In this tutorial, we will discuss on Spring’s @Primary annotation with examples, @Primary is available in Spring Framework since 3.0 version.
1. @Primary annotation in Spring
When there are multiple beans available of same type in Spring container, all of them are qualified to be autowired to single-valued dependency. That causes ambiguity and leads to throw an exception by framework. @Primary indicates that a bean should be given preference when multiple candidates are qualified to autowire a single-valued dependency.
There should be only one @Primary bean among same type of beans.
Let’s see an example.
package com.javabydeveloper.spring.primary;
public class Animal {
private String name;
public Animal(String name) {
this.name = name;
}
@Override
public String toString() {
return name;
}
}
@Configuration
@ComponentScan("com.javabydeveloper.spring.primary")
public class AppConfigForPrimary {
@Bean
public Animal tigerBean() {
return new Animal("Tiger");
}
@Bean
@Primary
public Animal kangarooBean() {
return new Animal("Kangaroo");
}
@Bean
public Animal foxBean() {
return new Animal("Fox");
}
// ... Other beans
}
package com.javabydeveloper.spring.primary;
// ...
public class PrimaryDITestBean {
@Autowired
private Animal animal;
public void printAnimal() {
System.out.println(animal);
}
// ...
}
Testing :
public class ApplicationPrimaryDemo {
public static void main(String[] args) {
ApplicationContext ctxt = new AnnotationConfigApplicationContext(AppConfigForPrimary.class);
PrimaryDITestBean pdits = ctxt.getBean(PrimaryDITestBean.class);
pdits.printAnimal();
// ...
}
}
Output :
Kangaroo //...
2. Using @Primary with @Component
In previous example we have seen how to use @Primary with @Bean. @Primary may be used on any class directly or indirectly annotated with @Component class.
Let’s see an example.
package com.javabydeveloper.spring.primary;
public interface DataService {
public void printData();
}
package com.javabydeveloper.spring.primary;
// ...
@Component
public class BinaryDataService implements DataService {
@Override
public void printData() {
System.out.println("Binary Data");
}
}
package com.javabydeveloper.spring.primary;
// ...
@Component
@Primary
public class TextDataService implements DataService {
@Override
public void printData() {
System.out.println("Text Data");
}
}
public class PrimaryDITestBean {
// ...
@Autowired
private DataService dataService;
public void printData() {
dataService.printData();
}
// ...
}
Testing :
public class ApplicationPrimaryDemo {
public static void main(String[] args) {
ApplicationContext ctxt = new AnnotationConfigApplicationContext(AppConfigForPrimary.class);
PrimaryDITestBean pdits = ctxt.getBean(PrimaryDITestBean.class);
pdits.printData();
// ...
}
}
Output :
Text Data //...
3. Spring @Primary vs @Qualifier
3.1. We use @Qualifier in Spring to autowire a specific bean among same type of beans, where as @Primary is used to give high preference to the specific bean among multiple beans of same type to inject to a bean.
3.2. Suppose that you have provided multiple beans of same type and Spring is responsible to autowire a bean for that type (example: multiple datasources in single application), you need to tell Spring which bean should get autowired by default by annotating bean with @Primary, and we can not use @Qualifier in that situation because Spring will lookup for dependency in configuration phase.
3.3. We can use @Qualifier and @Primary for the same bean. Use @Qualifier to inject specific bean otherwise Spring injects bean by default which is annotated with @Primary.
Let’s see an example.
package com.javabydeveloper.spring.primary;
public interface Bird {
}
package com.javabydeveloper.spring.primary;
public class Eagle implements Bird {
@Override
public String toString() {
return "Eagle";
}
}
public class Peacock implements Bird{
@Override
public String toString() {
return "Peacock";
}
}
@Configuration
@ComponentScan("com.javabydeveloper.spring.primary")
public class AppConfigForPrimary {
// ...
@Bean
@Primary
@Qualifier("peacockBean")
public Bird peacock() {
return new Peacock();
}
@Bean
@Qualifier("eagleBean")
public Bird eagle() {
return new Eagle();
}
}
public class PrimaryDITestBean {
@Autowired
@Qualifier("eagleBean")
private Bird bird;
// ...
public void printBird() {
System.out.println(bird);
}
}
Testing :
public class ApplicationPrimaryDemo {
public static void main(String[] args) {
ApplicationContext ctxt = new AnnotationConfigApplicationContext(AppConfigForPrimary.class);
PrimaryDITestBean pdits = ctxt.getBean(PrimaryDITestBean.class);
pdits.printBird();
// ...
}
}
Output :
Eagle // ...
Comment line @Qualifier("eagleBean") in PrimaryDITestBean class and test again.
Output :
Peacock // ...
4. Conclusion
In this quick tutorial we have seen how to use Spring’s @Primary annotation in Spring applications with examples.
You can get source code from our GitHub repository.
5. References
Other Related Tutorials:
