Introduction:
Spring @Primary annotation is used to give a higher preference to the marked bean when multiple beans of the same type exist.
Spring, by default, auto-wires by type. And so, when Spring attempts to autowire and there are multiple beans of the same type, we’ll get a NoUniqueBeanDefinitionException:
Caused by: org.springframework.beans.factory.NoUniqueBeanDefinitionException : No qualifying bean of type [com.programmergirl.Person] is defined: expected single matching bean but found 2: student, teacher ...
To solve this, we can choose to use Spring @Primary annotation, thereby marking one bean to be the primary one.
In this tutorial, we’ll explore the usage of this annotation in more detail.
@Primary In Configuration Class:
Let’s say we have the following configuration class:
@Configuration
public class UniversityConfig {
@Bean
@Primary
public Person student() {
return new Student();
}
@Bean
public Person teacher() {
return new Teacher();
}
}
Both Teacher and Student beans inherit from Person and so we have marked it as the return type of both of our @Bean annotated method.
However, please note that we have marked the Student bean to be the primary one using @Primary annotation. Now, let’s start up our application:
AnnotationConfigApplicationContext context = AnnotationConfigApplicationContext(UniversityConfig.class); Person student = context.getBean(Person.class); System.out.println(student.getClass());
We’ll see that a Student object got a preference while Spring attempted autowiring.
Spring Component With @Primary:
Let’s say we instead have our component scan enabled:
@Configuration
@ComponentScan(basePackages="com.programmergirl.beans")
public class UniversityConfig {
}
For such cases, we can directly annotate our Spring component class with @Primary:
@Primary
@Component
public class Student implements Person {
...
}
@Component
public class Teacher implements Person {
...
}
Now, when directly trying to inject a Person type without a @Qualifier, a Student bean will get injected:
@Service
public class StudentService {
// Student bean is primary and so it'll get injected
@Autowired
private Person student;
public void printStudentDetails() {
System.out.println(student.getClass());
...
}
}
Conclusion:
In this quick tutorial, we explored the usages of @Primary annotation in Spring.
As the name suggests, we can use @Primary annotation to define a primary one when having multiple beans of the same type.
Published on Java Code Geeks with permission by Shubhra Srivastava, partner at our JCG program. See the original article here: Spring @Primary Annotation Opinions expressed by Java Code Geeks contributors are their own. |
