Spring DI - Setter-Based


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Setter-based DI is accomplished by the container calling setter methods on your beans after invoking a no-argument constructor or no-argument static factory method to instantiate your bean.

Example

The following example shows a class TextEditor that can only be dependency-injected using pure setter-based injection.

Let's update the project created in Spring DI - Create Project chapter. We're adding following files −

  • TextEditor.java − A class containing a SpellChecker as dependency.

  • SpellChecker.java − A dependency class.

  • MainApp.java − Main application to run and test.

Here is the content of TextEditor.java file −

package com.tutorialspoint;
public class TextEditor {
   private SpellChecker spellChecker;

   // a setter method to inject the dependency.
   public void setSpellChecker(SpellChecker spellChecker) {
      System.out.println("Inside setSpellChecker." );
      this.spellChecker = spellChecker;
   }
   // a getter method to return spellChecker
   public SpellChecker getSpellChecker() {
      return spellChecker;
   }
   public void spellCheck() {
      spellChecker.checkSpelling();
   }
}

Here you need to check the naming convention of the setter methods. To set a variable spellChecker we are using setSpellChecker() method which is very similar to Java POJO classes. Let us create 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("applicationcontext.xml");

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

Following is the configuration file applicationcontext.xml which has configuration for the setter-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">
      <property name = "spellChecker" ref = "spellChecker"/>
   </bean>

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

You should note the difference in applicationcontext.xml file defined in the constructor-based injection and the setter-based injection. The only difference is inside the <bean> element where we have used <constructor-arg> tags for constructor-based injection and <property> tags for setter-based injection.

The second important point to note is that in case you are passing a reference to an object, you need to use ref attribute of <property> tag and if you are passing a value directly then you should use value attribute.

Output

Once you are done creating the 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 setSpellChecker.
Inside checkSpelling.
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