Constructor-Based Dependency Injection

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In Constructor based dependency injection, Spring Container uses one of the constructors defined in the class in order to inject dependencies for that class.

In constructor based DI, instead of <property name="prop_name" value="prop_value"/>  tag, you use <constructor-arg  name="arg_name"  value="arg_value"/> tag in the bean definition.

For example, suppose you have a Car class defined as:

public class Car {
    private String make;
    private String model;
    private int year;
    private Engine engine;

/*Spring Container will use this constructor to inject dependencies*/
public Car(String make,String model,int year){
    this.engine=engine;
    this.model=model;
    this.make=make;
    this.year=year;
}
//setters and getters...
}

In the above class there is a constructor which has three arguments namely make, model, year.

Now in order inject these dependencies you will define your bean as :

<bean id="myCar"  class="exampes.spring.di.Car">
    <constructor-arg name="make" value="Maruti Suzuki"/>
    <constructor-arg name="model" value="Wagon R"/>
   <constructor-arg name="year" value="2012"/>
</bean>

In above bean definition we have  injected only primited values . Suppose Car class has a constructor :

public Car(String make,String model,int year, Engine engine){
...
this.engine=engine;
}

Here constructor has one more argument which is a reference to an instance of some other class Engine.

Now your bean definition will get changed as:

<bean id="myCar"  class="exampes.spring.di.Car">
   <constructor-arg name="make" value="Maruti Suzuki"/>
   <constructor-arg name="model" value="Wagon R"/>
   <constructor-arg name="year" value="2012"/>
  <constructor-arg name="engine" ref="myEngine"/>
</bean>

where myEngine is some other bean defined in configuration file as:

<bean id="myEngine" class="examples.spring.di.Engine">
...
</bean>

You can find full example here.

TODO: ambiguities in constructor resolving.

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