Spring – Autowire using Annotations
Starting with Spring 2.5, the framework introduced a new style of Dependency Injection driven by @Autowired Annotations. This annotation allows Spring to resolve and inject collaborating beans into your bean.
To enable Annotation based autowiring we need to place below line in SpringConfig.xml
<context:annotation-config/>
Once annotation injection is enabled, autowiring can be used on properties, setters, and constructors.
1. @Autowired on Properties
public class Student {
private int sno;
private String name;
@Autowired
private Address address;
}
2.@Autowired on Setters
public class Student {
private int sno;
private String name;
private Address address;
@Autowired
public void setAddress(Address address) {
this.address = address;
}
}
3. @Autowired on Constructors
public class Student {
private int sno;
private String name;
private Address address;
@Autowired
public Student(Address address) {
this.address = address;
}
}
4.@Autowired and Optional Dependencies Spring expects @Autowired dependencies to be available when the dependent bean is being constructed. If the framework cannot resolve a bean for wiring, it will throw NoSuchBeanDefinitionException. To avoid this we have to use (required=false)
public class Student {
private int sno;
private String name;
@Autowired(required = false)
private Address address;
}
By default, the @Autowired annotation implies the dependency is required similar to @Required annotation, however, you can turn off the default behavior by using (required=false) option with @Autowired.
5.Autowiring by @Qualifier
By default, Spring resolves @Autowired entries by type. If more than one
beans of the same type are available in the container, the framework will throw
a fatal exception indicating that more than one bean is available for
autowiring.
To avoid this error we have to use @Qualifier.
Now we can update SpringConfig.xml by removing autowire="byType" , because we are using @Autowired annotation
<beans>
<bean id="student" class="core.Student">
<property name="sno" value="101"></property>
<property name="name" value="Satya Kaveti"></property>
</bean>
<bean id="address" class="core.Address">
<property name="hno" value="322"></property>
<property name="city" value="HYDERABAD"></property>
</bean>
<bean id="address1" class="core.Address">
<property name="hno" value="322"></property>
<property name="city" value="HYDERABAD"></property>
</bean>
</beans>
In above you have multiple bean of same type, Container will confuse which bean
should inject & throws NoSuchBeanDefinitionException:
by: *org.springframework.beans.factory.NoSuchBeanDefinitionException*: No unique
bean of type [core.Address] is defined: expected single matching bean but found
2: [address, address1]
To fix above problem, you need @Qualifier to tell Spring about which bean should autowired.
public class Student {
private int sno;
private String name;
@Qualifier("address")
private Address address;
}