Spring Boot 6-SpringBoot运行原理

springboot运行原理
springboot为我们做了自动配置。主要是spring-boot-autoconfigure.jar

关于springboot的原理，我们还是回归到@SpringbootApplication注解。他的核心功能主要由@EnableAutoConfiguration提供。

@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 {};

}

public class AutoConfigurationImportSelector
implements DeferredImportSelector, BeanClassLoaderAware, ResourceLoaderAware,
BeanFactoryAware, EnvironmentAware, Ordered {
　　 @Override
    public void process(AnnotationMetadata annotationMetadata,
            DeferredImportSelector deferredImportSelector) {
        Assert.state(
                deferredImportSelector instanceof AutoConfigurationImportSelector,
                () -> String.format("Only %s implementations are supported, got %s",
                        AutoConfigurationImportSelector.class.getSimpleName(),
                        deferredImportSelector.getClass().getName()));
        AutoConfigurationEntry autoConfigurationEntry = ((AutoConfigurationImportSelector) deferredImportSelector)
                .getAutoConfigurationEntry(getAutoConfigurationMetadata(),
                        annotationMetadata);
        this.autoConfigurationEntries.add(autoConfigurationEntry);
        for (String importClassName : autoConfigurationEntry.getConfigurations()) {
            this.entries.putIfAbsent(importClassName, annotationMetadata);
        }
    }
    protected AutoConfigurationEntry getAutoConfigurationEntry(
        AutoConfigurationMetadata autoConfigurationMetadata,
        AnnotationMetadata annotationMetadata) {
        if (!isEnabled(annotationMetadata)) {
            return EMPTY_ENTRY;
        }
        AnnotationAttributes attributes = getAttributes(annotationMetadata);
        List<String> configurations = getCandidateConfigurations(annotationMetadata,
                attributes);
        configurations = removeDuplicates(configurations);
        Set<String> exclusions = getExclusions(annotationMetadata, attributes);
        checkExcludedClasses(configurations, exclusions);
        configurations.removeAll(exclusions);
        configurations = filter(configurations, autoConfigurationMetadata);
        fireAutoConfigurationImportEvents(configurations, exclusions);
        return new AutoConfigurationEntry(configurations, exclusions);
    }
    protected List<String> getCandidateConfigurations(AnnotationMetadata metadata,
        AnnotationAttributes attributes) {
　　　　 //通过读取jar包下的spring.factories来自动配置。
        List<String> configurations = SpringFactoriesLoader.loadFactoryNames(
                getSpringFactoriesLoaderFactoryClass(), getBeanClassLoader());
        Assert.notEmpty(configurations,
                "No auto configuration classes found in META-INF/spring.factories. If you "
                        + "are using a custom packaging, make sure that file is correct.");
        return configurations;
    }
}