In this Article, we will see how to use Guice Dependency injection when working with existing application or service.
Let’s assume we have a service class.
public interface BookService {
List<Book> getBookMetadata();
}
and its implementation
public class BookServiceImpl implements BookService{
@Override
public List<Book> getBookMetadata() {
// return list of Book object
}
Binding Class used for the above service
public class ApplicationModule extends AbstractModule
{
protected void configure() {
bind(BookService.class).to(BookServiceImpl.class);
}
}
We need to create a Lambda which gets the book metadata from the BookService when triggered by S3Event.
public class TestLambda implements RequestHandler<S3Event, Object> {
private BookService bookService;
@injest
public TestLambda(BookService bookservice) { // error
this.bookService = bookservice;
}
public Object handleRequest(S3Event request, Context context) {
List<Book> booklist = bookService.getBookMetadata();
...
}
}
The above code gives the following error no public zero-argument constructor as the lambda expects constructor without any argument.
Instead you can use add the following to your Lambda class to use Guice dependency injection
Injector injector = Guice.createInjector(new ApplicationModule());
bookService = injector.getInstance(BookService.class);
Final Lambda code looks like the following
public class TestLambda implements RequestHandler<S3Event, Object> {
private BookService bookService;
public TestLambda(BookService bookservice) {
Injector injector = Guice.createInjector(new ApplicationModule());
this.bookService = injector.getInstance(BookService.class);
}
public Object handleRequest(S3Event request, Context context) {
List<Book> booklist = bookService.getBookMetadata();
...
}
}