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关于springboot:springboot开发笔记9自动装配与EnableAutoConfiguration和Conditinal

jiezi

上一节:springboot 开发笔记 -(8)- 配置文件的加载程序

springboot 主动拆卸原理和两个注解类的关系是深有千千结: @EnableAutoConfiguration 和 @Conditinal

咱们从 springboot 的启动类上的注解看下这千头万绪: 张宇的《一言难尽》不是有这么几句:

从哪里开始 从哪里失去
我一言难尽 忍不住伤心
掂量不出爱或不爱之间的间隔
模摸糊糊中 明确你的决定
不敢勉强你 只好尴尬本人
我尴尬我本人 …

9.1 从哪里开始: @SpringBootApplication

package com.niewj.springboot;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class ConfigApplication {public static void main(String[] args) {SpringApplication.run(ConfigApplication.class, args);
    }

}

@SpringBootApplication:

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@SpringBootConfiguration // 1
@EnableAutoConfiguration // 2
@ComponentScan(excludeFilters = { @Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),
      @Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })
public @interface SpringBootApplication {}

咱们来说下这两个注解的性能 :

// 1. 次要是讲此类标识为: @Configuration, 作为配置类, 让 spring 容器启动辨认;

// 2. @EnableAutoConfiguration 次要是主动拆卸; 上面细说 它;

9.2 主动拆卸去: @EnableAutoConfiguration:

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage // 3
@Import(AutoConfigurationImportSelector.class) // 4
public @interface EnableAutoConfiguration 
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@Import(AutoConfigurationPackages.Registrar.class) // 5
public @interface AutoConfigurationPackage {}

// 3. @AutoConfigurationPackage 次要做了什么? 看注解链:

@SpringBootApplication -> @EnableAutoConfiguration -> @AutoConfigurationPackage -> @Import(AutoConfigurationPackages.Registrar.class)

好了, 咱们就次要看看 AutoConfigurationPackages.Registrar.class做了什么咱们见不得的事件:

参见上面的 #9.2.1 而后小结:

把入口类的 package 名, 注册到容器的 Bean 定义中, 后续扫描注解用(@Component/@Service/@Controller/@Repository 等)

// 4. @Import(AutoConfigurationImportSelector.class) 咱们也另立门户:

参见上面的 #9.2.2 而后小结:

做了啥啥啥

9.2.1 AutoConfigurationPackages.Registrar.class

/**
 * {@link ImportBeanDefinitionRegistrar} to store the base package from the importing
 * configuration.
 */
static class Registrar implements ImportBeanDefinitionRegistrar, DeterminableImports {

    @Override
    public void registerBeanDefinitions(AnnotationMetadata metadata, BeanDefinitionRegistry registry) {register(registry, new PackageImport(metadata).getPackageName());
    }

    @Override
    public Set<Object> determineImports(AnnotationMetadata metadata) {return Collections.singleton(new PackageImport(metadata));
    }

}

实现了 ImportBeanDefinitionRegistrar接口, 也就是说, 是向容器中注册 Bean 的(不明确的能够参考:spring 注解驱动开发 -(5) 向 Spring 容器中注册组件的办法), 注册什么 Bean 呢?

register(registry, new PackageImport(metadata).getPackageName());

打个断点 debug 看看: 相当于向容器中注册了以后入口类的包名, 后续扫描包下注解的时候就能够用到了!

原来如此, 小结: AutoConfigurationPackages.Registrar.class的性能如其名: 把入口类的 package 名注册到容器的 Bean 定义中以供后续加载和扫描注解用; 比方 @Component/@Service/@Controller/@Repository 等这些;

9.2.2 @Import(AutoConfigurationImportSelector.class)

@Import 就不必介绍了 (能够参考文末的链接): 向容器中注册一个一般 Bean 的; 那么AutoConfigurationImportSelector 是一个什么样的一般 Bean 呢?

