Constructor versus setter injection

Originally, in XML-based application contexts, we used the constructor injection with mandatory dependencies and the setter injection with nonmandatory dependencies.

However, an important thing to note is that when we use @Autowired on a field or a method, the dependency is required by default. If no candidates are available for an @Autowired field, autowiring fails and throws an exception. So, the choice is not so clear anymore with Java application contexts.

Using the setter injection results in the state of the object changing during the creation. For fans of immutable objects, the constructor injection might be the way to go. Using the setter injection might sometimes hide the fact that a class has a lot of dependencies. Using the constructor injection makes it obvious, since the size of the constructor increases.