Spring Boot神奇之处详解--条件配置(一.原理详解)

谁借莪１个温暖的怀抱￠ 2022-05-14 08:52 177阅读 0赞

### 深入理解Spring Boot的运行原理 ###

> Spring 4.x 提供了基于条件配置Bean的能力,Spring Boot之所以神奇也是因为这一原理,本文主要内容是围绕这一原理对Spring Boot做一下较为深入的了解(关于新建项目及初识Spring Boot的一些内容请看官网的入门例子,非常精辟[SpringBoot Getting Started][])

#### 第一步 先看看Spring Boot为我们作了哪些自动配置的工作: ####

*  方法一: 在application.properties中设置属性  
    debug=true
 *  方法二: 在运行jar文件时 加上 -debug  
    java -jar xx.jar -debug  
    ![这里写图片描述][70]

#### 第二步 找到Spring Boot源码中spring-boot-autoconfigure-1.x.x.x.jar包是自动配置的核心包. ####

![这里写图片描述][70 1]

#### 第三步 从入口逐步深入 ####

1.  首先我们我们看Spring Boot的启动类  
    `@SpringBootApplication public class LocationApplication { public static void main(String[] args) { SpringApplication.run(LocationApplication.class, args); } }`
2.  此处我们应该关注的是`@SpringBootApplication` 这个注解

package org.springframework.boot.autoconfigure;
    
    import java.lang.annotation.Documented;
    import java.lang.annotation.ElementType;
    import java.lang.annotation.Inherited;
    import java.lang.annotation.Retention;
    import java.lang.annotation.RetentionPolicy;
    import java.lang.annotation.Target;
    
    import org.springframework.boot.SpringBootConfiguration;
    import org.springframework.boot.context.TypeExcludeFilter;
    import org.springframework.context.annotation.Bean;
    import org.springframework.context.annotation.ComponentScan;
    import org.springframework.context.annotation.ComponentScan.Filter;
    import org.springframework.context.annotation.Configuration;
    import org.springframework.context.annotation.FilterType;
    import org.springframework.core.annotation.AliasFor;
    
    /** * Indicates a {@link Configuration configuration} class that declares one or more * {@link Bean @Bean} methods and also triggers {@link EnableAutoConfiguration * auto-configuration} and {@link ComponentScan component scanning}. This is a convenience * annotation that is equivalent to declaring {@code @Configuration}, * {@code @EnableAutoConfiguration} and {@code @ComponentScan}. * * @author Phillip Webb * @author Stephane Nicoll * @since 1.2.0 */
    @Target(ElementType.TYPE)
    @Retention(RetentionPolicy.RUNTIME)
    @Documented
    @Inherited
    @SpringBootConfiguration
    @EnableAutoConfiguration
    @ComponentScan(excludeFilters = {
            @Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),
            @Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })
    public @interface SpringBootApplication { 
    
    
        @AliasFor(annotation = EnableAutoConfiguration.class, attribute = "exclude")
        Class<?>[] exclude() default {};
    
    
        @AliasFor(annotation = EnableAutoConfiguration.class, attribute = "excludeName")
        String[] excludeName() default {};
    
    
        @AliasFor(annotation = ComponentScan.class, attribute = "basePackages")
        String[] scanBasePackages() default {};
    
    
        @AliasFor(annotation = ComponentScan.class, attribute = "basePackageClasses")
        Class<?>[] scanBasePackageClasses() default {};
    
    }

`@SpringBootApplication` 是一个组合注解,它的组成部分有以下几种:

*  `@Configuration` 声明带此注解的类为定义Spring上下文Bean的配置类
 *  `@EnableAutoConfiguration` 该注解告诉Spring Boot开始依据classpath的设置,其他的bean和各种各样属性设置来为项目添加bean
 *  你可以手动添加 `@EnableWebMvc` 但是Spring boot一旦在classpath中发现**spring webmvc**,就标志这当前的项目为web应用,并且自动的为MVC配置一些核心的东西,例如 `DispatchServlet` 的配置
 *  `@ComponentScan` 告诉Spring 去自动扫描同启动类在同一个目录级别的包中的各种Spring组件, 例如Controller,Service,configuration等

