/**
    * 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 {};

}

　　这里面包含了@SpringBootConfiguration，@EnableAutoConfiguration，@ComponentScan，此处@ComponentScan由于没有指定扫描包，因此它默认扫描的是与该类同级的类或者同级包下的所有类，另外@SpringBootConfiguration，通过源码得知它是一个@Configuration:

/*
 * Copyright 2012-2016 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.boot;

import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

import org.springframework.context.annotation.Configuration;

/**
 * Indicates that a class provides Spring Boot application
 * {@link Configuration @Configuration}. Can be used as an alternative to the Spring's
 * standard {@code @Configuration} annotation so that configuration can be found
 * automatically (for example in tests).
 * <p>
 * Application should only ever include <em>one</em> {@code @SpringBootConfiguration} and
 * most idiomatic Spring Boot applications will inherit it from
 * {@code @SpringBootApplication}.
 *
 * @author Phillip Webb
 * @since 1.4.0
 */
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Configuration
public @interface SpringBootConfiguration {

}

　　由此我们可以推断出@SpringBootApplication等同于@Configuration @ComponentScan @EnableAutoConfiguration

1.2、@EnableAutoConfiguration

　　一旦加上此注解，那么将会开启自动装配功能，简单点讲，Spring会试图在你的classpath下找到所有配置的Bean然后进行装配。当然装配Bean时，会根据若干个(Conditional)定制规则来进行初始化。我们看一下它的源码：