CODEWITHSUNDEEP
javaspring
Flash
Flash

Reputation: 3021

Injecting bean into default constructor

Lets say i have a class like this :

public class A {

 private B b;

 public A() {

 }

 // Some code calling methods in b.
}

How can i inject an instance of B into A via XML configuration without adding a parameterized constructor in Spring ?

Can i have both parameterized constructor and a setter as well ?

 public class A {

     private B b;

     public A(B b) {
        this.b = b;
     }

     public void setB(B b) {
         this.b = b;
     }
     // Some code calling methods in b.
    }

Edit : Thanks for all the answers. My actual problem is that i have a class like this :

public class A {

 private B b;

 public A(B b) {
    this.b = b;
 }
 // Some code calling methods in b.
}

and i want to have a default constructor for the above class without removing the parameterized constructor for backward compatibility reasons.

So ,

  1. Does the bean get injected automatically into the default constructor when i add one ? Or should i have additional configuration in the XML file to inject it into it ?
  2. If i add a setter what additional configuration changes should i make in the XML and how ?

Consider that i have an xml file as below :

<?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.xsd">

    <bean id="name"
          class="A">
        <constructor-arg ref="B" />
    </bean>

</beans>

Upvotes: 0

Views: 1651

Answers (4)

sanbhat
sanbhat

Reputation: 17622

You can do it via Setter Injection. For this, you need to add a setter method for your member field b

Can i have both parameterized constructor and a setter as well ?

Yes you can have.

If the parameterized constructor is the only constructor in your bean, then you should go with constructor injection.

else create non-argument default constructor (along with the parameterized constructor) for your class and go with setter OR constructor injection.

Upvotes: 1

LaurentG
LaurentG

Reputation: 11807

The B object cannot be available in the constructor. The best option is to inject it and use a @PostConstruct method to initialize the bean. To inject the bean b, setter injection as mentionned by sanbhat. Alternatively, you could also use annotation-based configuration and simply add a @Inject or @Autowired annotation to the B field and define your bean B in the XML configuration.

public class A {

 @Inject
 private B b;

 @PostConstruct
 public init() {
    // inititalization code...
 }

 // Some code calling methods in b.
}

Upvotes: 0

radha krishna
radha krishna

Reputation: 230

you may pass the bean class B id to Class A via <constructor-arg> tag like this in (for example beans.xml)

<bean id="a" class="ClassA">
    <constructor-arg ref="beanB"/>
</bean>
<bean id="beanB" class="com.tutorialspoint.SpellChecker">
   </bean>

the following url may helps you alot in this regard

http://www.tutorialspoint.com/spring/constructor_based_dependency_injection.htm [edited]

Upvotes: 0

Evgeniy Dorofeev
Evgeniy Dorofeev

Reputation: 136122

If we are not allowed to change this class then there is no way, otherwise we can use Spring's Autowired annotation

public class A {
   @Autowired
   private B b
   ...

Upvotes: 0

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