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
 

  
  
 		 Sandy
  
    
 		 200
  
    
 		 22
  

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
 

 		 200

 
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.

 

Beans.xml

 

  
  
 		 Sandy
  
    
 		 200
  
    
 		 22
  
  

 

Now, the output is –

 
Hence the ambiguity in the parameter is resolved using more specific attributes alone with the ‘property’ tag like type to specify the intended data type of the value.