How do you implement multi-turn conversation with memory per user in a Spring REST API using LangChain4j?

// 1. AI Services interface with per-user memory
interface ChatAssistant {
    String chat(@MemoryId String userId, @UserMessage String message);
}

// 2. In-memory store for development (switch to Redis/DB for production)
Map<String, ChatMemory> memoryMap = new ConcurrentHashMap<>();

ChatMemoryProvider memoryProvider = memoryId ->
    memoryMap.computeIfAbsent(memoryId.toString(), id ->
        MessageWindowChatMemory.withMaxMessages(20));

// 3. Build the AI Service with the provider
ChatAssistant assistant = AiServices.builder(ChatAssistant.class)
    .chatLanguageModel(model)
    .chatMemoryProvider(memoryProvider)
    .build();

// 4. Spring REST controller
@RestController
@RequestMapping("/api/chat")
class ChatController {
    private final ChatAssistant assistant;

    ChatController(ChatAssistant assistant) {
        this.assistant = assistant;
    }

    @PostMapping("/{userId}")
    String chat(@PathVariable String userId, @RequestBody String message) {
        return assistant.chat(userId, message); // each user gets isolated memory
    }
}