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.
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