SpringBoot自动配置原理

结论：

@EnableAutoConfiguration注解内部使用@Import({AutoConfigurationImportSelector.class})加载配置类。

配置文件位置， META-INF/spring.factories 文件，该配置文件定义了大量的配置类，当SpringBoot启动时，会自动加载这些配置类，初始化Bean

并不是所有的Bean都会被初始化，在配置类中使用Condition来加载满足条件的Bean

我们点进去引导类上的注解@SpringBootApplication

@SpringBootApplication
public class SpringbootConditionApplication {

    public static void main(String[] args) {
        ConfigurableApplicationContext context = SpringApplication.run(SpringbootConditionApplication.class, args);
        Object user = context.getBean("user");
        System.out.println(user);

    }

}

点进去之后看到

@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 {

这其中有三个注解比较重要：

@SpringBootConfiguration 加了这个注解说明当前类为配置类。

@ComponentScan扫描当前类下的package

@EnableAutoConfiguration这个注解帮助我们自动载入应用程序所需要的配置。点进去

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import({AutoConfigurationImportSelector.class})
public @interface EnableAutoConfiguration {
    String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";

    Class<?>[] exclude() default {};

    String[] excludeName() default {};
}

我们发现它用@Import导入了AutoConfigurationImportSelector，点进去，我们看到它实现了这样的方法

public String[] selectImports(AnnotationMetadata annotationMetadata) {
  //表示没有需要导入的内容。所以如果功能未启用，直接返回这个空数组。      
if (!this.isEnabled(annotationMetadata)) {
            return NO_IMPORTS;
        } else {
            AutoConfigurationEntry autoConfigurationEntry = this.getAutoConfigurationEntry(annotationMetadata);
        //获取到自动配置项后，将其配置转换为字符串数组并返回。
            return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations());
        }
    }

这个方法返回了一个字符串数组，这个数组里定义了很多需要被加载的类。其中这句代码定义加载类,并返回一个Entry对象：

 AutoConfigurationEntry autoConfigurationEntry = this.getAutoConfigurationEntry(annotationMetadata);

我们查看getAutoConfigurationEntry方法

  protected AutoConfigurationEntry getAutoConfigurationEntry(AnnotationMetadata annotationMetadata) {
        if (!this.isEnabled(annotationMetadata)) {
            return EMPTY_ENTRY;
        } else {
            AnnotationAttributes attributes = this.getAttributes(annotationMetadata);
            List<String> configurations = this.getCandidateConfigurations(annotationMetadata, attributes);
            configurations = this.removeDuplicates(configurations);
            Set<String> exclusions = this.getExclusions(annotationMetadata, attributes);
            this.checkExcludedClasses(configurations, exclusions);
            configurations.removeAll(exclusions);
            configurations = this.getConfigurationClassFilter().filter(configurations);
            this.fireAutoConfigurationImportEvents(configurations, exclusions);
            return new AutoConfigurationEntry(configurations, exclusions);
        }
    }

 核心代码是 List<String> configurations = this.getCandidateConfigurations(annotationMetadata, attributes);跟进去getCandidateConfigurations

 protected List<String> getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) {
        List<String> configurations = SpringFactoriesLoader.loadFactoryNames(this.getSpringFactoriesLoaderFactoryClass(), this.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;
    }

这个方法的作用是通过 Spring 的工厂加载器从 META-INF/spring.factories 文件中加载自动配置类的名称，并将这些类名存储在列表中返回。