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