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如何实现自己的SpringBoot自动配置

SpringBoot基本原理

SpringBoot可以帮助我们更好的使用Spring,更快的配置一个可以运行的Spring应用。

SpringBoot实现自动配置实现的入口是@EnableAutoConfiguration注解,该注解又通过@Import注解导入了AutoConfigurationImportSelector,在该类中实现了对META-INF/spring.factoriesorg.springframework.boot.autoconfigure.EnableAutoConfiguration属性指定的所有配置,如org.springframework.boot.autoconfigure.cache.CacheAutoConfiguration

//文件:spring-boot-autoconfigure-2.1.3.RELEASE.jar!/META-INF/spring.factories

# Auto Configure
org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
org.springframework.boot.autoconfigure.admin.SpringApplicationAdminJmxAutoConfiguration,\
org.springframework.boot.autoconfigure.aop.AopAutoConfiguration,\
org.springframework.boot.autoconfigure.amqp.RabbitAutoConfiguration,\
org.springframework.boot.autoconfigure.batch.BatchAutoConfiguration,\
org.springframework.boot.autoconfigure.cache.CacheAutoConfiguration,\
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在CacheAutoConfiguration中,通过条件注解@ConditionalOnXXX,以及@Import注解来实现对缓存功能的相关Bean的自动配置。

// 文件:spring-boot-autoconfigure-2.1.3.RELEASE-sources.jar!/org/springframework/boot/autoconfigure/cache/CacheAutoConfiguration.java

@Configuration
@ConditionalOnClass(CacheManager.class)
@ConditionalOnBean(CacheAspectSupport.class)
@ConditionalOnMissingBean(value = CacheManager.class, name = "cacheResolver")
@EnableConfigurationProperties(CacheProperties.class)
@AutoConfigureAfter({ CouchbaseAutoConfiguration.class, HazelcastAutoConfiguration.class,
		HibernateJpaAutoConfiguration.class, RedisAutoConfiguration.class })
@Import(CacheConfigurationImportSelector.class)
public class CacheAutoConfiguration {

	@Bean
	@ConditionalOnMissingBean
	public CacheManagerCustomizers cacheManagerCustomizers(
			ObjectProvider<CacheManagerCustomizer<?>> customizers) {
		return new CacheManagerCustomizers(
				customizers.orderedStream().collect(Collectors.toList()));
	}
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实现自己的AutoConfiguration

在实际开发过程中,我们可能需要将公司内部的,或者Spring官方还不支持的第三方类库集成到我们的项目中。但是为了重用,最好能够按照SpringBoot的自动配置来配置。

第一步:引入Spring官方依赖

项目的pom文件:

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
	<modelVersion>4.0.0</modelVersion>
    <!-- 和Spring官方的自动配置做区分 -->
	<groupId>geektime.spring.hello</groupId>
	<artifactId>geektime-spring-boot-autoconfigure</artifactId>
	<version>0.0.1-SNAPSHOT</version>
	<name>geektime-spring-boot-autoconfigure</name>
	<description>Demo project for Spring Boot</description>

	<properties>
		<java.version>1.8</java.version>
		<spring-boot.version>2.1.3.RELEASE</spring-boot.version>
	</properties>

     <!-- 引入Spring官方的配置管理 -->
	<dependencyManagement>
		<dependencies>
			<dependency>
				<groupId>org.springframework.boot</groupId>
				<artifactId>spring-boot-dependencies</artifactId>
				<version>${spring-boot.version}</version>
				<type>pom</type>
				<scope>import</scope>
			</dependency>
		</dependencies>
	</dependencyManagement>

	<dependencies>
         <!-- 引入SpringBoot的自动配置模块 -->
		<dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-autoconfigure</artifactId>
		</dependency>

        <!-- 自己的定义第三方依赖 -->
		<dependency>
			<groupId>geektime.spring.hello</groupId>
			<artifactId>greeting</artifactId>
			<version>0.0.1-SNAPSHOT</version>
			<scope>provided</scope>
		</dependency>
	</dependencies>

</project>
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第二步:编写Java Configuration,并添加条件注解

Spring支持的条件注解:

  • 条件注解@Conditional
  • 类条件@ConditionalOnClass,@ConditionalOnMissingClass
  • 属性条件@ConditionalOnProperty
  • Bean 条件@ConditionalOnBean,@ConditionalOnMissingBean,@ConditionalOnSingleCandidate
  • 资源条件@ConditionalOnResource
  • Web 应⽤用条件@ConditionalOnWebApplication,@ConditionalOnNotWebApplication
  • 其他条件@ConditionalOnExpression,@ConditionalOnJava,@ConditionalOnJndi

Java Configuration代码如下:

@Configuration
@ConditionalOnClass(GreetingApplicationRunner.class)
public class GreetingAutoConfiguration {
    @Bean
    @ConditionalOnMissingBean(GreetingApplicationRunner.class)
    @ConditionalOnProperty(name = "greeting.enabled", havingValue = "true", matchIfMissing = true)
    public GreetingApplicationRunner greetingApplicationRunner() {
        return new GreetingApplicationRunner();
    }
}
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第三步:定位自动配置

在当前项目的resource目录下的META-INF目录下创建spring.factories文件,内容如下:

org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
geektime.spring.hello.greeting.GreetingAutoConfiguration
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第四步:定义自动配置的执行顺序

主要的注解有:

  • @AutoConfigureBefore
  • @AutoConfigureAfter
  • @AutoConfigureOrder

一般不需要,具体使用可以参考Spring官方的自动配置

如何在不支持SpringBoot的老版本Spring中实现SpringBoot

如果在老版本的Spring中实现类似SpringBoot的自动配置功能,则需要使用Spring的扩展点机制,原理可以参考博文Spring框架的扩展点

添加一个Configuration:

@Configuration
public class GreetingAutoConfiguration {
    @Bean
    public static GreetingBeanFactoryPostProcessor greetingBeanFactoryPostProcessor() {
        return new GreetingBeanFactoryPostProcessor();
    }
}
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自定义一个BeanFactoryPostProcessor实现类似条件注解的功能:

@Slf4j
public class GreetingBeanFactoryPostProcessor implements BeanFactoryPostProcessor {
    public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {
        boolean hasClass = ClassUtils.isPresent("geektime.spring.hello.greeting.GreetingApplicationRunner",
                GreetingBeanFactoryPostProcessor.class.getClassLoader());
        if (!hasClass) {
            log.info("GreetingApplicationRunner is NOT present in CLASSPATH.");
            return;
        }

        if (beanFactory.containsBeanDefinition("greetingApplicationRunner")) {
            log.info("We already have a greetingApplicationRunner bean registered.");
            return;
        }

        register(beanFactory);
    }

    private void register(ConfigurableListableBeanFactory beanFactory) {
        if (beanFactory instanceof BeanDefinitionRegistry) {
            GenericBeanDefinition beanDefinition = new GenericBeanDefinition();
            beanDefinition.setBeanClass(GreetingApplicationRunner.class);

            ((BeanDefinitionRegistry) beanFactory)
                    .registerBeanDefinition("greetingApplicationRunner",
                            beanDefinition);
        } else {
            beanFactory.registerSingleton("greetingApplicationRunner",
                    new GreetingApplicationRunner());
        }
    }
}
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参考资料

  1. Spring框架的扩展点
  2. Spring全家桶视频课程之SpringBoot
  3. Spring全家桶课程代码