--------------------

我们看到当中`@EnableAutoConfiguration` 使我们本文需要关注的点. 正是因为该注解,才让Spring Boot有了神奇的功能  
3. `@EnableAutoConfiguration` 源码分析

import java.lang.annotation.Documented;
    import java.lang.annotation.ElementType;
    import java.lang.annotation.Inherited;
    import java.lang.annotation.Retention;
    import java.lang.annotation.RetentionPolicy;
    import java.lang.annotation.Target;
    
    import org.springframework.boot.autoconfigure.condition.ConditionalOnBean;
    import org.springframework.boot.autoconfigure.condition.ConditionalOnClass;
    import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
    import org.springframework.boot.context.embedded.EmbeddedServletContainerFactory;
    import org.springframework.boot.context.embedded.tomcat.TomcatEmbeddedServletContainerFactory;
    import org.springframework.context.annotation.Conditional;
    import org.springframework.context.annotation.Configuration;
    import org.springframework.context.annotation.Import;
    import org.springframework.core.io.support.SpringFactoriesLoader;
    
    @SuppressWarnings("deprecation")
    @Target(ElementType.TYPE)
    @Retention(RetentionPolicy.RUNTIME)
    @Documented
    @Inherited
    @AutoConfigurationPackage
    @Import(EnableAutoConfigurationImportSelector.class)
    public @interface EnableAutoConfiguration { 
    
        String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";
    
        //排除一些从来不会用到的特殊的自动配置类 
        Class<?>[] exclude() default {};
    
       //排除的类的名字
        String[] excludeName() default {};
    
    }

我们注意到`@Import(EnableAutoConfigurationImportSelector.class)` 是该注解的核心.我们进入EnableAutoConfigurationImportSelector 这个类然后看父类AutoConfigurationImportSelector中的一个方法:

/** * 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) {
            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;
        }

我们可以看源码的注释, 返回那些需要被考虑在内的自动配置的类. 此处就涉及到另外一个关键的文档. spring.factories  
Spring Boot会在启动的时候去加载包中含有该文件的jar包, 我们看该文件里边到底有什么

> ##### Initializers #####
> 
> org.springframework.context.ApplicationContextInitializer=\\  
> org.springframework.boot.autoconfigure.SharedMetadataReaderFactoryContextInitializer,\\  
> org.springframework.boot.autoconfigure.logging.AutoConfigurationReportLoggingInitializer
> 
> ##### 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.OnClassCondition
> 
> ##### Auto Configure #####
> 
> `此处是相关自动配置需要去扫描的东西, 可以去点进去一个类查看一下里边的内容`  
> 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.CloudAutoConfiguration,\\  
> 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.CassandraRepositoriesAutoConfiguration,\\  
> org.springframework.boot.autoconfigure.data.couchbase.CouchbaseDataAutoConfiguration,\\  
> 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.jpa.JpaRepositoriesAutoConfiguration,\\  
> org.springframework.boot.autoconfigure.data.ldap.LdapDataAutoConfiguration,\\  
> org.springframework.boot.autoconfigure.data.ldap.LdapRepositoriesAutoConfiguration,\\  
> org.springframework.boot.autoconfigure.data.mongo.MongoDataAutoConfiguration,\\  
> 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.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.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.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.flyway.FlywayAutoConfiguration,\\  
> org.springframework.boot.autoconfigure.groovy.template.GroovyTemplateAutoConfiguration,\\  
> org.springframework.boot.autoconfigure.jersey.JerseyAutoConfiguration,\\  
> org.springframework.boot.autoconfigure.jooq.JooqAutoConfiguration,\\  
> 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.mobile.DeviceResolverAutoConfiguration,\\  
> org.springframework.boot.autoconfigure.mobile.DeviceDelegatingViewResolverAutoConfiguration,\\  
> org.springframework.boot.autoconfigure.mobile.SitePreferenceAutoConfiguration,\\  
> org.springframework.boot.autoconfigure.mongo.embedded.EmbeddedMongoAutoConfiguration,\\  
> org.springframework.boot.autoconfigure.mongo.MongoAutoConfiguration,\\  
> org.springframework.boot.autoconfigure.mustache.MustacheAutoConfiguration,\\  
> org.springframework.boot.autoconfigure.orm.jpa.HibernateJpaAutoConfiguration,\\  
> org.springframework.boot.autoconfigure.reactor.ReactorAutoConfiguration,\\  
> org.springframework.boot.autoconfigure.security.SecurityAutoConfiguration,\\  
> org.springframework.boot.autoconfigure.security.SecurityFilterAutoConfiguration,\\  
> org.springframework.boot.autoconfigure.security.FallbackWebSecurityAutoConfiguration,\\  
> org.springframework.boot.autoconfigure.security.oauth2.OAuth2AutoConfiguration,\\  
> org.springframework.boot.autoconfigure.sendgrid.SendGridAutoConfiguration,\\  
> org.springframework.boot.autoconfigure.session.SessionAutoConfiguration,\\  
> org.springframework.boot.autoconfigure.social.SocialWebAutoConfiguration,\\  
> org.springframework.boot.autoconfigure.social.FacebookAutoConfiguration,\\  
> org.springframework.boot.autoconfigure.social.LinkedInAutoConfiguration,\\  
> org.springframework.boot.autoconfigure.social.TwitterAutoConfiguration,\\  
> org.springframework.boot.autoconfigure.solr.SolrAutoConfiguration,\\  
> 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.DispatcherServletAutoConfiguration,\\  
> org.springframework.boot.autoconfigure.web.EmbeddedServletContainerAutoConfiguration,\\  
> org.springframework.boot.autoconfigure.web.ErrorMvcAutoConfiguration,\\  
> org.springframework.boot.autoconfigure.web.HttpEncodingAutoConfiguration,\\  
> org.springframework.boot.autoconfigure.web.HttpMessageConvertersAutoConfiguration,\\  
> org.springframework.boot.autoconfigure.web.MultipartAutoConfiguration,\\  
> org.springframework.boot.autoconfigure.web.ServerPropertiesAutoConfiguration,\\  
> org.springframework.boot.autoconfigure.web.WebClientAutoConfiguration,\\  
> org.springframework.boot.autoconfigure.web.WebMvcAutoConfiguration,\\  
> org.springframework.boot.autoconfigure.websocket.WebSocketAutoConfiguration,\\  
> org.springframework.boot.autoconfigure.websocket.WebSocketMessagingAutoConfiguration,\\  
> org.springframework.boot.autoconfigure.webservices.WebServicesAutoConfiguration
> 
> ##### Failure analyzers #####
> 
> org.springframework.boot.diagnostics.FailureAnalyzer=\\  
> org.springframework.boot.autoconfigure.diagnostics.analyzer.NoSuchBeanDefinitionFailureAnalyzer,\\  
> org.springframework.boot.autoconfigure.jdbc.DataSourceBeanCreationFailureAnalyzer,\\  
> org.springframework.boot.autoconfigure.jdbc.HikariDriverConfigurationFailureAnalyzer
> 
> ##### 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.JspTemplateAvailabilityProvider

上边的spring.factories 文件内容就是Spring Boot的大神们默认为Spring Boot所集成的自动配置的部分内容,当我们进入上边自动配置下的一个类中时.我们会发现如下的各种条件注解. 条件注解就像if else一样帮我们做一些简单的判断. 你将会看到如下的几种注解:

*  @ConditionalOnBean：当容器里有指定的Bean 的条件下。
 *  @ConditionalOnClass：当类路径下有指定的类的条件下。 @ConditionalOnExpression：基于SpEL  
    表达式作为判断条件。 @ConditionalOnJava：基于JVM 版本作为判断条件。
 *  @ConditionalOnJndi：在JNDI 存在的条件下查找指定的位置。
 *  @ConditionalOnMissingBean：当容器里没有指定Bean 的情况下。
 *  @ConditionalOnMissingClass：当类路径下没有指定的类的条件下。
 *  @ConditionalOnNotWebApplication：当前项目不是Web 项目的条件下。
 *  @ConditionalOnProperty：指定的属性是否有指定的值。
 *  @ConditionalOnResource：类路径是否有指定的值。
 *  @ConditionalOnSingleCandidate：当指定Bean 在容器中只有一个，或者虽然有多个但是指定首选的Bean。
 *  @ConditionalOnWebApplication：当前项目是Web 项目的条件下。

真的是太令人兴奋了我们简单分析一个较为简单的OnWebApplicationCondition.java,看看判断的逻辑:

/* * Copyright 2012-2016 the original author or authors. * * Licensed under the Apache License, Version 2.0 (the "License"); * you may not use this file except in compliance with the License. * You may obtain a copy of the License at * * http://www.apache.org/licenses/LICENSE-2.0 * * Unless required by applicable law or agreed to in writing, software * distributed under the License is distributed on an "AS IS" BASIS, * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. * See the License for the specific language governing permissions and * limitations under the License. */
    
    package org.springframework.boot.autoconfigure.condition;
    
    import org.springframework.context.annotation.Condition;
    import org.springframework.context.annotation.ConditionContext;
    import org.springframework.core.Ordered;
    import org.springframework.core.annotation.Order;
    import org.springframework.core.type.AnnotatedTypeMetadata;
    import org.springframework.util.ClassUtils;
    import org.springframework.util.ObjectUtils;
    import org.springframework.web.context.WebApplicationContext;
    import org.springframework.web.context.support.StandardServletEnvironment;
    
    /** * {@link Condition} that checks for the presence or absence of * {@link WebApplicationContext}. * * @author Dave Syer * @see ConditionalOnWebApplication * @see ConditionalOnNotWebApplication */
    @Order(Ordered.HIGHEST_PRECEDENCE + 20)
    class OnWebApplicationCondition extends SpringBootCondition {
    
        private static final String WEB_CONTEXT_CLASS = "org.springframework.web.context."
                + "support.GenericWebApplicationContext";
    
        @Override
        public ConditionOutcome getMatchOutcome(ConditionContext context,
                AnnotatedTypeMetadata metadata) {
            boolean required = metadata
                    .isAnnotated(ConditionalOnWebApplication.class.getName());
            ConditionOutcome outcome = isWebApplication(context, metadata, required);
            if (required && !outcome.isMatch()) {
                return ConditionOutcome.noMatch(outcome.getConditionMessage());
            }
            if (!required && outcome.isMatch()) {
                return ConditionOutcome.noMatch(outcome.getConditionMessage());
            }
            return ConditionOutcome.match(outcome.getConditionMessage());
        }
    
        private ConditionOutcome isWebApplication(ConditionContext context,
                AnnotatedTypeMetadata metadata, boolean required) {
            ConditionMessage.Builder message = ConditionMessage.forCondition(
                    ConditionalOnWebApplication.class, required ? "(required)" : "");
            if (!ClassUtils.isPresent(WEB_CONTEXT_CLASS, context.getClassLoader())) {
                return ConditionOutcome
                        .noMatch(message.didNotFind("web application classes").atAll());
            }
            if (context.getBeanFactory() != null) {
                String[] scopes = context.getBeanFactory().getRegisteredScopeNames();
                if (ObjectUtils.containsElement(scopes, "session")) {
                    return ConditionOutcome.match(message.foundExactly("'session' scope"));
                }
            }
            if (context.getEnvironment() instanceof StandardServletEnvironment) {
                return ConditionOutcome
                        .match(message.foundExactly("StandardServletEnvironment"));
            }
            if (context.getResourceLoader() instanceof WebApplicationContext) {
                return ConditionOutcome.match(message.foundExactly("WebApplicationContext"));
            }
            return ConditionOutcome.noMatch(message.because("not a web application"));
        }
    
    }

从上边 isWebApplication方法中我们可以看到四个if判断做了如下事情

1.  GenericWebApplicationContext 是否在类路径中；
2.  容器里是否有名为session 的scope；
3.  当前容器的Enviroment 是否为StandardServletEnvironment；
4.  当前的ResourceLoader 是否为WebApplicationContext （ ResourceLoader  
    是ApplicationContext 的顶级接口之一)；
    
    最后通过ConditionOutcome这个类来输出是否匹配上的true 或者false

[SpringBoot Getting Started]: https://spring.io/guides/gs/spring-boot/
[70]: /images/20220514/a33d51154387441e93e291f3bfbc0d48.png
[70 1]: /images/20220514/b66e51fb967a4f0dbd52de8442511826.png