LangChain4j Cheat Sheet

LangChain4j essentials in one page: AiServices, chat models, memory, embedding stores, retrievers and tools, with the annotations and builders you need.

AI Services

Declare an interface
The implementation is generated against the model.
```
interface Assistant { String chat(String message); }
```

Build it

Wire the model, memory, tools and retriever.

AiServices.builder(Assistant.class).chatModel(model).build();

@SystemMessage
Static role instructions for every call.
```
@SystemMessage("You are a Java expert.")
```

@UserMessage

Template with {{it}} or named @V parameters.

@UserMessage("Summarise: {{text}}") String summarise(@V("text") String text);

Typed return
Return a POJO, enum, List or boolean and it is parsed for you.
```
Sentiment analyse(String review);  // enum Sentiment
```

Memory

Message window
Keep the last N messages — simple and predictable.
```
MessageWindowChatMemory.withMaxMessages(20)
```
Token window
Bound memory by tokens instead of message count.
```
TokenWindowChatMemory.withMaxTokens(2000, tokenizer)
```

Per user

Memory keyed by a @MemoryId argument.

.chatMemoryProvider(id -> MessageWindowChatMemory.withMaxMessages(20))

Retrieval

Load documents

Filesystem, URL, S3, GitHub — plus Tika for mixed formats.

FileSystemDocumentLoader.loadDocuments(dir, new ApacheTikaDocumentParser());

Split
Recursive splitter with overlap preserves sentence boundaries.
```
DocumentSplitters.recursive(600, 120)
```

Ingest

Split, embed and store in one pipeline.

EmbeddingStoreIngestor.builder().documentSplitter(splitter).embeddingModel(m).embeddingStore(s).build().ingest(docs);

Retriever

Attach to an AI Service to get RAG automatically.

EmbeddingStoreContentRetriever.builder().embeddingStore(s).maxResults(5).minScore(0.6).build();

Tools

Define

Annotate a method; the description drives model selection.

@Tool("Adds two numbers") int add(int a, int b) { return a + b; }

Pass the object holding the tool methods.

AiServices.builder(Assistant.class).tools(new Calculator()).build();

Guard the loop
Cap tool iterations so a confused agent cannot spin.
```
.maxSequentialToolsInvocations(5)
```