@Override
public String[] selectImports(AnnotationMetadata annotationMetadata) {if (!isEnabled(annotationMetadata)) {return NO_IMPORTS;}
    AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader
        .loadMetadata(this.beanClassLoader);
    // ==> 要害在这里: getAutoConfigurationEntry()
    AutoConfigurationEntry autoConfigurationEntry = getAutoConfigurationEntry(autoConfigurationMetadata,
                                                                              annotationMetadata);
    return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations());
}

/**
 * Return the {@link AutoConfigurationEntry} based on the {@link AnnotationMetadata}
 * of the importing {@link Configuration @Configuration} class.
 * @param autoConfigurationMetadata the auto-configuration metadata
 * @param annotationMetadata the annotation metadata of the configuration class
 * @return the auto-configurations that should be imported
 */
protected AutoConfigurationEntry getAutoConfigurationEntry(AutoConfigurationMetadata autoConfigurationMetadata,
                                                           AnnotationMetadata annotationMetadata) {if (!isEnabled(annotationMetadata)) {return EMPTY_ENTRY;}
    AnnotationAttributes attributes = getAttributes(annotationMetadata);
    // ==> 要害在这里:getCandidateConfigurations()
    List<String> configurations = getCandidateConfigurations(annotationMetadata, attributes);
    configurations = removeDuplicates(configurations);
    Set<String> exclusions = getExclusions(annotationMetadata, attributes);
    checkExcludedClasses(configurations, exclusions);
    configurations.removeAll(exclusions);
    configurations = filter(configurations, autoConfigurationMetadata);
    fireAutoConfigurationImportEvents(configurations, exclusions);
    return new AutoConfigurationEntry(configurations, exclusions);
}

/**
 * Return the auto-configuration class names that should be considered. By default
 * this method will load candidates using {@link SpringFactoriesLoader} with
 * {@link #getSpringFactoriesLoaderFactoryClass()}.
 * @param metadata the source metadata
 * @param attributes the {@link #getAttributes(AnnotationMetadata) annotation
 * attributes}
 * @return a list of candidate configurations
 */
protected List<String> getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) {// ==> 要害在这里: SpringFactoriesLoader.loadFactoryNames()
    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;
}

那么外围问题来了, SpringFactoriesLoader 的 loadFactoryNames()是作甚的?

List<String> configurations = SpringFactoriesLoader.loadFactoryNames(getSpringFactoriesLoaderFactoryClass(),
      getBeanClassLoader());

/**
 * Load the fully qualified class names of factory implementations of the
 * given type from {@value #FACTORIES_RESOURCE_LOCATION}, using the given
 * class loader.
 * @param factoryType the interface or abstract class representing the factory
 * @param classLoader the ClassLoader to use for loading resources; can be
 * {@code null} to use the default
 * @throws IllegalArgumentException if an error occurs while loading factory names
 * @see #loadFactories
 */
public static List<String> loadFactoryNames(Class<?> factoryType, @Nullable ClassLoader classLoader) {String factoryTypeName = factoryType.getName();
    return loadSpringFactories(classLoader).getOrDefault(factoryTypeName, Collections.emptyList());
}

Load the fully qualified class names of factory implementations of the given type from “META-INF/spring.factories”, using the given class loader.

工具人翻译:

从 ”META-INF/spring.factories” 配置文件里, 加载给定类型的工厂 factoryType 实现的全限定类名, 应用给定的类加载器。

例如 org.springframework.boot.autoconfigure 包下的 spring-boot-autoconfigure-2.2.2.RELEASE.jar 中的 ”META-INF/spring.factories”

# Initializers
org.springframework.context.ApplicationContextInitializer=\
org.springframework.boot.autoconfigure.SharedMetadataReaderFactoryContextInitializer,\
org.springframework.boot.autoconfigure.logging.ConditionEvaluationReportLoggingListener

# Application Listeners
org.springframework.context.ApplicationListener=\
org.springframework.boot.autoconfigure.BackgroundPreinitializer

# Auto Configuration Import Listeners
org.springframework.boot.autoconfigure.AutoConfigurationImportListener=\
org.springframework.boot.autoconfigure.condition.ConditionEvaluationReportAutoConfigurationImportListener

# Auto Configuration Import Filters
org.springframework.boot.autoconfigure.AutoConfigurationImportFilter=\
org.springframework.boot.autoconfigure.condition.OnBeanCondition,\
org.springframework.boot.autoconfigure.condition.OnClassCondition,\
org.springframework.boot.autoconfigure.condition.OnWebApplicationCondition

