Spring Boot之实现自动配置

GITHUB地址:https://github.com/zhangboqing/springboot-learning

一、Spring Boot自动配置原理

　　自动配置功能是由@SpringBootApplication中的@EnableAutoConfiguration注解提供的。

 @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 {};

 }

　　这里的关键功能是@Import(AutoConfigurationImportSelector.class)，它导入了AutoConfigurationImportSelector类，该类使用了SpringFactoriesLoader.loadFactoryNames方法来扫描具有META-INF/spring.factories文件的jar包。

 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;
     }

　　并且在META-INF/spring.factories中定义了需要自动配置的具体类是什么

 org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
   com.zbq2.autoconfig.HelloServiceAutoConfiguration

二、如何实现自动配置功能

1.新建stater maven项目
增加Spring Boot自身的自动配置maven依赖
<!-- https://mvnrepository.com/artifact/org.springframework.boot/spring-boot-autoconfigure -->
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-autoconfigure</artifactId>
<version>2.0.3.RELEASE</version>
</dependency>

2.属性配置
HelloServiceProperties
自定义一个获取属性的类,可以获取在application.properties中设置的自定义属性

 /**
  * @author zhangboqing
  * @date 2018/6/29
  */
 @Data
 @ConfigurationProperties(prefix = "hello")
 public class HelloServiceProperties {

     private static final String MSG = "world";

     private String msg = MSG;
 }

3.判断依据类
HelloService
定义一个类，通过判断该类是否存在，来创建该类的Bean,该类可以是第三方库的类如DataSource

 /**
  * @author zhangboqing
  * @date 2018/6/29
  */
 @Data
 public class HelloService {

     private String msg;

     public String sayHello() {
         return new StringBuilder("Hello").append(msg).toString();
     }
 }

4.自动配置类
HelloServiceAutoConfiguration
根据HelloServiceProperties提供的参数，并通过@ConditionalOnClass判断HelloService这个类在类路径中是否存在，且当容器中没有这个Bean的情况下自动配置这个Bean

 /**
  * @author zhangboqing
  * @date 2018/6/29
  */
 @Configuration
 @EnableConfigurationProperties(HelloServiceProperties.class)
 @ConditionalOnClass(HelloService.class)
 @ConditionalOnProperty(prefix = "hello",matchIfMissing = true)
 public class HelloServiceAutoConfiguration {

     @Autowired
     private HelloServiceProperties helloServiceProperties;

     @Bean
     @ConditionalOnMissingBean(HelloService.class)
     public HelloService helloService() {
         HelloService helloService = new HelloService();
         helloService.setMsg(helloServiceProperties.getMsg());
         return helloService;
     }

 }

5.注册配置
若想自动配置生效，需要注册自动配置类。在src/main/resources下新建META-INF/spring.factories
如下：

# Auto Configure
org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
org.springframework.boot.autoconfigure.admin.SpringApplicationAdminJmxAutoConfiguration,\
org.springframework.boot.autoconfigure.aop.AopAutoConfiguration

若有多个自动配置，则用','隔开，此处'\'是为了换行后仍能读到属性

org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
  com.zbq2.autoconfig.HelloServiceAutoConfiguration

6.使用starter 作为其他项目的依赖
引入maven 坐标依赖