Difference Between @ComponentScan and @EnableAutoConfiguration in Spring Boot

1. Introduction

In this quick tutorial, we'll learn about the differences between @ComponentScan and @EnableAutoConfiguration annotations in the Spring Framework.

2. Spring Annotations

Annotations make it easier to configure the dependency injection in Spring. Instead of using XML configuration files, we can use Spring Bean annotations on classes and methods to define beans. After that, the Spring IoC container configures and manages the beans.

Here's an overview of the annotations that we are going to discuss in this article:

• @ComponentScan scans for annotated Spring components
• @EnableAutoConfiguration is used to enable the auto-configuration

Let's now look into the difference between these two annotations.

3. How They Differ

The main difference between these annotations is that @ComponentScan scans for Spring components while @EnableAutoConfiguration is used for auto-configuring beans present in the classpath in Spring Boot applications.

Now, let's go through them in more detail.

3.1. @ComponentScan

While developing an application, we need to tell the Spring framework to look for Spring-managed components. @ComponentScan enables Spring to scan for things like configurations, controllers, services, and other components we define.

In particular, the @ComponentScan annotation is used with @Configuration annotation to specify the package for Spring to scan for components:

@Configuration
@ComponentScan
public class EmployeeApplication {
    public static void main(String[] args) {
        ApplicationContext context = SpringApplication.run(EmployeeApplication.class, args);
        // ...
    }
}

Alternatively, Spring can also start scanning from the specified package, which we can define using basePackageClasses() or basePackages(). If no package is specified, then it considers the package of the class declaring the @ComponentScan annotation as the starting package:

package com.baeldung.annotations.componentscanautoconfigure;
// ...
@Configuration
@ComponentScan(basePackages = {"com.baeldung.annotations.componentscanautoconfigure.healthcare",
  "com.baeldung.annotations.componentscanautoconfigure.employee"},
  basePackageClasses = Teacher.class)
public class EmployeeApplication {
    public static void main(String[] args) {
        ApplicationContext context = SpringApplication.run(EmployeeApplication.class, args);
        // ...
    }
}

In the example, Spring will scan the healthcare and employee packages and the Teacher class for components.

Spring searches the specified packages along with all its sub-packages for classes annotated with @Configuration. Additionally, the Configuration classes can contain @Bean annotations, which register the methods as beans in the Spring application context. After that, the @ComponentScan annotation can auto-detect such beans:

@Configuration
public class Hospital {
    @Bean
    public Doctor getDoctor() {
        return new Doctor();
    }
}

Furthermore, the @ComponentScan annotation can also scan, detect, and register beans for classes annotated with @Component, @Controller, @Service, and @Repository.

For example, we can create an Employee class as a component which can be scanned by the @ComponentScan annotation:

@Component("employee")
public class Employee {
    // ...
}

3.2. @EnableAutoConfiguration

The @EnableAutoConfiguration annotation enables Spring Boot to auto-configure the application context. Therefore, it automatically creates and registers beans based on both the included jar files in the classpath and the beans defined by us.

For example, when we define the spring-boot-starter-web dependency in our classpath, Spring boot auto-configures Tomcat and Spring MVC. However, this auto-configuration has less precedence in case we define our own configurations.

The package of the class declaring the @EnableAutoConfiguration annotation is considered as the default. Therefore, we should always apply the @EnableAutoConfiguration annotation in the root package so that every sub-packages and class can be examined:

@Configuration
@EnableAutoConfiguration
public class EmployeeApplication {
    public static void main(String[] args) {
        ApplicationContext context = SpringApplication.run(EmployeeApplication.class, args);
        // ...
    }
}

Furthermore, the @EnableAutoConfiguration annotation provides two parameters to manually exclude any parameter:

We can use exclude to disable a list of classes that we do not want to be auto-configured:

@Configuration
@EnableAutoConfiguration(exclude={JdbcTemplateAutoConfiguration.class})
public class EmployeeApplication {
    public static void main(String[] args) {
        ApplicationContext context = SpringApplication.run(EmployeeApplication.class, args);
        // ...
    }
}

We can use excludeName to define a fully qualified list of class names that we want to exclude from the auto-configuration:

@Configuration
@EnableAutoConfiguration(excludeName = {"org.springframework.boot.autoconfigure.jdbc.JdbcTemplateAutoConfiguration"})
public class EmployeeApplication {
    public static void main(String[] args) {
        ApplicationContext context = SpringApplication.run(EmployeeApplication.class, args);
        // ...
    }
}

Since Spring Boot 1.2.0, we can use the @SpringBootApplication annotation, which is a combination of the three annotations @Configuration, @EnableAutoConfiguration, and@ComponentScan with their default attributes:

@SpringBootApplication
public class EmployeeApplication {
    public static void main(String[] args) {
        ApplicationContext context = SpringApplication.run(EmployeeApplication.class, args);
        // ...
    }
}

4. Conclusion

In this article, we learned about the differences between @ComponentScan and @EnableAutoConfiguration in Spring Boot.

As always, the code for these examples is available over on GitHub.

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