Spring Dependency Injection Styles – Why I love Java based configuration
blog.codecentric.de
blog.codecentric.de
Spring supports two methods of dependency injection: constructor injection and setter injection. Guice (in comparison) favours constructor injection. I prefer constructor injection too. It means objects can't be instatiated in an invalid state (assuming you enforce preconditions) whereas with setter injection you have to rely on lifecycle methods (eg @PostConstruct) to enforce that.
With Java-based Spring configuration you create instances rather than define dependencies in terms of classes (as Guice does it). This is a problem for constructor injection but not for setter injection.
This means when you create an instance to satisfy a dependency you need to explicitly supply all the constructor arguments. Spring would otherwise figure out setter injection.
With Guice you associate classes instead so you don't need to explicitly call a constructor with all the arguments. For Java-based configuration IMHO Guice is much better in this regard.
Also, Java-based Spring configuration relies on the cglib library, which may or may not be a problem in certain environments (eg its not officially supported on AppEngine but in my experience works fine for this).
So if you like constructor injection you have two options: component scanning or XML.
Component scanning is slow. Too slow. For even a small number of objects (<100) I've reduced startup times from ~30 seconds to ~10 seconds just by switching to explicit construction.
XML is really the best choice here. One thing you may not realize is you don't need to specify all the dependencies in XML (setter or constructor). You simply do a definition like:
<bean class="com.example.Foo" />
public class Foo {
@Inject
public class Foo(Bar bar, AnotherBar anotherBar) { ... }
}
IMHO this is far better than Spring's Java-based configuration.
EDIT: to clarify, @Inject here is JSR330 DI annotations [1] so java.named.Inject not com.google.inject.Inject. As that page notes Guice now supports either form.
I only half agree that this is a limitation. I use as little component scanning as possible. I think it's better to scan for the configuration classes (hopefully there aren't many) and explicitly define the rest. I like this for two reasons, first for speed and second because everything is defined in a fairly central place. I can't tell you how many times I've been surprised by annotation based configuration.
Personally I like Spring's Java Configuration over Guice's. It's more flexible for my needs. Although I suppose that an argument could be made that that's the problem: configuration over convention.
I'm glad you mentioned the Guice style @inject is available with Spring now. I wasn't aware of that.
So, approaches may vary.
All the rest is flavor we could discuss about for hours without getting to an end, but Spring's Java based configuration definitely doesn't have a problem with constructor injection.
Regards, Tobias
The difference is mainly that Guice tries to be a library and Spring tries to be a framework. I, for one, like knowing that I'm not pulling in "the world" just to get dependency injection.
Guice at its core does one thing, and one thing well: dependency injection (and by extension, AOP). It does dependency injection far better than any Spring version I've used (see: constructor injection).
Well yeah, several.
You don't have to depend on a huge API (spring-core and everything else needed for spring-di) just to get one piece of functionality.
You can avoid using XML as application logic.
You can avoid singletons-as-default-behavior and mutable beans all over the place.
It will likely be faster.
>In software engineering, Inversion of Control (IoC) is an object-oriented programming practice where the object coupling is bound at run time by an assembler object and is typically not known at compile time using static analysis.
One is using @Autowired, @Component, and friends to build up your context. One could argue this violates IoC, although I disagree, but that's actually not the interesting meaning.
The interesting meaning is the use of @Configuration and @Bean to replace XML-driven config with Java-driven config. Basically you can replace a Spring XML file with a Java class that creates the same beans and assembles them in the same way, but in cases where you need to do something awkward to create a Bean instance that would be trivial in Java, you just do the trivial thing in Java.
So you haven't lost any of the abstractness provided by IoC. Bean classes don't change at all. It's simply how they're assembled that changes. Instead of writing a giant set of XML files to create a giant set of POJOs and wire them together, you write a similar set of Java classes and have one less language to deal with.
This is how you would do it without spring. I suppose there must be an advantage of using @Configuration and @Bean instead of plain java, but what is it?