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

<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

巴特西

Spring Boot之实现自动配置

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