Spring Boot 自动配置

Spring Boot 启动方法如下

@SpringBootApplication
public class ActuatorApplication {

    public static void main(String[] args) {
        SpringApplication.run(ActuatorApplication.class, args);
    }

}

@SpringBootApplication 其实包含3个注解

○@SpringBootConfiguration：标记当前类为配置类
○@EnableAutoConfiguration：开启自动配置
○@ComponentScan：扫描主类所在的同级包以及下级包里的Bean

关键是@EnableAutoConfiguration
我们来看看@EnableAutoConfiguration的源代码：

@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 {};
}

关键是@Import(AutoConfigurationImportSelector.class)导入的配置功能，
AutoConfigurationImportSelector中的方法getCandidateConfigurations，得到待配置的class的类名集合

/**
 * Return the auto-configuration class names that should be considered. By default
 * this method will load candidates using {@link SpringFactoriesLoader} with
 * {@link #getSpringFactoriesLoaderFactoryClass()}.
 * @param metadata the source metadata
 * @param attributes the {@link #getAttributes(AnnotationMetadata) annotation
 * attributes}
 * @return a list of candidate configurations
 */
    protected List<String> getCandidateConfigurations(AnnotationMetadata metadata,
            AnnotationAttributes attributes) {
        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;
}

下一步通过SpringFactoriesLoader.loadFactoryNames() 扫描所有 jar 包类路径下的META-INF/spring.factories，将扫描到的文件内容包装成properties对象，从properties中获取到EnableAutoConfiguration.class类名对应的值，然后把他们添加到容器中。