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。

 1 @Target(ElementType.TYPE)
 2 @Retention(RetentionPolicy.RUNTIME)
 3 @Documented
 4 @Inherited
 5 @AutoConfigurationPackage
 6 @Import(AutoConfigurationImportSelector.class)
 7 public @interface EnableAutoConfiguration {
 8 
 9     String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";
10 
11     /**
12      * Exclude specific auto-configuration classes such that they will never be applied.
13      * @return the classes to exclude
14      */
15     Class<?>[] exclude() default {};
16 
17     /**
18      * Exclude specific auto-configuration class names such that they will never be
19      * applied.
20      * @return the class names to exclude
21      * @since 1.3.0
22      */
23     String[] excludeName() default {};
24 
25 }

　　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。

1 protected List<String> getCandidateConfigurations(AnnotationMetadata metadata,
2             AnnotationAttributes attributes) {
3         List<String> configurations = SpringFactoriesLoader.loadFactoryNames(
4                 getSpringFactoriesLoaderFactoryClass(), getBeanClassLoader());
5         Assert.notEmpty(configurations,
6                 "No auto configuration classes found in META-INF/spring.factories. If you "
7                         + "are using a custom packaging, make sure that file is correct.");
8         return configurations;
9     }

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

1 org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
2   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

 1 /**
 2  * @author zhangboqing
 3  * @date 2018/6/29
 4  */
 5 @Data
 6 @ConfigurationProperties(prefix = "hello")
 7 public class HelloServiceProperties {
 8 
 9     private static final String MSG = "world";
10 
11     private String msg = MSG;
12 }

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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

 1 /**
 2  * @author zhangboqing
 3  * @date 2018/6/29
 4  */
 5 @Data
 6 public class HelloService {
 7 
 8     private String msg;
 9 
10     public String sayHello() {
11         return new StringBuilder("Hello").append(msg).toString();
12     }
13 }

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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

 1 /**
 2  * @author zhangboqing
 3  * @date 2018/6/29
 4  */
 5 @Configuration
 6 @EnableConfigurationProperties(HelloServiceProperties.class)
 7 @ConditionalOnClass(HelloService.class)
 8 @ConditionalOnProperty(prefix = "hello",matchIfMissing = true)
 9 public class HelloServiceAutoConfiguration {
10 
11     @Autowired
12     private HelloServiceProperties helloServiceProperties;
13 
14     @Bean
15     @ConditionalOnMissingBean(HelloService.class)
16     public HelloService helloService() {
17         HelloService helloService = new HelloService();
18         helloService.setMsg(helloServiceProperties.getMsg());
19         return helloService;
20     }
21 
22 }

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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

Spring Boot之实现自动配置

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