Spring
Setter vs Constructor Injection
Spring supports two types of dependency Injection, using setter method
e.g. setXXX() where XXX is dependency or via constructor argument. First
way of dependency injection is known
as setter injection while later is known as constructor injection.
Both approaches of Injecting dependency on Spring bean has there pros and
cons, which we will see in this Spring framework article. Difference between
Setter Injection and Constructor Injection in Spring is also a popular Spring framework interview question.Some time interviewer also ask as When do you use Setter Injection over Constructor
injection in Spring or simply benefits of using setter vs constructor injection
in Spring framework. Points discussed in this article not only help you to
understand Setter vs Constructor Injection but also Spring's dependency
Injection process. By the way if you are new in Spring framework and learning it, you may want to take a look at my list of 5 good books to learn Spring framework. That will certainly help on your learning process. Since Spring is now a must have skill for Java programmers, it worth putting time and effort to learn this powerful framework
Difference between Setter and Constructor
Injection in Spring framework
As I said earlier Spring supports both setter and constructor Injectionwhich are two standard way of injecting dependency on beans managed by IOC
constructor. Spring framework doesn't support Interface Injection on which
dependency is injected by implementing a particular interface. In this section we
will see couple of difference between setter and constructor Injection, which
will help you decide when to use setter Injection over constructor Injection in
Spring and vice-versa.
1) Fundamental difference between setter and constructor injection, as
there name implies is How dependency is injected. Setter injection in Spring uses setter methods
like setDependency() to inject dependency on any bean managed by
Spring's IOC container. On the other hand constructor injection uses constructor to inject
dependency on any Spring managed bean.
2) Because of using setter method, setter Injection in more readable than
constructor injection in Spring configuration file usually applicationContext.xml . Since
setter method has name e.g. setReporotService() by reading
Spring XML config file you know which
dependency you are setting. While in constructor injection, since it uses index
to inject dependency, its not as readable as setter injection and you need to
refer either Java documentation or code to find which index corresponds to which
property.
3) Another difference between setter vs constructor injection in Spring
and one of the drawback of setter injection
is that it does not ensures dependency Injection. You can
not guarantee that certain dependency is injected or not, which means you may
have an object with incomplete dependency. On other hand constructor Injection
does not allow you to construct object, until your dependencies are ready.
4) One more drawback of setter Injection is Security. By using setter
injection, you can override certain dependency
which is not possible which is not possible with constructor injection because
every time you call constructor, a new object is gets created.
5) One of our reader Murali Mohan Reddy, pointed out one more difference between Setter and Constructor injection in Spring, where later can help, if there is a circular dependency between two object A and B.
If Object A and B are dependent each other i.e A is depends ob B and
vice-versa. Spring throws ObjectCurrentlyInCreationException while
creating objects of A and B bcz A object cannot be created until B is
created and vice-versa. So spring can resolve circular dependencies
through setter-injection. Objects constructed before setter methods
invoked.
When
to use Setter Injection over Constructor Injection in Spring
Setter Injection has upper hand over Constructor Injection in terms of
readability. Since for configuring Spring we use XML files, readability is much
bigger concern. Also drawback of setter Injection around ensuring mandatory
dependency injected or not can be handled by configuring Spring to check
dependency using "dependency-check" attribute
of
Another worth noting point to remember while comparing Setter Injection vs
Constructor Injection is that, once number of dependency crossed a threshold
e.g. 5 or 6 its handy manageable to passing dependency via constructor. Setter
Injection is preferred choice when number of dependency to be injected is lot
more than normal, if some of those arguments is optional than using Builder design pattern is also
a good option.
In Summary both Setter Injection and Constructor Injection has there own
advantage and disadvantage. Good thing about Spring is that it doesn't restrict
you to use either Setter Injection or Constructor Injection and you are free to
use both of them in one Spring configuration file. Use Setter injection when
number of dependency is more or you need readability. Use Constructor Injection
when Object must be created with all of its dependency.
Further Reading on Spring Framework
Spring Recipes: A Problem-Solution Approach By Gary Mak
Expert Spring MVC and Web Flow By Seth Ladd, Darren Davison, Steven Devijver, and Colin Yates
Other Spring tutorials from Javarevisited Blog
Source:http://javarevisited.blogspot.com/2012/11/difference-between-setter-injection-vs-constructor-injection-spring-framework.html
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