns

This skill provides guidance for building declarative AI Services with LangChain4j using interface-based patterns, annotations for system and user messages, memory management, tools integration, and advanced AI application patterns that abstract away low-level LLM interactions.

Overview

LangChain4j AI Services define AI functionality using Java interfaces with annotations, providing type-safe, declarative AI with minimal boilerplate.

When to Use

Use this skill when:

• Building declarative AI services with minimal boilerplate using Java interfaces
• Creating type-safe conversational AI with memory management
• Implementing AI agents with function/tool calling capabilities
• Designing AI services returning structured data (enums, POJOs, lists)
• Integrating RAG patterns declaratively

Instructions

Follow these steps to create declarative AI Services with LangChain4j:

1. Define AI Service Interface

Create a Java interface with method signatures for AI interactions:

interface Assistant {
    String chat(String userMessage);
}

2. Add Annotations for System and User Messages

Use

@SystemMessage

and

@UserMessage

annotations to define prompts:

interface CustomerSupportBot {
    @SystemMessage("You are a helpful customer support agent for TechCorp")
    String handleInquiry(String customerMessage);

    @UserMessage("Analyze sentiment: {{it}}")
    Sentiment analyzeSentiment(String feedback);
}

3. Create AI Service Instance

Use

AiServices

builder or create to instantiate the service:

// Simple creation
Assistant assistant = AiServices.create(Assistant.class, chatModel);

// Or with builder for advanced configuration
Assistant assistant = AiServices.builder(Assistant.class)
    .chatModel(chatModel)
    .build();

4. Configure Memory for Multi-turn Conversations

Add memory management using

@MemoryId

for multi-user scenarios:

interface MultiUserAssistant {
    String chat(@MemoryId String userId, String userMessage);
}

Assistant assistant = AiServices.builder(MultiUserAssistant.class)
    .chatModel(model)
    .chatMemoryProvider(userId -> MessageWindowChatMemory.withMaxMessages(10))
    .build();

5. Integrate Tools for Function Calling

Register tools using

@Tool

annotation to enable AI function execution:

class Calculator {
    @Tool("Add two numbers") double add(double a, double b) { return a + b; }
}

interface MathGenius {
    String ask(String question);
}

MathGenius mathGenius = AiServices.builder(MathGenius.class)
    .chatModel(model)
    .tools(new Calculator())
    .build();

6. Validate and Test

Test AI services with concrete validation patterns:

// 1. Test with sample inputs
String response = assistant.chat("Hello, how are you?");
assert response != null && !response.isEmpty();

// 2. Validate structured outputs with assertions
Sentiment result = bot.analyzeSentiment("Great product!");
assert result == Sentiment.POSITIVE;

// 3. Log tool calls with side effects for audit
MathGenius math = AiServices.builder(MathGenius.class)
    .chatModel(model)
    .tools(new Calculator())
    .build();

// 4. Test memory isolation between users
String userA = assistant.chat("User A message", "session-a");
String userB = assistant.chat("User B message", "session-b");
assert !userA.equals(userB); // Verify memory isolation

Examples

See examples.md for comprehensive practical examples including:

• Basic chat interfaces
• Stateful assistants with memory
• Multi-user scenarios
• Structured output extraction
• Tool calling and function execution
• Streaming responses
• Error handling
• RAG integration
• Production patterns

API Reference

Complete API documentation, annotations, interfaces, and configuration patterns are available in references.md.

Best Practices

1. Use type-safe interfaces instead of string-based prompts
2. Implement proper memory management with appropriate limits
3. Design clear tool descriptions with parameter documentation
4. Handle errors gracefully with custom error handlers
5. Use structured output for predictable responses
6. Implement validation for user inputs
7. Monitor performance for production deployments

Dependencies

<!-- Maven -->
<dependency>
    <groupId>dev.langchain4j</groupId>
    <artifactId>langchain4j</artifactId>
    <version>1.8.0</version>
</dependency>
<dependency>
    <groupId>dev.langchain4j</groupId>
    <artifactId>langchain4j-open-ai</artifactId>
    <version>1.8.0</version>
</dependency>

// Gradle
implementation 'dev.langchain4j:langchain4j:1.8.0'
implementation 'dev.langchain4j:langchain4j-open-ai:1.8.0'

References

• LangChain4j Documentation
• LangChain4j AI Services - API References
• LangChain4j AI Services - Practical Examples

Constraints and Warnings

• AI Services rely on LLM responses which are non-deterministic; tests should account for variability.
• Memory providers store conversation history; ensure proper cleanup for multi-user scenarios.
• Tool execution can be expensive; implement rate limiting and timeout handling.
• Never pass sensitive data (API keys, passwords) in system or user messages.
• Large context windows can lead to high token costs; implement message pruning strategies.
• Streaming responses require proper error handling for partial failures.
• AI-generated outputs should be validated before use in production systems.
• Be cautious with tools that have side effects; AI models may call them unexpectedly.
• Token limits vary by model; ensure prompts and context fit within model constraints.