@SpringBootApplication

在一個springboot應用程序的入口，用到的@SpringbootApplication注解的源碼如下：

package org.springframework.boot.autoconfigure;

import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Inherited;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
import org.springframework.boot.SpringBootConfiguration;
import org.springframework.boot.context.TypeExcludeFilter;
import org.springframework.context.annotation.ComponentScan;
import org.springframework.context.annotation.FilterType;
import org.springframework.context.annotation.ComponentScan.Filter;
import org.springframework.core.annotation.AliasFor;

@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 {
    @AliasFor(
        annotation = EnableAutoConfiguration.class,
        attribute = "exclude"
    )
    Class<?>[] exclude() default {};

    @AliasFor(
        annotation = EnableAutoConfiguration.class,
        attribute = "excludeName"
    )
    String[] excludeName() default {};

    @AliasFor(
        annotation = ComponentScan.class,
        attribute = "basePackages"
    )
    String[] scanBasePackages() default {};

    @AliasFor(
        annotation = ComponentScan.class,
        attribute = "basePackageClasses"
    )
    Class<?>[] scanBasePackageClasses() default {};
}

@SpringBootConfiguration的源碼

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;

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Configuration
public @interface SpringBootConfiguration {
}

從上面兩段源碼可知，@SpringBootApplication是@Configuration，EnableAutoConfiguration，@ComponentScan的組合。

換言之 SpringBoot 提供了統一的注解來替代以上三個注解，簡化程序的配置。

@Configuration

Srping官方文檔：

@Configuration is a class-level annotation indicating that an object is a source of bean definitions.

@Configuration classes declare beans via public @Bean annotated methods. Calls to @Bean methods on @Configuration classes can also be used to define inter-bean dependencies.

@Configuration 是一個類級注釋，指示對象是一個bean定義的源。@Configuration 類通過 @bean 注解的公共方法聲明bean。

@Bean 注釋是用來表示一個方法實例化，配置和初始化是由 Spring IoC 容器管理的一個新的對象。

通俗的講 @Configuration 一般與 @Bean 注解配合使用，用 @Configuration 注解類等價與 XML 中配置 beans，用 @Bean 注解方法等價于 XML 中配置 bean。

舉個栗子：
xml配置

<beans>
    <bean id = "UserService" class="com.user.Userservice">
        <property name="userDAO" ref = "userDAO"></property>
    </bean>
    <bean id = "userDAO" class="com.user.UserDAO"></bean>
</beans>

java代碼：

import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;

@Configuration
public class Config {
    @Bean
    public UserService getUserService(){
        UserService userService = new UserService();
        userService.setUserDAO(null);
        return userService;
    }

    @Bean
    public UserDAO getUserDAO(){
        return new UserDAO();
    }
}

package org.spring.com.user;

public class UserService {
    private UserDAO userDAO;

    public UserDAO getUserDAO() {
        return userDAO;
    }

    public void setUserDAO(UserDAO userDAO) {
        this.userDAO = userDAO;
    }
}

package org.spring.com.user;

public class UserDAO {
}

小結：

使用 public 修飾 @Bean 注解的方法；UserService、UserDAO 類無需聲明為 @Component、@Service、@Repository、@Controller；

@EnableAutoConfiguration

spring官方文檔：