SpringBoot 自定义 starter

1. spring 框架中的 starter

spring 自动配置的依赖,：

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

springboot 框架本身大量组件的自动配置就是通过spring-boot-autoconfigure 实现的：

spring.factories 文件 EnableAutoConfiguration 后面配置了大量的自动配置类：

拿 jpa 的starter 举个例子：

@Configuration
@ConditionalOnBean(DataSource.class)
@ConditionalOnClass(JpaRepository.class)
@ConditionalOnMissingBean({ JpaRepositoryFactoryBean.class,
        JpaRepositoryConfigExtension.class })
@ConditionalOnProperty(prefix = "spring.data.jpa.repositories", name = "enabled", havingValue = "true", matchIfMissing = true)
@Import(JpaRepositoriesAutoConfigureRegistrar.class)
@AutoConfigureAfter(HibernateJpaAutoConfiguration.class)
public class JpaRepositoriesAutoConfiguration {
}

spring.factories 配置了上述配置类，JpaRepositoriesAutoConfiguration 会根据各类条件选择性加载。

org.springframework.boot.autoconfigure.EnableAutoConfiguration=org.springframework.boot.autoconfigure.data.jpa.JpaRepositoriesAutoConfiguration

通过 conditionOnXXx 注解，先判断spring容器是否已经加载相应的资源，且配置开关是开启的，spring 容器 jpa相关 bean 还没初始化，才会就行jpa配置的初始化；AutoConfigureAfter 注解配置了这个类的初始化顺序

jpa 配置是在HibernateJpaAutoConfiguration初始化之后再执行的：

@Configuration
@ConditionalOnClass({ LocalContainerEntityManagerFactoryBean.class, EntityManager.class })
@Conditional(HibernateEntityManagerCondition.class)
@EnableConfigurationProperties(JpaProperties.class)
@AutoConfigureAfter({ DataSourceAutoConfiguration.class })
@Import(HibernateJpaConfiguration.class)
public class HibernateJpaAutoConfiguration {
    @Order(Ordered.HIGHEST_PRECEDENCE + 20)
    static class HibernateEntityManagerCondition extends SpringBootCondition {
        private static final String[] CLASS_NAMES = {
                "org.hibernate.ejb.HibernateEntityManager",
                "org.hibernate.jpa.HibernateEntityManager" };
        @Override
        public ConditionOutcome getMatchOutcome(ConditionContext context,
                AnnotatedTypeMetadata metadata) {
            ConditionMessage.Builder message = ConditionMessage
                    .forCondition("HibernateEntityManager");
            for (String className : CLASS_NAMES) {
                if (ClassUtils.isPresent(className, context.getClassLoader())) {
                    return ConditionOutcome
                            .match(message.found("class").items(Style.QUOTE, className));
                }
            }
            return ConditionOutcome.noMatch(message.didNotFind("class", "classes")
                    .items(Style.QUOTE, Arrays.asList(CLASS_NAMES)));
        }
    }

2. 自定义 starter

pom 依赖：

          <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
            <optional>true</optional>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-configuration-processor</artifactId>
            <optional>true</optional>
        </dependency>

在 META-INF/spring.factories 中配 org.springframework.boot.autoconfigure.EnableAutoConfiguration =自己的配置类

配置类使用的注解有：
Configuration 注解
EnableConfigurationProperties 注解
NestedConfigurationProperty 注解
import 注解
ConditionalOnProperty注解
ConditionalOnClass注解
ConditionalOnMissingBean 注解
AutoConfigureAfter 注解