如何實現本身的SpringBoot自動配置

SpringBoot基本原理

SpringBoot能夠幫助咱們更好的使用Spring，更快的配置一個能夠運行的Spring應用。

SpringBoot實現自動配置實現的入口是@EnableAutoConfiguration註解，該註解又經過@Import註解導入了AutoConfigurationImportSelector，在該類中實現了對META-INF/spring.factories中 org.springframework.boot.autoconfigure.EnableAutoConfiguration屬性指定的全部配置，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，經過條件註解@ConditionalOnXXX，以及@Import註解來實現對緩存功能的相關Bean的自動配置。

// 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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實現本身的AutoConfiguration

在實際開發過程當中，咱們可能須要將公司內部的，或者Spring官方還不支持的第三方類庫集成到咱們的項目中。可是爲了重用，最好可以按照SpringBoot的自動配置來配置。

第一步：引入Spring官方依賴

項目的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官方的自動配置作區分 -->
	<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的自動配置模塊 -->
		<dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-autoconfigure</artifactId>
		</dependency>

        <!-- 本身的定義第三方依賴 -->
		<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：編寫Java Configuration，並添加條件註解

Spring支持的條件註解：

• 條件註解@Conditional
• 類條件@ConditionalOnClass，@ConditionalOnMissingClass
• 屬性條件@ConditionalOnProperty
• Bean 條件@ConditionalOnBean，@ConditionalOnMissingBean，@ConditionalOnSingleCandidate
• 資源條件@ConditionalOnResource
• Web 應⽤用條件@ConditionalOnWebApplication，@ConditionalOnNotWebApplication
• 其餘條件@ConditionalOnExpression，@ConditionalOnJava，@ConditionalOnJndi

Java Configuration代碼以下：

@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：定位自動配置

在當前項目的resource目錄下的META-INF目錄下建立spring.factories文件，內容以下：

org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
geektime.spring.hello.greeting.GreetingAutoConfiguration
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第四步：定義自動配置的執行順序

主要的註解有：

• @AutoConfigureBefore
• @AutoConfigureAfter
• @AutoConfigureOrder

通常不須要，具體使用能夠參考Spring官方的自動配置。

如何在不支持SpringBoot的老版本Spring中實現SpringBoot

若是在老版本的Spring中實現相似SpringBoot的自動配置功能，則須要使用Spring的擴展點機制，原理能夠參考博文Spring框架的擴展點。

添加一個Configuration：

@Configuration
public class GreetingAutoConfiguration {
    @Bean
    public static GreetingBeanFactoryPostProcessor greetingBeanFactoryPostProcessor() {
        return new GreetingBeanFactoryPostProcessor();
    }
}
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自定義一個BeanFactoryPostProcessor實現相似條件註解的功能：

@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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參考資料

1. Spring框架的擴展點
2. Spring全家桶視頻課程之SpringBoot
3. Spring全家桶課程代碼