可以在 Kotlin/Java 中创建 AWS lambda 处理程序而不实现 RequestHandler 吗？

Question

TLDR: In CDK, a the lambda handler is a method reference, so for Java/Kotlin is it required that you implement the RequestHandler class?

The typical approach for defining a lambda in Java/Kotlin is to extend the request handler class and override the handleRequest function. However, when you have many similar functions in Java, this results in many similar files/class definitions.

The recommended definition looks like this:

public class Handler implements RequestHandler<Event, String>{

  @Override
  public String handleRequest(Event event, Context context)
  {
      ...
  }
}

And the simplest I can come up with in Kotlin is naturally not a significant improvement. It looks like this:

class Handler : RequestHandler<Event, String> {
    override fun handleRequest(request: Event, context: Context) : String {
       ...
    }
}

In CDK, a the lambda handler is a method reference, so is the "RequestHandler" implementation necessary? This is really the key question, putting this at the top.

Answer 1

In Java the simplest Lambda handler looks like:

public class SampleHandler implements RequestStreamHandler {

    public void handleRequest(InputStream inputStream,
                              OutputStream outputStream,
                              Context context) throws IOException {
        context.getLogger().log("in the Lambda handler");
    }
}

within the handleRequest method you will have to read from the inputStream . One example could be:

ObjectMapper mapper = new ObjectMapper();

YourObject yourObject = mapper.readValue(inputStream, YourObject.class);

Now you have the object that Lambda would normally deserialize for you. To "return" something you just need to write to the outputStream :

OutputStreamWriter writer = new OutputStreamWriter(outputStream, StandardCharsets.UTF_8);
writer.write("{\"result\":\"everything is awesome\"}");

writer.close();

Of course you could also use something like mapper.writeValue(outputStream, yourResultObject) to not do the JSON serialization yourself.

Answer 2

You don't have to implement the RequestHandler interface. According to the official documentation , you just need to tell AWS which function to invoke if your class doesn't implement RequestHandler interface:

example.Handler::handleRequest

There's nothing special about defining such a function in a CDK application:

Function handlerFunction = Function.Builder.create(this, "ExampleHandlerFunction")
        .functionName("example-handler-function")
        .handler("example.Handler::handleRequest")
        .runtime(Runtime.JAVA_11)
        .code(Code.fromAsset("path/to/the/jar"))
        .build();