Constructor injection – ambiguities

We are familiar ambiguities that may arise due to  parameter type mismatch while making a call to a simple method in java. Similarly, care needs to be taken while passing values to a class using constructor injection. What about classes that contain multiple constructors with ambiguous , partially compatible dataTypes. Which exact parameters are about to match the provided constructor definition needs to be clear.
While injecting values to the properties of a bean through constructor injection, we need to make sure that the values are set as intended, on the right properties. Let us take an example.

Step 1 : Create the POJO

File : InjectionAmbiguity

package com.simpleCodeStuffs;
public class InjectionAmbiguity {
private String name;
private String score;
private int age;
//constructor 1
public InjectionAmbiguity(String name, String score,int age){
this.name=name;
this.age=age;
this.score=score;
}
//constructor 2
public InjectionAmbiguity(String name, int age, String score){
this.name=name;
this.age=age;
this.score=score;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
public String getScore() {
return score;
}
public void setScore(String score) {
this.score = score;
}
}

Step 2 : Provide the Bean definition in the xml file

File : Beans.xml

<?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">

  <bean id="ambiguity" class="com.simpleCodeStuffs.InjectionAmbiguity">
  <constructor-arg>     
                 <value>Sandy</value>   
  </constructor-arg>
    <constructor-arg>   
                 <value>200</value>     
  </constructor-arg>
    <constructor-arg>   
                 <value>22</value>      
  </constructor-arg>
</bean>
</beans>

Note :
Here I have simply assumed that the 3 property values will get set in the order they are passed to the constructor. i.e., name,Score,Age and hence constructor 1 will get called.

Step 3: Create the Main Class

File : MainClass.java

package com.simpleCodeStuffs;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;
public class MainClass {
public static void main(String[] args) {
ApplicationContext context = new ClassPathXmlApplicationContext 

("Beans.xml");
InjectionAmbiguity amb=(InjectionAmbiguity)context.getBean("ambiguity");
System.out.println("name "+amb.getName());
System.out.println("Age "+amb.getAge());
System.out.println("Score "+amb.getScore());
}
}

Step 4 : Lets check if the output is as expected

Oopsies !!!! This is because, the constructor-arg value has been passed as
 

        <constructor-arg>       
                 <value>200</value>     </constructor-arg>

This value has been converted from String to int and hence constructor 2 was called.

Step 5 :

To avoid such unexpected results, it is always better to make the constructor-arg injection as clear as possible. The use of type is adviced.

Usage of ‘type’,’index’ attributes along with ‘constructor-arg’ tag avoids ambiguity in the constructor being called

 

Beans.xml

<?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">

  <bean id="ambiguity" class="com.simpleCodeStuffs.InjectionAmbiguity">
  <constructor-arg type="java.lang.String">     
                 <value>Sandy</value>   
  </constructor-arg>
    <constructor-arg type="java.lang.String">   
                 <value>200</value>     
  </constructor-arg>
    <constructor-arg type="int">        
                 <value>22</value>      
  </constructor-arg>
  </bean>
</beans>