Integrating Google Guice in AWS Lambda

Question

I'm trying to integrate Google Guice in AWS Lambda but for some reasons, injection is not working well. It give me null whenever i try to call

Handler Code:

public class FirstLamdba implements RequestHandler<Request, Object>{   

        private UserService userService;

        @Inject
        public void seUserService(UserService userService) {
            this.userService = userService;
        }

        public Object handleRequest(Request request, Context context){

            userService.persistData();
}

UserService

public interface UserService {
    List<String> persistData();
}

UserServiceImpl

public class UserServiceImpl implements UserService{


    @Override
    public List<String> persistData() {

        System.out.println("***Working*********");
}

Binding Class:

public class MessageGuiceModule extends AbstractModule
{
  protected void configure() {

    bind(UserService.class).to(UserServiceImpl.class);
  }
}

Test Class:

 @Test
        public void testLambdaFunctionHandler() {
Request request = new Request();
            request.setName("Name");
            FirstLamdba handler = new FirstLamdba();
            Context ctx = createContext();

            Object output = handler.handleRequest(request, ctx);

            // TODO: validate output here if needed.
            if (output != null) {
                System.out.println(output.toString());
            }
        }

For some reasons, UserService userService is sets as null in FirstLamdba.

Any idea?

Answer 1

The first time a lambda function is invoked, the environment will be created.

public class FirstLamdba implements RequestHandler<Request, Object>{   

        Injector injector = Guice.createInjector(new MessageGuiceModule());
        private UserService userService = injector.getInstance(UserService.class);


        //setter for testing purpose
        public void setUserService(UserService userService) {
            this.userService = userService;
        }

        public Object handleRequest(Request request, Context context){

            userService.persistData();
}


@Test
public void testLambdaFunctionHandler() {
        Request request = new Request();
        request.setName("Name");
        FirstLamdba handler = new FirstLamdba();
        handler.setUserService(mockUserService);

        Context ctx = createContext();

        Object output = handler.handleRequest(request, ctx);

        // TODO: validate output here if needed.
        if (output != null) {
            System.out.println(output.toString());
        }
}

Answer 2

The Lambda RequestHandler instance is not instrumented with Guice, so using @Inject directly inside the RequestHandler class is not going to work. This is why you your userService property is always null.

I haven't tried using Guice with Lambda, but I believe you will have to explicitly call Guice.createInjector() at some point in order to bootstrap Guice dependency injection.

In general when developing AWS Lambda functions I recommend starting with a POJO that does things like bootstrapping your libraries, and exposes a single high-level method like persistUser() that you can easily run and test independently from any Lambda specific code. Once you get that working your Lambda function would simply be a few lines of code that instantiates an instance of this POJO and calls the persistUser() method.