Introduction

• @ComponentScan annotation was introduced in Spring 2.5, which is used to support "Annotation-based Configuration".
  
  
• It enables Spring’s "Component-Scanning Feature", which allows Spring to automatically look for components, configurations and services to manage as Spring beans.
• It scans one or more base packages and their sub-packages for classes annotated with component-related stereotypes (like @Component, @Service, @Repository, and @Controller) and registers these classes as beans within the Spring Context.
  
  
• Syntax :
  Below is the syntax of @ComponentScan which is placed above a class definition:

  import org.springframework.context.annotation.ComponentScan;
  import org.springframework.context.annotation.Configuration;
  
  @Configuration
  @ComponentScan(basePackages = "in.sp.package")
  public class AppConfig
  {
      // Application configuration
  }

• NOTE : @ComponentScan annotation is commonly used in conjunction with @Configuration to register components or it’s embedded within @SpringBootApplication in Spring Boot.
  
  
• Click Here for Spring Program using Annotation-Based Configurations in which @ComponentScan is used.

Common Usages of @ComponentScan Annotation

1. Standard Scanning for a Base Package

• The most common use case is scanning a single base package where most of our components exists. This registers all @Component-annotated classes (including @Service, @Repository, and @Controller).
• Syntax :
  

  @Configuration
  @ComponentScan(basePackages = "in.sp.package")
  public class AppConfig
  {
      // Scans in.sp.package and its sub-packages
  }

2. Multiple Base Packages

• If we have components spread across different packages, then we can use "basePackages" to specify multiple packages.
• Syntax :
  

  @Configuration
  @ComponentScan(basePackages = {"in.sp.service", "in.sp.repository"})
  public class AppConfig
  {
      // Scans two different packages
  }

3. Scanning Using basePackageClasses

• Instead of hardcoding package names, we can use basePackageClasses which defines a class whose package Spring will scan.
• Syntax :

  @Configuration
  @ComponentScan(basePackageClasses = {ServiceClass.class, RepositoryClass.class})
  public class AppConfig
  {
      // Scans packages of ServiceClass and RepositoryClass
  }
  

4. Custom Filtering with includeFilters and excludeFilters

• Filters allow selective inclusion or exclusion of components during scanning.
• includeFilters specifies types or annotations to include, while excludeFilters specifies those to exclude.
• Syntax :

  @Configuration
  @ComponentScan(
      basePackages = "in.sp.package",
      includeFilters = @ComponentScan.Filter(type = FilterType.ANNOTATION, classes = MyAnnotation.class),
      excludeFilters = @ComponentScan.Filter(type = FilterType.ASSIGNABLE_TYPE, classes = UnwantedClass.class)
  )
  public class AppConfig
  {
      // Includes classes annotated with MyAnnotation but excludes UnwantedClass
  }

5. Lazy Initialization of Beans

• We can set lazyInit to true if we want Spring to delay bean instantiation until the bean is needed, which can improve startup time for large applications.
• Syntax :

  @Configuration
  @ComponentScan(basePackages = "in.sp.package", lazyInit = true)
  public class AppConfig
  {
      // Scans in.sp.package but initializes beans lazily
  }