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 exploring Spring API you may find my post on using Spring stopwatch to calculate time difference and Spring’s HtmlUtils to escape HTML special character in Java useful and informative.
Difference between Setter and Constructor Injection in Spring framework
As I said earlier Spring supports both setter and constructor Injection which 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.
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
tag or tag.
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.
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