AnnotationAutoWiring in Spring

Summary

• Annotation in Spring is used to automatically inject values at runtime.
• The @Configuration annotation in Spring is used to indicate that a class is a source of configuration for the Spring IoC (Inversion of Control) container.
• In simple words, we can say that it tells Spring that this class is intended to be used as a configuration class in place of XML.
• Annotations are used for java-based configuration opposite to XML configuration.

Steps

• Create a class with @Component annotation which will act as bean.
• Create a class with @Configuration annotation which will inform the Spring container that this class can be used as Configuration class.
• Along with @Configuration also use @ComponentScan annotation with the basePackages attribute containing names of one or more base packages where Spring should look for these annotated components.
• Start the Spring IoC container by instantiating AnnotationConfigApplicationContext passing it the name of Configuration class.
• Finally, retrieve the bean using the getBean() method.

Syntax (Using basePackages)

`

@Configuration
@ComponentScan(basePackages = "com.autowire.demo.beans")
public class AppConfig {

public AppConfig() {
    System.out.println("AppConfig cons called");
}

}

`

Conclusion

• Note, If our bean classes and Configuration class are in the same package, then we can avoid using the basePackages attribute in the @ComponentScan annotation.
• If we want to inject a bean not in the current package, then use attribute basePackages or basePackageClasses.
• The attribute basePackages will take argument as a string object separated by comma.
• The attribute basePackageClasses will take the argument as Classes as an object separated by comma.
• The name will be null since we are not using @Autowired for constructor having name as argument.

Note:

• The jar spring-aop is required for annotation processing and let spring automatically scan the bean from Configuration class.