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AI / LangChain4j interview questions

How do you test LangChain4j AI Services without making real LLM API calls?

Testing AI Services without hitting real LLM endpoints is essential for fast, cost-free, deterministic unit tests. LangChain4j supports this through mock model implementations and the AiServices builder accepting any ChatLanguageModel — including test doubles you create yourself.

The most direct approach is to implement a simple mock that returns predetermined responses:

// Simple lambda mock
ChatLanguageModel mockModel = (messages, toolSpecifications) ->
    new AiMessage("The capital of France is Paris.");

GeographyAssistant assistant = AiServices.builder(GeographyAssistant.class)
    .chatLanguageModel(mockModel)
    .build();

String answer = assistant.ask("What is the capital of France?");
assertThat(answer).isEqualTo("The capital of France is Paris.");

For more complex scenarios, Mockito works naturally since ChatLanguageModel is an interface:

@ExtendWith(MockitoExtension.class)
class TranslatorTest {

    @Mock
    ChatLanguageModel mockModel;

    @Test
    void translatesText() {
        AiMessage fakeResponse = new AiMessage("Bonjour le monde");
        when(mockModel.generate(anyList())).thenReturn(new Response<>(fakeResponse));

        Translator translator = AiServices.builder(Translator.class)
            .chatLanguageModel(mockModel).build();

        assertThat(translator.translate("Hello world", "French"))
            .isEqualTo("Bonjour le monde");
    }
}

For integration tests that require a real LLM but want cost control, use Ollama with a small local model (e.g., tinyllama) via Testcontainers. This gives you real model behavior without OpenAI billing and can run in CI pipelines. The langchain4j-ollama module combined with the Testcontainers Ollama image enables fully automated integration test suites with no API keys required.

What is the simplest way to create a mock ChatLanguageModel for LangChain4j unit tests?
Which tool lets you run real LLM integration tests in CI without OpenAI API keys or billing?

More Related questions...

What is LangChain4j and what problem does it solve for Java developers? What are the core modules of LangChain4j? What is the AI Services feature in LangChain4j and how do you define one? How does ChatMemory work in LangChain4j and what types are available? What is Retrieval-Augmented Generation (RAG) in LangChain4j and how do you build a pipeline? What are Tools in LangChain4j and how does tool calling work? How do you integrate LangChain4j with Spring Boot? What is the EmbeddingModel in LangChain4j and which providers are supported? What EmbeddingStores does LangChain4j support and how do you choose one? What is document splitting in LangChain4j and why is it necessary? What is the @SystemMessage and @UserMessage annotation in LangChain4j AI Services? How does streaming work in LangChain4j and when should you use it? What is the ContentRetriever and RetrievalAugmentor in LangChain4j advanced RAG? How does LangChain4j handle structured output from LLMs? What is the PromptTemplate in LangChain4j and how does it differ from @UserMessage? What LLM providers does LangChain4j support and how do you switch between them? What is an Agent in LangChain4j and how does it differ from a simple AI Services call? How do you implement multi-turn conversation with memory per user in a Spring REST API using LangChain4j? What is the ImageModel in LangChain4j and which providers support image generation? How do you handle errors and retries in LangChain4j? How do you test LangChain4j AI Services without making real LLM API calls? What is the DocumentLoader API in LangChain4j and what sources does it support? What is the @Moderate annotation in LangChain4j and how does content moderation work? How does LangChain4j support vision (multi-modal) LLMs that accept images as input? What is the difference between synchronous and asynchronous execution in LangChain4j? What is LangChain4j's support for Quarkus and how does it differ from Spring Boot integration? How does LangChain4j implement the ReAct agent pattern and what are its limitations? What is the ModerationModel interface in LangChain4j and how can you implement a custom one? What is the Tokenizer interface in LangChain4j and why does it matter for memory management? How do you persist ChatMemory across application restarts in LangChain4j? What are the best practices for prompt engineering within LangChain4j AI Services? How does LangChain4j integrate with observability tools like OpenTelemetry? What is the InMemoryEmbeddingStore and when should you migrate to a real vector database? What are common LangChain4j anti-patterns to avoid in production applications? How does LangChain4j support multi-modal input processing for audio or documents beyond text and images? How do you implement a custom Tool with complex parameter types in LangChain4j? What is the HypotheticalDocumentEmbedder (HyDE) technique and how does LangChain4j support it? How do you handle LLM output parsing failures gracefully in LangChain4j? What is LangChain4j's support for graph-based RAG or knowledge graph integration? What is the LangChain4j EvaluationResult API and how do you measure RAG pipeline quality?
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