SpringBoot Starter的用法以及原理小结

<!--添加子模块-->  
<modules>  
<module>hello-spring-boot-starter</module>  
<module>hello-spring-boot-starter-autoconfigure</module>  
</modules>  
<!--定义的参数-->  
<properties>  
<maven.compiler.source>17</maven.compiler.source>  
<maven.compiler.target>17</maven.compiler.target>  
<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>  
<spring-boot.version>3.0.7</spring-boot.version>  
</properties>  
<!--依赖的配置约定-->  
<dependencyManagement>  
<dependencies>  
<dependency>  
<groupId>org.springframework.boot</groupId>  
<artifactId>spring-boot-autoconfigure</artifactId>  
<version>${spring-boot.version}</version>  
</dependency>  
<dependency>  
<groupId>org.springframework.boot</groupId>  
<artifactId>spring-boot-configuration-processor</artifactId>  
<version>3.5.11</version>  
</dependency>  
</dependencies>  
</dependencyManagement> 

<!--设置父模块-->  
<parent>  
<groupId>edu.whut</groupId>  
<artifactId>hello-spring-boot-starter-project</artifactId>  
<version>1.0-SNAPSHOT</version>  
</parent>
<dependencies>  
<dependency>  
<groupId>org.springframework.boot</groupId>  
<artifactId>spring-boot-autoconfigure</artifactId>  
</dependency>  
<dependency>  
<groupId>org.springframework.boot</groupId>  
<artifactId>spring-boot-configuration-processor</artifactId>  
</dependency>  
</dependencies>

public class HelloService {  
private final HelloProperties properties;  
  
public HelloService(HelloProperties properties) {  
this.properties = properties;  
}  
  
public void sayHello() {  
System.out.println("Hello,"+properties.getObject());  
}  
}

@ConfigurationProperties(prefix = "hello")  
public class HelloProperties {  
private String object;  
  
public String getObject() {  
return object;  
}  
  
public void setObject(String object) {  
this.object = object;  
}  
}

//标识自动配置类  
@AutoConfiguration  
@EnableConfigurationProperties(HelloProperties.class)  
public class HelloAutoConfiguration {  
@Bean  
//在application文件中配置了hello.object配置才会构造这个bean  
@ConditionalOnProperty(prefix = "hello", name = "object")  
public HelloService helloService(HelloProperties properties) {  
return new HelloService(properties);  
}  
}

edu.whut.HelloAutoConfiguration

<!--指定父模块-->  
<parent>  
<groupId>edu.whut</groupId>  
<artifactId>hello-spring-boot-starter-project</artifactId>  
<version>1.0-SNAPSHOT</version>  
</parent>
<dependencies>  
<dependency>  
<groupId>edu.whut</groupId>  
<artifactId>hello-spring-boot-starter-autoconfigure</artifactId>  
<version>1.0-SNAPSHOT</version>  
</dependency>  
</dependencies>

public static void main(String[] args) {  
ConfigurableApplicationContext applicationContext = SpringApplication.run(DemoApplication.class, args);  
Object bean = applicationContext.getBean("helloService");  
System.out.println(bean);  
}

hello.object="world"

edu.whut.HelloService@4463d9d3

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(AutoConfigurationImportSelector.class) // 核心关键
public @interface EnableAutoConfiguration {
    // ...
}

String[] selectImports(AnnotationMetadata importingClassMetadata);

public String[] selectImports(AnnotationMetadata annotationMetadata) {  
// 1. 检查是否开启了自动装配（默认是开启的）
if (!this.isEnabled(annotationMetadata)) {  
return NO_IMPORTS;  
} else {  
AutoConfigurationEntry autoConfigurationEntry = this.getAutoConfigurationEntry(annotationMetadata);  
return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations());  
}  
}

protected AutoConfigurationEntry getAutoConfigurationEntry(AnnotationMetadata annotationMetadata) {  
if (!this.isEnabled(annotationMetadata)) {  
return EMPTY_ENTRY;  
} else {  
AnnotationAttributes attributes = this.getAttributes(annotationMetadata); 
//加载所有候选配置类
List<String> configurations = this.getCandidateConfigurations(annotationMetadata, attributes);  //移除重复和显式排除的类
configurations = this.removeDuplicates(configurations);  
Set<String> exclusions = this.getExclusions(annotationMetadata, attributes);  
this.checkExcludedClasses(configurations, exclusions);  
configurations.removeAll(exclusions);  
//进行条件注解的筛选
configurations = this.getConfigurationClassFilter().filter(configurations);  
this.fireAutoConfigurationImportEvents(configurations, exclusions);  
return new AutoConfigurationEntry(configurations, exclusions);  
}  
}