SpringBoot 自定义 starter 与 EnableAutoConfiguration

一、自定义 starter

1. 命名

官方命名：

• spring-boot-starter-XXX
• spring-boot-starter-jdbc
• spring-boot-starter-data-redis

非官方命名：

• XXX-spring-boot-starter
• mybatis-spring-boot-starter
• druid-spring-boot-starter

2.maven 依赖

spring-boot-autoconfigure 是必须的

spring-boot-configuration-processor 在编译后会生成 META-INF/spring-configuration-metadata.json 文件，起到提示的功能

<groupId>com.example</groupId>
<artifactId>animal-spring-boot-starter</artifactId>
<version>1.0-SNAPSHOT</version>

<dependencies>

    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-autoconfigure</artifactId>
        <version>2.2.2.RELEASE</version>
    </dependency>

    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-configuration-processor</artifactId>
        <version>2.2.2.RELEASE</version>
    </dependency>
</dependencies>

3.创建参数类

@ConfigurationProperties(prefix = "animal")
public class AnimalProperties {

    private String type = "animal";

    private final Dog dog = new Dog();

    private final Cat cat = new Cat();

    public static class Dog {

        private String name;

        public String getName() {
            return name;
        }

        public void setName(String name) {
            this.name = name;
        }
    }

    public static class Cat {

        private String name;

        public String getName() {
            return name;
        }

        public void setName(String name) {
            this.name = name;
        }
    }

    public String getType() {
        return type;
    }

    public void setType(String type) {
        this.type = type;
    }

    public Dog getDog() {
        return dog;
    }

    public Cat getCat() {
        return cat;
    }
}

4.创建业务实现类

public class AnimalService {

    private AnimalProperties animalProperties;

    public AnimalService(AnimalProperties animalProperties) {
        this.animalProperties = animalProperties;
    }

    public String getMsg() {
        return String.format("type: %s; dog: %s; cat: %s;",
                animalProperties.getType(),
                animalProperties.getDog().getName(),
                animalProperties.getCat().getName());
    }
}

5.创建自动配置类

@Configuration(proxyBeanMethods = false)
@EnableConfigurationProperties(AnimalProperties.class)
@ConditionalOnClass(AnimalProperties.class)
public class AnimalAutoConfiguration {

    @Autowired
    private AnimalProperties animalProperties;

    @Bean
    @ConditionalOnMissingBean(AnimalService.class)
    public AnimalService animalService() {
        return new AnimalService(animalProperties);
    }
}

6.配置spring.factories

新建META-INF目录，然后在META-INF目录下创建spring.factories文件

org.springframework.boot.autoconfigure.EnableAutoConfiguration=com.example.AnimalAutoConfiguration

二、使用 starter

1.maven 依赖

<groupId>com.example</groupId>
<artifactId>spring-boot-service</artifactId>
<version>1.0-SNAPSHOT</version>

<dependencies>

    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter</artifactId>
        <version>2.2.2.RELEASE</version>
    </dependency>

    <dependency>
        <groupId>com.example</groupId>
        <artifactId>animal-spring-boot-starter</artifactId>
        <version>1.0-SNAPSHOT</version>
    </dependency>
</dependencies>

2.配置文件

application.yml 文件

animal:
  type: dog and cat
  dog:
    name: xiao qiang
  cat:
    name: xiao hua

3.启动类

@SpringBootApplication
public class Boot {

    public static void main(String[] args) {
        ApplicationContext context = SpringApplication.run(Boot.class, args);

        AnimalService service = context.getBean(AnimalService.class);
        String msg = service.getMsg();
        System.out.println(msg);
    }
}

结果：

type: dog and cat; dog: xiao qiang; cat: xiao hua;

三、原理

启动类有 @SpringBootApplication 注解， @SpringBootApplication 有 @EnableAutoConfiguration 注解，@EnableAutoConfiguration 引入 AutoConfigurationImportSelector

// SpringBootApplication.java
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@SpringBootConfiguration
@EnableAutoConfiguration
@ComponentScan(excludeFilters = { @Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),
        @Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })
public @interface SpringBootApplication {
}

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

SpringBoot 会解析 AutoConfigurationImportSelector，调用 AutoConfigurationImportSelector$AutoConfigurationGroup 的 process 方法

SpringFactoriesLoader.loadFactoryNames 会读取 META-INF/spring.factories 文件内容，读取到

org.springframework.boot.autoconfigure.EnableAutoConfiguration=com.example.AnimalAutoConfiguration

注册 AnimalAutoConfiguration 到 spring

// AutoConfigurationImportSelector.java
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;
}

protected Class<?> getSpringFactoriesLoaderFactoryClass() {
    return EnableAutoConfiguration.class;
}

@Override
public Class<? extends Group> getImportGroup() {
    return AutoConfigurationGroup.class;
}

// AutoConfigurationImportSelector$AutoConfigurationGroup.java
@Override
public void process(AnnotationMetadata annotationMetadata, DeferredImportSelector deferredImportSelector) {
    Assert.state(deferredImportSelector instanceof AutoConfigurationImportSelector,
            () -> String.format("Only %s implementations are supported, got %s",
                    AutoConfigurationImportSelector.class.getSimpleName(),
                    deferredImportSelector.getClass().getName()));
    AutoConfigurationEntry autoConfigurationEntry = ((AutoConfigurationImportSelector) deferredImportSelector)
            .getAutoConfigurationEntry(getAutoConfigurationMetadata(), annotationMetadata);
    this.autoConfigurationEntries.add(autoConfigurationEntry);
    for (String importClassName : autoConfigurationEntry.getConfigurations()) {
        this.entries.putIfAbsent(importClassName, annotationMetadata);
    }
}