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Lightrun – Third Party Code

We rely on other people’s code in our own work. Every day. It might be the language you’re writing in, the framework you’re building on, or some esoteric piece of software that does one thing so well you never found the need to implement it yourself.

The problem is, of course, when things fall apart in production - debugging the implementation of a 3rd party library you have no intimate knowledge of is, to say the least, tricky. It’s difficult to understand what talks to what and, specifically, which part of the underlying library is at fault.

Lightrun is a new kind of debugger.

It's one geared specifically towards real-life production environments. Using Lightrun, you can drill down into running applications, including 3rd party dependencies, with real-time logs, snapshots, and metrics. No hotfixes, redeployments, or restarts required.

Learn more in this quick, 5-minute Lightrun tutorial:

>> The Essential List of Spring Boot Annotations and Their Use Cases

 

1. Overview

In this quick tutorial, we'll discuss Spring's @Primary annotation which was introduced with version 3.0 of the framework.

Simply put, we use @Primary to give higher preference to a bean when there are multiple beans of the same type.

Let's describe the problem in detail.

2. Why Is @Primary Needed?

In some cases, we need to register more than one bean of the same type.

In this example we have JohnEmployee() and TonyEmployee() beans of the Employee type:

@Configuration
public class Config {

    @Bean
    public Employee JohnEmployee() {
        return new Employee("John");
    }

    @Bean
    public Employee TonyEmployee() {
        return new Employee("Tony");
    }
}

Spring throws NoUniqueBeanDefinitionException if we try to run the application.

To access beans with the same type we usually use @Qualifier(“beanName”) annotation.

We apply it at the injection point along with @Autowired. In our case, we select the beans at the configuration phase so @Qualifier can't be applied here. We can learn more about @Qualifier annotation by following the link.

To resolve this issue Spring offers the @Primary annotation.

3. Use @Primary With @Bean

Let's have a look at configuration class:

@Configuration
public class Config {

    @Bean
    public Employee JohnEmployee() {
        return new Employee("John");
    }

    @Bean
    @Primary
    public Employee TonyEmployee() {
        return new Employee("Tony");
    }
}

We mark TonyEmployee() bean with @Primary. Spring will inject TonyEmployee() bean preferentially over the JohnEmployee().

Now, let's start the application context and get the Employee bean from it:

AnnotationConfigApplicationContext context
  = new AnnotationConfigApplicationContext(Config.class);

Employee employee = context.getBean(Employee.class);
System.out.println(employee);

After we run the application:

Employee{name='Tony'}

From the output, we can see that the TonyEmployee() instance has a preference while autowiring.

4. Use @Primary With @Component

We can use @Primary directly on the beans. Let's have a look at the following scenario:

public interface Manager {
    String getManagerName();
}

We have a Manager interface and two subclass beans, DepartmentManager:

@Component
public class DepartmentManager implements Manager {
    @Override
    public String getManagerName() {
        return "Department manager";
    }
}

And the GeneralManager bean:

@Component
@Primary
public class GeneralManager implements Manager {
    @Override
    public String getManagerName() {
        return "General manager";
    }
}

They both override the getManagerName() of the Manager interface. Also, note that we mark the GeneralManager bean with @Primary.

This time, @Primary only makes sense when we enable the component scan:

@Configuration
@ComponentScan(basePackages="org.baeldung.primary")
public class Config {
}

Let's create a service to use dependency injection while finding the right bean:

@Service
public class ManagerService {

    @Autowired
    private Manager manager;

    public Manager getManager() {
        return manager;
    }
}

Here, both beans DepartmentManager and GeneralManager are eligible for autowiring.

As we marked GeneralManager bean with @Primary, it will be selected for dependency injection:

ManagerService service = context.getBean(ManagerService.class);
Manager manager = service.getManager();
System.out.println(manager.getManagerName());

The output is “General manager”.

5. Conclusion

In this article, we learned about Spring's @Primary annotation. With the code examples, we demonstrated the need and the use cases of the @Primary.

As usual, the complete code for this article is available over on GitHub project.

 

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