spring-boot 是如何实现自动配置的

时间 2019-12-22

原文原文链接

spring-boot 实现了零配置集成组件，若是须要使用jpa，引入对应的starter，就在项目中自动配置好了。java

IOC容器中的条件Bean

spring从4.0开始支持条件bean，当判断条件为true时，bean的定义才有效。linux

先看看条件bean的定义：spring

@Configuration
class MyConfiguration {

@Bean(name = "emailerService")
@Conditional(WindowsCondition.class)
public EmailService windowsEmailerService() {
    return new WindowsEmailService();
}


@Bean(name = "emailerService")
@Conditional(LinuxCondition.class)
public EmailService linuxEmailerService() {
    return new LinuxEmailService();
}

}

上面代码是两个条件bean的定义，两个bean有相同的类型，一个在Windows下有效，一个在Linux下有效。windows

class WindowsCondition implements Condition {
 
	@Override
	public boolean matches(ConditionContext context,        AnnotatedTypeMetadata metadata) {
		return context.getEnvironment().getProperty("os.name").contains("Windows");
	}
}
 
class LinuxCondition implements Condition {
	@Override
	public boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) {
		return context.getEnvironment().getProperty("os.name").contains("Linux");
	}
}

在IOC加载bean定义的时候，会执行@Conditional注解指定的Condition，当Condition为true的时候该bean才有效，上面代码是两个实现了Condition接口的类，分别根据配置os.name是Windows仍是Linux判断是否为true。springboot

springboot中的条件bean，顾名思义

@ConditionalOnClass @ConditionalOnMissingClass
@ConditionalOnBean @ConditionalOnMissingBean
@ConditionalOnProperty
@ConditionalOnResource
@ConditionalOnWebApplication and @ConditionalOnNotWebApplication
@ConditionalOnExpression

定义Java配置的bean（在指定条件下才有效）

@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 {


}

以上为JPA的自动配置ide

@ConditionalOnBean(DataSource.class)：在容器中有DataSource实例的时候有效
@ConditionalOnClass(JpaRepository.class)：在classpath中有JpaRepository这个类的时候有效spring-boot
。。。spa

自动导入AutoConfiguration

在META-INF/spring.factories中添加配置:code

# Auto Configure
org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
org.springframework.boot.autoconfigure.data.jpa.JpaRepositoriesAutoConfiguration,\

spring-boot会自动扫描全部classpath下的META-INF/spring.factories，找到org.springframework.boot.autoconfigure.EnableAutoConfiguration对应的value，并导入value对应的配置。接口