There is a quite good explanation written by the Spring team in their documentation, where it says:
Constructor-based or setter-based DI?
Since you can mix constructor-based and setter-based DI, it is a good
rule of thumb to use constructors for mandatory dependencies and
setter methods or configuration methods for optional dependencies.
Note that use of the @Required annotation on a setter method can be
used to make the property be a required dependency; however,
constructor injection with programmatic validation of arguments is
preferable.
The Spring team generally advocates constructor injection, as it lets
you implement application components as immutable objects and ensures
that required dependencies are not null. Furthermore,
constructor-injected components are always returned to the client
(calling) code in a fully initialized state. As a side note, a large
number of constructor arguments is a bad code smell, implying that the
class likely has too many responsibilities and should be refactored to
better address proper separation of concerns.
Setter injection should primarily only be used for optional
dependencies that can be assigned reasonable default values within the
class. Otherwise, not-null checks must be performed everywhere the
code uses the dependency. One benefit of setter injection is that
setter methods make objects of that class amenable to reconfiguration
or re-injection later. Management through JMX MBeans is therefore a
compelling use case for setter injection.
Use the DI style that makes the most sense for a particular class.
Sometimes, when dealing with third-party classes for which you do not
have the source, the choice is made for you. For example, if a
third-party class does not expose any setter methods, then constructor
injection may be the only available form of DI.
So in general it is recommended to use constructor injection for mandatory fields and setter injection for optional fields, but be careful, because if you are using setter injection, you renounce the goodness of immutables. What is discouraged is definetely field injection, as it needs to use java reflection mechanism to populate your fields, which has this consequence that you couple your code very tightly with spring's mechanisms and also it makes your unit tests much harder, as you cannot so easilly pass a mock object via a constructor or setter.
