Explain why constructor inject is better than other options

java spring dependency-injection required
minil picture minil · Jan 19, 2014 · Viewed 119.9k times · Source

In a Pro Spring 3 Book, Chapter 4 - Introduction IOC and DI in Spring - Page 59, In "Setter Injection vs. Constructor Injection" section, a paragraph says

Spring included, provide a mechanism for ensuring that all dependencies are defined when you use Setter Injection, but by using Constructor Injection, you assert the requirement for the dependency in a container-agnostic manner"

Could you explain with examples

Answer

Emerson Farrugia picture Emerson Farrugia · Jan 19, 2014

A class that takes a required dependency as a constructor argument can only be instantiated if that argument is provided (you should have a guard clause to make sure the argument is not null.) A constructor therefore enforces the dependency requirement whether or not you're using Spring, making it container-agnostic.

If you use setter injection, the setter may or may not be called, so the instance may never be provided with its dependency. The only way to force the setter to be called is using @Required or @Autowired , which is specific to Spring and is therefore not container-agnostic.

So to keep your code independent of Spring, use constructor arguments for injection.

Update: Spring 4.3 will perform implicit injection in single-constructor scenarios, making your code more independent of Spring by potentially not requiring an @Autowired annotation at all.

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