# 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,\
org.springframework.boot.autoconfigure.cassandra.CassandraAutoConfiguration,\
org.springframework.boot.autoconfigure.cloud.CloudServiceConnectorsAutoConfiguration,\
org.springframework.boot.autoconfigure.context.ConfigurationPropertiesAutoConfiguration,\
org.springframework.boot.autoconfigure.context.MessageSourceAutoConfiguration,\
org.springframework.boot.autoconfigure.context.PropertyPlaceholderAutoConfiguration,\
org.springframework.boot.autoconfigure.couchbase.CouchbaseAutoConfiguration,\
org.springframework.boot.autoconfigure.dao.PersistenceExceptionTranslationAutoConfiguration,\
org.springframework.boot.autoconfigure.data.cassandra.CassandraDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.cassandra.CassandraReactiveDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.cassandra.CassandraReactiveRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.cassandra.CassandraRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.couchbase.CouchbaseDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.couchbase.CouchbaseReactiveDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.couchbase.CouchbaseReactiveRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.couchbase.CouchbaseRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchAutoConfiguration,\
org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.elasticsearch.ReactiveElasticsearchRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.elasticsearch.ReactiveRestClientAutoConfiguration,\
org.springframework.boot.autoconfigure.data.jdbc.JdbcRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.jpa.JpaRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.ldap.LdapRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.mongo.MongoDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.mongo.MongoReactiveDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.mongo.MongoReactiveRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.mongo.MongoRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.neo4j.Neo4jDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.neo4j.Neo4jRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.solr.SolrRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.redis.RedisAutoConfiguration,\
org.springframework.boot.autoconfigure.data.redis.RedisReactiveAutoConfiguration,\
org.springframework.boot.autoconfigure.data.redis.RedisRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.rest.RepositoryRestMvcAutoConfiguration,\
org.springframework.boot.autoconfigure.data.web.SpringDataWebAutoConfiguration,\
org.springframework.boot.autoconfigure.elasticsearch.jest.JestAutoConfiguration,\
org.springframework.boot.autoconfigure.elasticsearch.rest.RestClientAutoConfiguration,\
org.springframework.boot.autoconfigure.flyway.FlywayAutoConfiguration,\
org.springframework.boot.autoconfigure.freemarker.FreeMarkerAutoConfiguration,\
org.springframework.boot.autoconfigure.gson.GsonAutoConfiguration,\
org.springframework.boot.autoconfigure.h2.H2ConsoleAutoConfiguration,\
org.springframework.boot.autoconfigure.hateoas.HypermediaAutoConfiguration,\
org.springframework.boot.autoconfigure.hazelcast.HazelcastAutoConfiguration,\
org.springframework.boot.autoconfigure.hazelcast.HazelcastJpaDependencyAutoConfiguration,\
org.springframework.boot.autoconfigure.http.HttpMessageConvertersAutoConfiguration,\
org.springframework.boot.autoconfigure.http.codec.CodecsAutoConfiguration,\
org.springframework.boot.autoconfigure.influx.InfluxDbAutoConfiguration,\
org.springframework.boot.autoconfigure.info.ProjectInfoAutoConfiguration,\
org.springframework.boot.autoconfigure.integration.IntegrationAutoConfiguration,\
org.springframework.boot.autoconfigure.jackson.JacksonAutoConfiguration,\
org.springframework.boot.autoconfigure.jdbc.DataSourceAutoConfiguration,\
org.springframework.boot.autoconfigure.jdbc.JdbcTemplateAutoConfiguration,\
org.springframework.boot.autoconfigure.jdbc.JndiDataSourceAutoConfiguration,\
org.springframework.boot.autoconfigure.jdbc.XADataSourceAutoConfiguration,\
org.springframework.boot.autoconfigure.jdbc.DataSourceTransactionManagerAutoConfiguration,\
org.springframework.boot.autoconfigure.jms.JmsAutoConfiguration,\
org.springframework.boot.autoconfigure.jmx.JmxAutoConfiguration,\
org.springframework.boot.autoconfigure.jms.JndiConnectionFactoryAutoConfiguration,\
org.springframework.boot.autoconfigure.jms.activemq.ActiveMQAutoConfiguration,\
org.springframework.boot.autoconfigure.jms.artemis.ArtemisAutoConfiguration,\
org.springframework.boot.autoconfigure.groovy.template.GroovyTemplateAutoConfiguration,\
org.springframework.boot.autoconfigure.jersey.JerseyAutoConfiguration,\
org.springframework.boot.autoconfigure.jooq.JooqAutoConfiguration,\
