Constructor-based Dependency Injection

Constructor-based DI is accomplished when the container invokes a class constructor with a number of arguments, each representing a dependency on other class.

Example:

The following example shows a class TextEditor that can only be dependency-injected with constructor injection.

Let us have working Eclipse IDE in place and follow the following steps to create a Spring application:

Here is the content of TextEditor.java file:

package com.tutorialspoint;

public class TextEditor {
   private SpellChecker spellChecker;

   public TextEditor(SpellChecker spellChecker) {
      System.out.println("Inside TextEditor constructor." );
      this.spellChecker = spellChecker;
   }
   public void spellCheck() {
      spellChecker.checkSpelling();
   }
}

Following is the content of another dependent class file SpellChecker.java:

package com.tutorialspoint;

public class SpellChecker {
   public SpellChecker(){
      System.out.println("Inside SpellChecker constructor." );
   }

   public void checkSpelling() {
      System.out.println("Inside checkSpelling." );
   }

}

Following is the content of the MainApp.java file:

package com.tutorialspoint;

import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

public class MainApp {
   public static void main(String[] args) {
      ApplicationContext context = 
             new ClassPathXmlApplicationContext("Beans.xml");

      TextEditor te = (TextEditor) context.getBean("textEditor");

      te.spellCheck();
   }
}

Following is the configuration file Beans.xml which has configuration for the constructor-based injection:

<?xml version="1.0" encoding="UTF-8"?>

<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://www.springframework.org/schema/beans
    http://www.springframework.org/schema/beans/spring-beans-3.0.xsd">

   <!-- Definition for textEditor bean -->
   <bean id="textEditor" class="com.tutorialspoint.TextEditor">
      <constructor-arg ref="spellChecker"/>
   </bean>

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

</beans>

Once you are done with creating source and bean configuration files, let us run the application. If everything is fine with your application, this will print the following message:

Inside SpellChecker constructor.
Inside TextEditor constructor.
Inside checkSpelling.

Constructor arguments resolution:

There may be a ambiguity exist while passing arguments to the constructor in case there are more than one parameters. To resolve this ambiguity, the order in which the constructor arguments are defined in a bean definition is the order in which those arguments are supplied to the appropriate constructor. Consider the following class:

package x.y;

public class Foo {
   public Foo(Bar bar, Baz baz) {
      // ...
   }
}

The following configuration works fine:

<beans>
   <bean id="foo" class="x.y.Foo">
      <constructor-arg ref="bar"/>
      <constructor-arg ref="baz"/>
   </bean>

   <bean id="bar" class="x.y.Bar"/>
   <bean id="baz" class="x.y.Baz"/>
</beans>

Let us check one more case where we pass different types to the constructor. Consider the following class:

package x.y;

public class Foo {
   public Foo(int year, String name) {
      // ...
   }
}

The container can also use type matching with simple types if you explicitly specify the type of the constructor argument using the type attribute. For example:

<beans>

   <bean id="exampleBean" class="examples.ExampleBean">
      <constructor-arg type="int" value="2001"/>
      <constructor-arg type="java.lang.String" value="Zara"/>
   </bean>

</beans>

Finally and the best way to pass constructor arguments, use the index attribute to specify explicitly the index of constructor arguments. Here the index is 0 based. For example:

<beans>

   <bean id="exampleBean" class="examples.ExampleBean">
      <constructor-arg index="0" value="2001"/>
      <constructor-arg index="1" value="Zara"/>
   </bean>

</beans>

A final note, in case you are passing a reference to an object, you need to use ref attribute of <constructor-arg> tag and if you are passing a value directly then you should use value attribute as shown above.