依赖注入如何在没有构造函数的情况下实例化一个类？

Question

I followed the following example of dependency injection: http://www.tutorialspoint.com/spring/spring_autowired_annotation.htm

For example the TextEditor class (from the above link):

public class TextEditor {
   private SpellChecker spellChecker;

   @Autowired
   public void setSpellChecker( SpellChecker spellChecker ){
      this.spellChecker = spellChecker;
   }
   public SpellChecker getSpellChecker( ) {
      return spellChecker;
   }
   public void spellCheck() {
      spellChecker.checkSpelling();
   }
}

How can these dependencies/classes be instantiated, while they don't have any constructor?

Is Java simply making an object of that type, that is empty? Like an empty parameter constructor without any code?

Thanks for making this more clear!

Answer 1

Unless specified otherwise, every Java class has the default constructor. So here, you have a default public TextEditor() constructor, even though you haven't coded for it. (You could code it if you needed to change its visibility from public, declare a thrown exception, etc.)

So yes, Spring calls this default constructor - then calls the setSpellChecker method (as annotated, and through reflection) to populate it.

Answer 2

If no constructor is defined, a class can be instantiated via the no-argument default constructor.

So, the framework calls that constructor (supposedly using reflection) and then uses the set method to set the one field of the freshly created class.

Answer 3

The example above is using Spring annotations and Spring context file and those are the main and most important parts of the project, considering the DI.

So in the context file you have following line:

<!-- Definition for spellChecker bean -->
<bean id="spellChecker" class="com.tutorialspoint.SpellChecker">
</bean>

this defines a class with reference spellChecker that mapps to a class com.tutorialspoint.SpellChecker and once the compiler find such property in a method that is marked as @Autowired on instantiation of the object it injects/sets the relevant version of the required dependency.

In cases where a property doesn't match a reference tag in the applicationContext.xml file Spring is trying to map the type eg property with name mySpecialSpellChecker which has type of com.tutorialspoint.SpellChecker still will be mapped to bean with id="spellChecker" if there are more than one of same type Spring won't instantiate your object and you might get compile time error as Spring can't know which version of the two (or more) is the correct one so this requires developer input.

This is the order of execution:

1. instantiate textEditor , this has default constructor that is not visible in the code public TextEditor ()
2. the new instance is set in a pool of available objects with reference textEditor
3. instantiate spellChecker and add to the pool of available object with relevant reference/label
4. all @Autowired properties/methods are set/called with relevant objects in this case Spring calls: setSpellChecker(spellChecker)