Langchain4j

LangChain4j is an open source Java library for building applications on large language models. It gives Java developers a unified API to many commercial and open source LLMs and vector stores, so applications can switch providers without rewriting their integration code. The library covers low level pieces such as prompt templating, chat memory management and output parsing, and higher level patterns such as agents and retrieval augmented generation. It offers integrations with Quarkus, Spring Boot and Helidon. Integration runs in both directions: Java code can call LLMs, and LLMs can call back into Java code as tools. It is aimed at Java developers who want to add chatbots, assistants or agent workflows to existing applications while staying in the Java ecosystem. The project documentation lists integrations for chat models, embedding models and vector stores, so the same application code can run against different providers. It also describes tool calling, where the model decides when to invoke a Java method and receives the result, and retrieval augmented generation, where application data is retrieved and supplied to the model with the prompt. The documentation site publishes no pricing, and the project is presented as open source software.