Constructor injection in spring

Summary

• We have changed the order of constructor.
• Observe the output.

Syntax 1

`

<bean class="com.constructor.injection.beans.Employee" id="employee">
    <constructor-arg  type="float" value="10000" />
    <constructor-arg  type="String"  value="Sumit" />
</bean>

`

Output

`

bean initialized using str-float constructor
Name:null
Designation:Sumit
salary:10000.0

`

Syntax

`

<bean class="com.constructor.injection.beans.Employee" id="employee">
    <constructor-arg  type="float" index="0" value="10000" />
    <constructor-arg  type="String"  value="Sumit" />
</bean>

`

Output

`

bean initialized using float-str constructor
Name:Sumit
Designation:null
salary:10000.0

`

Conclusion

• For 1st use case output, observe the output, why we are we getting wrong output ?
  • That's because internally Spring uses ConstructorResolver class to decide which constructor to use to initialize our beans.
  • This class uses Reflection technique which selects the first matching constructor from the constructor list available and this list is random.
  • That's why if we change the order of constructor in the class , the selected constructor changes.
• To resolve this, add index attribute
  • To resolve this problem the Spring team strongly recommends to use another attribute called index with type attribute in <constructor-arg>.
  • This attribute instructs the container to select the constructor based upon the index /order /position of supplied arguments.