How do you persist ChatMemory across application restarts in LangChain4j?

LangChain4j's built-in MessageWindowChatMemory and TokenWindowChatMemory use in-memory storage — conversations vanish when the application restarts or when a new pod starts in a Kubernetes cluster. For production persistence you need a persistent ChatMemoryStore implementation.

LangChain4j defines the ChatMemoryStore interface with three methods:

public interface ChatMemoryStore {
    List<ChatMessage> getMessages(Object memoryId);
    void updateMessages(Object memoryId, List<ChatMessage> messages);
    void deleteMessages(Object memoryId);
}

You implement this against any persistence backend and plug it into the memory configuration:

// Redis-backed implementation example
@Component
class RedisChatMemoryStore implements ChatMemoryStore {
    private final RedisTemplate<String, String> redis;
    private final ObjectMapper mapper;

    @Override
    public List<ChatMessage> getMessages(Object memoryId) {
        String json = redis.opsForValue().get("chat:" + memoryId);
        if (json == null) return new ArrayList<>();
        return mapper.readValue(json, new TypeReference<>(){});
    }

    @Override
    public void updateMessages(Object memoryId, List<ChatMessage> messages) {
        redis.opsForValue().set("chat:" + memoryId,
            mapper.writeValueAsString(messages),
            Duration.ofHours(24));
    }

    @Override
    public void deleteMessages(Object memoryId) {
        redis.delete("chat:" + memoryId);
    }
}

// Wire into memory
ChatMemoryProvider provider = memoryId ->
    MessageWindowChatMemory.builder()
        .id(memoryId)
        .maxMessages(20)
        .chatMemoryStore(redisChatMemoryStore)
        .build();

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