springboot functioning principle of @EnableAutoConfiguration

Usually we start a project to increase @SpringBootApplicatoin springboot annotation startup method, the notes included in the @EnableAutoConfiguration

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@SpringBootConfiguration
@EnableAutoConfiguration
@ComponentScan(excludeFilters = {
        @Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),
        @Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })
public @interface SpringBootApplication {

    /**
     * Exclude specific auto-configuration classes such that they will never be applied.
     * @return the classes to exclude
     */
    @AliasFor(annotation = EnableAutoConfiguration.class)
    Class<?>[] exclude() default {};

    /**
     * Exclude specific auto-configuration class names such that they will never be
     * applied.
     * @return the class names to exclude
     * @since 1.3.0
     */
    @AliasFor(annotation = EnableAutoConfiguration.class)
    String[] excludeName() default {};

    /**
     * Base packages to scan for annotated components. Use {@link #scanBasePackageClasses}
     * for a type-safe alternative to String-based package names.
     * @return base packages to scan
     * @since 1.3.0
     */
    @AliasFor(annotation = ComponentScan.class, attribute = "basePackages")
    String[] scanBasePackages() default {};

    /**
     * Type-safe alternative to {@link #scanBasePackages} for specifying the packages to
     * scan for annotated components. The package of each class specified will be scanned.
     * <p>
     * Consider creating a special no-op marker class or interface in each package that
     * serves no purpose other than being referenced by this attribute.
     * @return base packages to scan
     * @since 1.3.0
     */
    @AliasFor(annotation = ComponentScan.class, attribute = "basePackageClasses")
    Class<?>[] scanBasePackageClasses() default {};

}

 

So @EnableAutoConfiguration how it works automatically. Let's take a look at the definition of @EnableAutoConfiguration

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(AutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {

    String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";

    /**
     * Exclude specific auto-configuration classes such that they will never be applied.
     * @return the classes to exclude
     */
    Class<?>[] exclude() default {};

    /**
     * Exclude specific auto-configuration class names such that they will never be
     * applied.
     * @return the class names to exclude
     * @since 1.3.0
     */
    String[] excludeName() default {};

}

We look at the definition of notation EnableAutoConfiguration:

Enable auto-configuration of the Spring Application Context, attempting to guess and
* configure beans that you are likely to need. Auto-configuration classes are usually
* applied based on your classpath and what beans you have defined.

Probably it means: try to guess bean container you want to configure, and then automatically loaded.

Very pleased to find that the @EnableAutoConfiguration inside references @Import (AutoConfigurationImportSelector. Class), intuition looks like this AutoConfigurationImportSelector which we introduced those auto-configuration (data source, interceptor) class.

Let's look at the definition of AutoConfigurationImportSelector:

public class AutoConfigurationImportSelector
implements DeferredImportSelector, BeanClassLoaderAware, ResourceLoaderAware,
BeanFactoryAware, EnvironmentAware, Ordered

The original AutoConfigurationImportSelector class implements ORG . Springframework . Context . Annotation .DeferredImportSelector

After then realized the class public String [] selectImports (AnnotationMetadata annotationMetadata), spring while parsing the bean, it will automatically call this method to expand the class load, which is the method returns a list of names selectImports to load classes.

From which it is easy to think of selectImports return those data sources AutoEnableConfiguration, the interceptor class configuration. !

Then look at the specific selectImports implementation: