springboot开发笔记-(9)-自动装配与@EnableAutoConfiguration和@Conditinal

springboot自动装配原理和两个注解类的关系是深有千千结: @EnableAutoConfiguration和@Conditinal

我们从springboot的启动类上的注解看下这千丝万缕: 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
衡量不出爱或不爱之间的距离
隐隐约约中明白你的决定
不敢勉强你只好为难自己
我为难我自己...

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 {}

我们来说下这两个注解的功能 :

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

// 2. @EnableAutoConfiguration主要是自动装配; 下面细说它;

9.2 自动装配去: @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 主要做了什么? 看注解链:

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

好了, 我们就主要看看 AutoConfigurationPackages.Registrar.class做了什么我们见不得的事情:

参见下面的 #9.2.1 然后小结:

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) 我们也另立门户:

参见下面的 #9.2.2 然后小结:

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(不明白的可以参考:spring注解驱动开发-(5) 向Spring容器中注册组件的方法), 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当前入口类的包名, 后续扫描包下注解的时候就可以用到了!

原来如此, 小结: AutoConfigurationPackages.Registrar.class的功能如其名: 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; 比如 @Component/@Service/@Controller/@Repository等这些;

9.2.2 @Import(AutoConfigurationImportSelector.class)

@Import就不用介绍了(可以参考文末的链接): 向容器中注册一个普通Bean的; 那么AutoConfigurationImportSelector是一个什么样的普通Bean呢?

@Override
public String[] selectImports(AnnotationMetadata annotationMetadata) {
    if (!isEnabled(annotationMetadata)) {
        return NO_IMPORTS;
    }
    AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader
        .loadMetadata(this.beanClassLoader);
    // ==>关键在这里: 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);
    // ==>关键在这里: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) {    
    // ==>关键在这里: 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;
}

那么核心问题来了, 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, 使用给定的类加载器。

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? 回忆下上面那句歌词:从哪里开始从哪里失去 感觉是不是很恐怖? 好了, 我们收住!

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

由于 ImportSelector 接口就是返回一个类名的字符串数组String[], 然后交给spring容器让它来把这些字符串形式的类初始化了, 然后都注册到容器中;
这里就是, 使用 @EnableAutoConfiguration 的全限定名去它所在的jar包spring-boot-autoconfigure.jar下去找 META-INF/spring.factories, 然后去读取那124个类, 转换成 String[](当然不是全部, 下面的@Conditinal就是限定满足条件的才会放进去! 如下图的debug状态, 经过filter后, 只剩下了24个)
那么这24个类都做了什么呢? 其实就类似于我们自己写一个配置类(标注@Configuration), 然后提供初始化Bean的方法(标注@Bean), 让它注册到容器中被容器管理; 只不过是这样的类多了, 难免混乱, 这种管理方式只是为了统一, 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

紧接着上面的124个类-->那么问题来了:

这些类是干嘛的?
向容器中提供注册Bean的, 各种功能的Bean, 类似于我们自己的@Configuration + @Bean;
这些类124个都加载注册到容器中吗?
不是的, 是有条件的:@Conditinal
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”组合实现的动态加载机制), 很多地方有用到:
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: 高级开发必须理解的Java中SPI机制

9.3.1 Conditional相关注解一览

springboot中常见的 Conditinal以及其用意:

@Conditional
@ConditionalOnJava // 系统的java版本是否符合要求
@ConditionalOnBean // 容器中存在指定的Bean
@ConditionalOnClass // 系统中有指定的类
@ConditionalOnProperty // 系统中指定的属性是否有指定的值
@ConditionalOnWebApplication // 当前是Web环境
@ConditionalOnNotWebApplication // 当前不是web环境
@ConditionalOnResource // 类路径下是否存在指定的资源文件

@ConditionalOnMissingBean // 容器中不存在指定的Bean
@ConditionalOnMissingClass // 系统中不存在指定类
@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

下面我们找几个springboot中的XXXAutoConfiguration的示例:

示例1: AOP自动装配类

AopAutoConfiguration源码(注释删去了)

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限定是否一定要匹配为true, 这里说明不为true 也注册;

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

类上注解: 只有在系统中能找到类文件 Advice.class 时才注册AspectJAutoProxyingConfiguration

示例2: DispatcherServlet自动装配类

DispatcherServletAutoConfiguration源码局部(删去部分注释)

@AutoConfigureOrder(Ordered.HIGHEST_PRECEDENCE)
@Configuration(proxyBeanMethods = false)
// #1 只在web环境下才能装配, 且是servlet的环境下;
@ConditionalOnWebApplication(type = Type.SERVLET) 
// #2 只有系统中有类 DispatcherServlet 存在才能装配;
@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 下面所有都是系统有ServletRegistration类才装配
    @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;
        }

    }

    ......省略后面部分......

}

说明见类注释中;

另外说明:

我们在基于注解的spring开发中没有配置 DispatcherServlet, 就是因为这个DispatcherServlet自动装配类在起作用, 这里向容器中注册了一个Bean:public DispatcherServlet dispatcherServlet()这块! 相关的配置信息都在参数类: WebMvcProperties中(自然它也是容器中已注册的.)

示例3: WebMvc自动装配类

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

参考文档:

springboot开发笔记-(9)-自动装配与@EnableAutoConfiguration和@Conditinal

9.1 从哪里开始: @SpringBootApplication

9.2 自动装配去: @EnableAutoConfiguration:

9.2.1 AutoConfigurationPackages.Registrar.class

9.2.2 @Import(AutoConfigurationImportSelector.class)

9.3 @Conditional和xxxAutoConfiguration类

9.3.1 Conditional相关注解一览

9.3.2 几个示例

示例1: AOP自动装配类

示例2: DispatcherServlet自动装配类

示例3: WebMvc自动装配类

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