org.springframework.boot.autoconfigure.jsonb.JsonbAutoConfiguration,\
org.springframework.boot.autoconfigure.kafka.KafkaAutoConfiguration,\
org.springframework.boot.autoconfigure.ldap.embedded.EmbeddedLdapAutoConfiguration,\
org.springframework.boot.autoconfigure.ldap.LdapAutoConfiguration,\
org.springframework.boot.autoconfigure.liquibase.LiquibaseAutoConfiguration,\
org.springframework.boot.autoconfigure.mail.MailSenderAutoConfiguration,\
org.springframework.boot.autoconfigure.mail.MailSenderValidatorAutoConfiguration,\
org.springframework.boot.autoconfigure.mongo.embedded.EmbeddedMongoAutoConfiguration,\
org.springframework.boot.autoconfigure.mongo.MongoAutoConfiguration,\
org.springframework.boot.autoconfigure.mongo.MongoReactiveAutoConfiguration,\
org.springframework.boot.autoconfigure.mustache.MustacheAutoConfiguration,\
org.springframework.boot.autoconfigure.orm.jpa.HibernateJpaAutoConfiguration,\
org.springframework.boot.autoconfigure.quartz.QuartzAutoConfiguration,\
org.springframework.boot.autoconfigure.rsocket.RSocketMessagingAutoConfiguration,\
org.springframework.boot.autoconfigure.rsocket.RSocketRequesterAutoConfiguration,\
org.springframework.boot.autoconfigure.rsocket.RSocketServerAutoConfiguration,\
org.springframework.boot.autoconfigure.rsocket.RSocketStrategiesAutoConfiguration,\
org.springframework.boot.autoconfigure.security.servlet.SecurityAutoConfiguration,\
org.springframework.boot.autoconfigure.security.servlet.UserDetailsServiceAutoConfiguration,\
org.springframework.boot.autoconfigure.security.servlet.SecurityFilterAutoConfiguration,\
org.springframework.boot.autoconfigure.security.reactive.ReactiveSecurityAutoConfiguration,\
org.springframework.boot.autoconfigure.security.reactive.ReactiveUserDetailsServiceAutoConfiguration,\
org.springframework.boot.autoconfigure.security.rsocket.RSocketSecurityAutoConfiguration,\
org.springframework.boot.autoconfigure.security.saml2.Saml2RelyingPartyAutoConfiguration,\
org.springframework.boot.autoconfigure.sendgrid.SendGridAutoConfiguration,\
org.springframework.boot.autoconfigure.session.SessionAutoConfiguration,\
org.springframework.boot.autoconfigure.security.oauth2.client.servlet.OAuth2ClientAutoConfiguration,\
org.springframework.boot.autoconfigure.security.oauth2.client.reactive.ReactiveOAuth2ClientAutoConfiguration,\
org.springframework.boot.autoconfigure.security.oauth2.resource.servlet.OAuth2ResourceServerAutoConfiguration,\
org.springframework.boot.autoconfigure.security.oauth2.resource.reactive.ReactiveOAuth2ResourceServerAutoConfiguration,\
org.springframework.boot.autoconfigure.solr.SolrAutoConfiguration,\
org.springframework.boot.autoconfigure.task.TaskExecutionAutoConfiguration,\
org.springframework.boot.autoconfigure.task.TaskSchedulingAutoConfiguration,\
org.springframework.boot.autoconfigure.thymeleaf.ThymeleafAutoConfiguration,\
org.springframework.boot.autoconfigure.transaction.TransactionAutoConfiguration,\
org.springframework.boot.autoconfigure.transaction.jta.JtaAutoConfiguration,\
org.springframework.boot.autoconfigure.validation.ValidationAutoConfiguration,\
org.springframework.boot.autoconfigure.web.client.RestTemplateAutoConfiguration,\
org.springframework.boot.autoconfigure.web.embedded.EmbeddedWebServerFactoryCustomizerAutoConfiguration,\
org.springframework.boot.autoconfigure.web.reactive.HttpHandlerAutoConfiguration,\
org.springframework.boot.autoconfigure.web.reactive.ReactiveWebServerFactoryAutoConfiguration,\
org.springframework.boot.autoconfigure.web.reactive.WebFluxAutoConfiguration,\
org.springframework.boot.autoconfigure.web.reactive.error.ErrorWebFluxAutoConfiguration,\
org.springframework.boot.autoconfigure.web.reactive.function.client.ClientHttpConnectorAutoConfiguration,\
org.springframework.boot.autoconfigure.web.reactive.function.client.WebClientAutoConfiguration,\
org.springframework.boot.autoconfigure.web.servlet.DispatcherServletAutoConfiguration,\
org.springframework.boot.autoconfigure.web.servlet.ServletWebServerFactoryAutoConfiguration,\
org.springframework.boot.autoconfigure.web.servlet.error.ErrorMvcAutoConfiguration,\
org.springframework.boot.autoconfigure.web.servlet.HttpEncodingAutoConfiguration,\
org.springframework.boot.autoconfigure.web.servlet.MultipartAutoConfiguration,\
org.springframework.boot.autoconfigure.web.servlet.WebMvcAutoConfiguration,\
org.springframework.boot.autoconfigure.websocket.reactive.WebSocketReactiveAutoConfiguration,\
org.springframework.boot.autoconfigure.websocket.servlet.WebSocketServletAutoConfiguration,\
org.springframework.boot.autoconfigure.websocket.servlet.WebSocketMessagingAutoConfiguration,\
org.springframework.boot.autoconfigure.webservices.WebServicesAutoConfiguration,\
org.springframework.boot.autoconfigure.webservices.client.WebServiceTemplateAutoConfiguration

