Question

I found many examples about setter dependency injection for class member variables, but found it difficult to find any examples about setter dependency injection for local variables inside methods using spring.

How can I do this?

Answer 1

Your given example of a SpellChecker being used by a TextEditor class could look like this before it is capable of Dependency Injection. You are creating a new instance of the class SpellChecker inside the method where you are using it.

public class TextEditor {

    public void doSomething() {
        new SpellChecker().checkSpelling();
    }

}

If you want to decouple the code and be able to replace the SpellChecker by a mock for unit testing purposes or by another implementation provided by your Dependency Injection container you have to change the class and provide a field where this instance can be injected.

public class TextEditor {

    @Autowired
    private SpellChecker spellChecker;

    public void doSomething() {
        spellChecker.checkSpelling();
    }

}

Since this is still not easy to test because you would need to use Reflection to inject a mock into that private field you can provide a Setter or make the field package visible by removing the modifier private from the field. So a unit test in the same package can access this field and provide the mock.