ProgrammingPages

Dependency injection and a service locator aren't necessarily mutually exclusive concepts.

With service locator the application class asks for it explicitly by a message to the locator. With injection there is no explicit request, the service appears in the application class - hence the inversion of control.

with a Service Locator every user of a service has a dependency to the locator. The locator can hide dependencies to other implementations, but you do need to see the locator.

In this kind of scenario I don't see the injector's inversion as providing anything compelling.The difference comes if the lister is a component that I'm providing to an application that other people are writing. In this case I don't know much about the APIs of the service locators that my customers are going to use. Each customer might have their own incompatible service locators

Since with an injector you don't have a dependency from a component to the injector, the component cannot obtain further services from the injector once it's been configured.

frameworks should minimize their impact upon application code, and particularly should not do things that slow down the edit-execute cycle. Using plugins to substitute heavyweight components does a lot help this process, which is vital for practices such as Test Driven Development.

a more general issue with object-oriented programming - should you fill fields in a constructor or with setters.

Another advantage with constructor initialization is that it allows you to clearly hide any fields that are immutable by simply not providing a setter. I think this is important - if something shouldn't change then the lack of a setter communicates this very well

The problem with classic Factory Methods for components assembly is that they are usually seen as static methods, and you can't have those on interfaces. You can make a factory class, but then that just becomes another service instance. A factory service is often a good tactic, but you still have to instantiate the factory using one of the techniques here

Factory Methods

start with constructor injection, but be ready to switch to setter injection

One case is where you have a simple application that's not got a lot of deployment variation. In this case a bit of code can be clearer than separate XML file.

configuration files or code on an API to wire up services

the Java world at the moment is a cacophony of configuration files,

using aspect oriented ideas with these containers

When building application classes the two are roughly equivalent, but I think Service Locator has a slight edge due to its more straightforward behavior. However if you are building classes to used in multiple applications then Dependency Injection is a better choice.

POJO-based programming model

EJB interception