# Failure analyzers
org.springframework.boot.diagnostics.FailureAnalyzer=\
org.springframework.boot.autoconfigure.diagnostics.analyzer.NoSuchBeanDefinitionFailureAnalyzer,\
org.springframework.boot.autoconfigure.flyway.FlywayMigrationScriptMissingFailureAnalyzer,\
org.springframework.boot.autoconfigure.jdbc.DataSourceBeanCreationFailureAnalyzer,\
org.springframework.boot.autoconfigure.jdbc.HikariDriverConfigurationFailureAnalyzer,\
org.springframework.boot.autoconfigure.session.NonUniqueSessionRepositoryFailureAnalyzer

# Template availability providers
org.springframework.boot.autoconfigure.template.TemplateAvailabilityProvider=\
org.springframework.boot.autoconfigure.freemarker.FreeMarkerTemplateAvailabilityProvider,\
org.springframework.boot.autoconfigure.mustache.MustacheTemplateAvailabilityProvider,\
org.springframework.boot.autoconfigure.groovy.template.GroovyTemplateAvailabilityProvider,\
org.springframework.boot.autoconfigure.thymeleaf.ThymeleafTemplateAvailabilityProvider,\
org.springframework.boot.autoconfigure.web.servlet.JspTemplateAvailabilityProvider

到这里是不是感觉快迷失了? 回顾下下面那句歌词:从哪里开始 从哪里失去 感觉是不是很恐怖? 好了, 咱们收住!

小结一下: @Import(AutoConfigurationImportSelector.class)次要做的事:

  1. 因为 ImportSelector 接口就是返回一个 类名的字符串数组String[], 而后交给 spring 容器让它来把这些字符串模式的类初始化了, 而后都注册到容器中;
  2. 这里就是, 应用 @EnableAutoConfiguration 的全限定名去它所在的 jar 包 spring-boot-autoconfigure.jar 上来找 META-INF/spring.factories, 而后去读取那 124 个类, 转换成 String[](当然不是全副, 上面的 @Conditinal 就是限定满足条件的才会放进去! 如下图的 debug 状态, 通过 filter 后, 只剩下了 24 个)
  3. 那么这 24 个类都做了什么呢? 其实就相似于咱们本人写一个配置类(标注 @Configuration), 而后提供初始化 Bean 的办法(标注 @Bean), 让它注册到容器中被容器治理; 只不过是这样的类多了, 不免凌乱, 这种治理形式只是为了对立, 简洁!

也就是说, 如果加载类是: org.springframework.boot.autoconfigure.EnableAutoConfiguration就能读取到这么多全限定类名, 没错, 我数了下, 一共 124 个类

filter 图:

9.3 @Conditional 和 xxxAutoConfiguration 类

紧接着下面的 124 个类 –> 那么问题来了:

  1. 这些类是干嘛的?

    向容器中提供注册 Bean 的, 各种性能的 Bean, 相似于咱们本人的 @Configuration + @Bean;

  2. 这些类 124 个都加载注册到容器中吗?

    不是的, 是有条件的:@Conditinal

  3. 这是什么操作? 为什么有 SpringFactoriesLoader ? 为什么有META-INF/spring.factories? 这都是什么鬼?

    SPI 机制 (Java SPI 实际上是“ 基于接口的编程+策略模式+配置文件”组合实现的动静加载机制), 很多中央有用到:

    1. JDBC 加载不同类型数据库的驱动
    2. 日志门面类和具体的日志实现
    3. Dubbo

    参考: 高级开发必须了解的 Java 中 SPI 机制

9.3.1 Conditional 相干注解一览

springboot 中常见的 Conditinal 以及其用意:

@Conditional
@ConditionalOnJava // 零碎的 java 版本是否符合要求
@ConditionalOnBean // 容器中存在指定的 Bean
@ConditionalOnClass // 零碎中有指定的类
@ConditionalOnProperty // 零碎中指定的属性是否有指定的值
@ConditionalOnWebApplication // 以后是 Web 环境
@ConditionalOnNotWebApplication // 以后不是 web 环境
@ConditionalOnResource // 类门路下是否存在指定的资源文件

@ConditionalOnMissingBean // 容器中不存在指定的 Bean
@ConditionalOnMissingClass // 零碎中不存在指定类
@ConditionalOnSingleCandidate // 容器中只有一个指定 Bean, 或者这个 Bean 是首选 Bean(primary 的)

9.3.2 几个示例

上面咱们找几个 springboot 中的 XXXAutoConfiguration 的示例:

示例 1: AOP 主动拆卸类

AopAutoConfiguration 源码(正文删去了)

package org.springframework.boot.autoconfigure.aop;

import org.aspectj.weaver.Advice;

import org.springframework.aop.config.AopConfigUtils;
import org.springframework.beans.factory.BeanFactory;
import org.springframework.beans.factory.support.BeanDefinitionRegistry;
import org.springframework.boot.autoconfigure.condition.ConditionalOnClass;
import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingClass;
import org.springframework.boot.autoconfigure.condition.ConditionalOnProperty;
import org.springframework.context.annotation.Configuration;
import org.springframework.context.annotation.EnableAspectJAutoProxy;

@Configuration(proxyBeanMethods = false)
@ConditionalOnProperty(prefix = "spring.aop", name = "auto", havingValue = "true", matchIfMissing = true) // ===> #1.
public class AopAutoConfiguration {@Configuration(proxyBeanMethods = false)
    @ConditionalOnClass(Advice.class)  // ==> #2
    static class AspectJAutoProxyingConfiguration {@Configuration(proxyBeanMethods = false)
        @EnableAspectJAutoProxy(proxyTargetClass = false)
        @ConditionalOnProperty(prefix = "spring.aop", name = "proxy-target-class", havingValue = "false", matchIfMissing = false)
        static class JdkDynamicAutoProxyConfiguration { }

        @Configuration(proxyBeanMethods = false)
        @EnableAspectJAutoProxy(proxyTargetClass = true)
        @ConditionalOnProperty(prefix = "spring.aop", name = "proxy-target-class", havingValue = "true",
                matchIfMissing = true)
        static class CglibAutoProxyConfiguration {}}

    @Configuration(proxyBeanMethods = false)
    @ConditionalOnMissingClass("org.aspectj.weaver.Advice") // ==> #3
    @ConditionalOnProperty(prefix = "spring.aop", name = "proxy-target-class", havingValue = "true",
            matchIfMissing = true)
    static class ClassProxyingConfiguration {ClassProxyingConfiguration(BeanFactory beanFactory) {if (beanFactory instanceof BeanDefinitionRegistry) {BeanDefinitionRegistry registry = (BeanDefinitionRegistry) beanFactory;
                AopConfigUtils.registerAutoProxyCreatorIfNecessary(registry);
                AopConfigUtils.forceAutoProxyCreatorToUseClassProxying(registry);
            }
        }

    }

}

// #1. @ConditionalOnProperty(prefix = “spring.aop”, name = “auto”, havingValue = “true”, matchIfMissing = true)

类上注解: 是说只有在配置文件里能找到: spring.aop.auto=true 此类才注册到容器; matchIfMissing 限定是否肯定要匹配为 true, 这里阐明 不为 true 也注册;

// #2. @ConditionalOnClass(Advice.class)

类上注解: 只有在零碎中能找到类文件 Advice.class 时 才注册AspectJAutoProxyingConfiguration

示例 2: DispatcherServlet 主动拆卸类

DispatcherServletAutoConfiguration 源码部分(删去局部正文)

@AutoConfigureOrder(Ordered.HIGHEST_PRECEDENCE)
@Configuration(proxyBeanMethods = false)
// #1 只在 web 环境下能力拆卸, 且是 servlet 的环境下;
@ConditionalOnWebApplication(type = Type.SERVLET) 
// #2 只有零碎中有类 DispatcherServlet 存在能力拆卸;
@ConditionalOnClass(DispatcherServlet.class)
@AutoConfigureAfter(ServletWebServerFactoryAutoConfiguration.class)
public class DispatcherServletAutoConfiguration {

    /*
     * The bean name for a DispatcherServlet that will be mapped to the root URL "/"
     */
    public static final String DEFAULT_DISPATCHER_SERVLET_BEAN_NAME = "dispatcherServlet";

    /*
     * The bean name for a ServletRegistrationBean for the DispatcherServlet "/"
     */
    public static final String DEFAULT_DISPATCHER_SERVLET_REGISTRATION_BEAN_NAME = "dispatcherServletRegistration";

    @Configuration(proxyBeanMethods = false)
     // #3 括号中的类 DefaultDispatcherServletCondition 定义了条件
    @Conditional(DefaultDispatcherServletCondition.class)
    // #4 上面所有都是零碎有 ServletRegistration 类才拆卸
    @ConditionalOnClass(ServletRegistration.class) 
    @EnableConfigurationProperties({HttpProperties.class, WebMvcProperties.class})
    protected static class DispatcherServletConfiguration {@Bean(name = DEFAULT_DISPATCHER_SERVLET_BEAN_NAME)
        public DispatcherServlet dispatcherServlet(HttpProperties httpProperties, WebMvcProperties webMvcProperties) {DispatcherServlet dispatcherServlet = new DispatcherServlet();
            dispatcherServlet.setDispatchOptionsRequest(webMvcProperties.isDispatchOptionsRequest());
            dispatcherServlet.setDispatchTraceRequest(webMvcProperties.isDispatchTraceRequest());
            dispatcherServlet.setThrowExceptionIfNoHandlerFound(webMvcProperties.isThrowExceptionIfNoHandlerFound());
            dispatcherServlet.setPublishEvents(webMvcProperties.isPublishRequestHandledEvents());
            dispatcherServlet.setEnableLoggingRequestDetails(httpProperties.isLogRequestDetails());
            return dispatcherServlet;
        }

        @Bean
        @ConditionalOnBean(MultipartResolver.class)    // ==>#5
        @ConditionalOnMissingBean(name = DispatcherServlet.MULTIPART_RESOLVER_BEAN_NAME) // ==>#6
        public MultipartResolver multipartResolver(MultipartResolver resolver) {
            // Detect if the user has created a MultipartResolver but named it incorrectly
            return resolver;
        }

    }

    ...... 省略前面局部......

}

阐明见类正文中;

另外阐明:

咱们在基于注解的 spring 开发中没有配置 DispatcherServlet, 就是因为这个 DispatcherServlet 主动拆卸类在起作用, 这里向容器中注册了一个 Bean:public DispatcherServlet dispatcherServlet()这块! 相干的配置信息都在参数类: WebMvcProperties中(天然它也是容器中已注册的.)

示例 3: WebMvc 主动拆卸类

@Configuration(proxyBeanMethods = false)
// 1. 要求 servlet 的 web 环境;
@ConditionalOnWebApplication(type = Type.SERVLET) 
// 2. 要求能找到 Servlet/DispatcherServlet/WebMvcConfigure 这几个类才拆卸
@ConditionalOnClass({Servlet.class, DispatcherServlet.class, WebMvcConfigurer.class}) 
// 3. 要求容器中没有已注册的 Bean WebMvcConfigurationSupport 的示例才会拆卸;
@ConditionalOnMissingBean(WebMvcConfigurationSupport.class)
@AutoConfigureOrder(Ordered.HIGHEST_PRECEDENCE + 10)
@AutoConfigureAfter({ DispatcherServletAutoConfiguration.class, TaskExecutionAutoConfiguration.class,
        ValidationAutoConfiguration.class })
public class WebMvcAutoConfiguration {}

参考文档:

  1. 【Spring Boot 源码剖析】@EnableAutoConfiguration 注解（一）@AutoConfigurationImportSelector 注解的解决
  2. 高级开发必须了解的 Java 中 SPI 机制
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