这里是一个学习过程笔记的汇总：Spring Boot学习汇总
前面的例子已经提到过，Spring Boot快捷开发，我们只需要导入一个需要的模块对应的starter，就可以将所有需要组件导入到容器，那么这些Spring Boot是怎么完成的呢？下面就啃一下Spring Boot的源码。
从启动类入手：
@SpringBootApplication
public class DemoApplication {
public static void main(String[] args) {
SpringApplication.run(DemoApplication.class, args);
}
}启动类上就一个@SpringBootApplication注解，点进去细看：
@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 {倒数第二个注解@EnableAutoConfiguration，前面文章已经提到过，它作用是开启自动配置功能，开启后，所有需要用到的配置，都将自动配置好。所以今天就主要围绕这个注解啃。
继续往下走，看下@EnableAutoConfiguration有什么：
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(AutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {看到@Import注解，后面跟着一个类AutoConfigurationImportSelector，这个类是一个选择器，类里面有个方法selectImports()，选择需要被导入到容器中的组件， 继续往下走。
先说一下整个流程：
AutoConfigurationImportSelector类中有一个选择器selectImports, 用来获取所有候选配置类EnableAutoConfiguration，
这里使用了内部工具SpringFactoriesLoader，查找classpath上所有jar包中的META-INF\spring.factories，找出其中key为org.springframework.boot.autoconfigure.EnableAutoConfiguration 的属性定义的所有工厂类名称。然后经过去重，过滤（这个过滤主要是通过一个注解，@Conditionan，后面也会说到），最后筛选出项目真正需要的组件，并注入到容器。
具体的如下，这是所有的自动注入配置文件（即spring-boot-autoconfiguration-2.1.0.RELEASE.jar包下的META-INF\spring.factories文件中的）：
这里每一个xxxAutoConfiguration类都是容器中的一个组件，用他们来做自动配置。
# 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.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.reactor.core.ReactorCoreAutoConfiguration,\
org.springframework.boot.autoconfigure.security.servlet.SecurityAutoConfiguration,\
org.springframework.boot.autoconfigure.security.servlet.SecurityRequestMatcherProviderAutoConfiguration,\
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.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复制出部分源码，开啃（// 是我理解的一些注释，仅供参考）：
@Override
public String[] selectImports(AnnotationMetadata annotationMetadata) {
if (!isEnabled(annotationMetadata)) {
return NO_IMPORTS;
}
// 从配置文件中加载自动配置相关的元数据
AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader
.loadMetadata(this.beanClassLoader);
// 这里调的是下面的方法
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);
// 获取所有候选配置类的EnableAutoConfiguration
// 下面进入到这个方法细看
List<String> configurations = getCandidateConfigurations(annotationMetadata,
attributes);
configurations = removeDuplicates(configurations);
Set<String> exclusions = getExclusions(annotationMetadata, attributes);
checkExcludedClasses(configurations, exclusions);
configurations.removeAll(exclusions);
// 应用过滤器AutoConfigurationImportFilter，
// 对于 spring boot autoconfigure，定义了一个需要被应用的过滤器 ：
// org.springframework.boot.autoconfigure.condition.OnClassCondition，
// 此过滤器检查候选配置类上的注解@ConditionalOnClass，如果要求的类在classpath
// 中不存在，则这个候选配置类会被排除掉
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) {
// 调用SpringFactoriesLoader加载工厂类名称
// 有两个参数，第一个：getSpringFactoryLoaderFactoryClass(),这个是获取要加载 的类的名称，如下面方法显示，这里正是要获取EnableAutoConfiguration自动配置类的数据
// 下面继续看一下loadFactoryNames这个方法
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;
}
/**
* Return the class used by {@link SpringFactoriesLoader} to load configuration
* candidates.
* @return the factory class
*/
protected Class<?> getSpringFactoriesLoaderFactoryClass() {
return EnableAutoConfiguration.class;
}
// 这里传过来的参数，factoryClass即EnableAutoConfiguration
public static List<String> loadFactoryNames(Class<?> factoryClass, @Nullable ClassLoader classLoader) {
// EnableAutoConfiguration
String factoryClassName = factoryClass.getName();
// loadSpringFactories()方法是查找classpath上所有jar包中的META-INF\spring.factories下的所有工厂类名称。
// 然后调用getOrDefault()方法，第一个参数是EnableAutoConfiguration,即在
// loadApringFactories()的基础上找出key为org.springframework.boot.autoconfigure.EnableAutoConfiguration的所有的工厂类名称。
return loadSpringFactories(classLoader).getOrDefault(factoryClassName, Collections.emptyList());
}
private static Map<String, List<String>> loadSpringFactories(@Nullable ClassLoader classLoader) {
MultiValueMap<String, String> result = cache.get(classLoader);
if (result != null) {
return result;
}
try {
// 可以点进去看到 FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories";
// 获取到所有的类路径
Enumeration<URL> urls = (classLoader != null ?
classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :
ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));
result = new LinkedMultiValueMap<>();
while (urls.hasMoreElements()) {
URL url = urls.nextElement();
// 获取到相应的资源
UrlResource resource = new UrlResource(url);
// 将资源封装成Properties配置文件
Properties properties = PropertiesLoaderUtils.loadProperties(resource);
// 遍历properties文件，将所有的资源获取到，如下面的截图1所示
for (Map.Entry<?, ?> entry : properties.entrySet()) {
String factoryClassName = ((String) entry.getKey()).trim();
for (String factoryName : StringUtils.commaDelimitedListToStringArray((String) entry.getValue())) {
result.add(factoryClassName, factoryName.trim());
}
}
}
cache.put(classLoader, result);
return result;
}
catch (IOException ex) {
throw new IllegalArgumentException("Unable to load factories from location [" +
FACTORIES_RESOURCE_LOCATION + "]", ex);
}
}截图1：
到这，这个流程算是清楚了，啃完了，知道项目所需的自动配置类是怎么筛选出来的，那么接下来就看下那些组件是怎么自动导入到容器中的。
就随便挑选一个举例自动配置原理，比如说: HttpEncodingAutoConfiguration（Http编码自动配置）, 先看一下这个类（// 是自己写的一些注释，仅供参考）：
/*
* Copyright 2012-2018 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.web.servlet;
import org.springframework.boot.autoconfigure.EnableAutoConfiguration;
import org.springframework.boot.autoconfigure.condition.ConditionalOnClass;
import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
import org.springframework.boot.autoconfigure.condition.ConditionalOnProperty;
import org.springframework.boot.autoconfigure.condition.ConditionalOnWebApplication;
import org.springframework.boot.autoconfigure.http.HttpProperties;
import org.springframework.boot.autoconfigure.http.HttpProperties.Encoding.Type;
import org.springframework.boot.context.properties.EnableConfigurationProperties;
import org.springframework.boot.web.server.WebServerFactoryCustomizer;
import org.springframework.boot.web.servlet.filter.OrderedCharacterEncodingFilter;
import org.springframework.boot.web.servlet.server.ConfigurableServletWebServerFactory;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.core.Ordered;
import org.springframework.web.filter.CharacterEncodingFilter;
/**
* {@link EnableAutoConfiguration Auto-configuration} for configuring the encoding to use
* in web applications.
*
* @author Stephane Nicoll
* @author Brian Clozel
* @since 1.2.0
*/
// 表示这是一个配置类，可以给容器中添加组件，和平时写的配置类一样
@Configuration
// 启动指定类的ConfigurationProperties功能；将配置文件中的值与HttpProperties绑定起来。
@EnableConfigurationProperties(HttpProperties.class)
// 这是一个Spring的底层注解@Conditional的衍生注解，作用是在满足指定条件的情况下，整个配置类中的配置才会生效。 此处的@ConditionalOnWebApplication就是判断当前应用是否是web应用，如果是，当前的配置才会生效
@ConditionalOnWebApplication(type = ConditionalOnWebApplication.Type.SERVLET)
// 这个同样是@Conditional的一个衍生注解，判断当前项目有没有CharacterEncodingFilter这个类，有的话当前配置才会生效。 CharaacterEncodingFilter类是SpringMVC中的解决乱码的过滤器
@ConditionalOnClass(CharacterEncodingFilter.class)
// 判断配置文件中是否存在 spring.http.encoding.enabled, 如果存在，则判断成立， 后面的matchIfMissing=true代表计时其那面的spring.http.encoding.enabled不存在，也默认配置生效
@ConditionalOnProperty(prefix = "spring.http.encoding", value = "enabled", matchIfMissing = true)
public class HttpEncodingAutoConfiguration {
// 这里点进去看HttpProperties类，可以看到这个已经和SpringBoot的配置文件映射了，就像我们第一篇讲的那样。
private final HttpProperties.Encoding properties;
public HttpEncodingAutoConfiguration(HttpProperties properties) {
this.properties = properties.getEncoding();
}
@Bean // 给容器添加一个组件，这个组件的某些值需要从properties中获取，而properties又是和Spring Boot的配置文件进行绑定的
@ConditionalOnMissingBean // 容器中不存在这个组件
public CharacterEncodingFilter characterEncodingFilter() {
CharacterEncodingFilter filter = new OrderedCharacterEncodingFilter();
filter.setEncoding(this.properties.getCharset().name());
filter.setForceRequestEncoding(this.properties.shouldForce(Type.REQUEST));
filter.setForceResponseEncoding(this.properties.shouldForce(Type.RESPONSE));
return filter;
}
@Bean
public LocaleCharsetMappingsCustomizer localeCharsetMappingsCustomizer() {
return new LocaleCharsetMappingsCustomizer(this.properties);
}
private static class LocaleCharsetMappingsCustomizer implements
WebServerFactoryCustomizer<ConfigurableServletWebServerFactory>, Ordered {
private final HttpProperties.Encoding properties;
LocaleCharsetMappingsCustomizer(HttpProperties.Encoding properties) {
this.properties = properties;
}
@Override
public void customize(ConfigurableServletWebServerFactory factory) {
if (this.properties.getMapping() != null) {
factory.setLocaleCharsetMappings(this.properties.getMapping());
}
}
@Override
public int getOrder() {
return 0;
}
}
}上面这个HttpEncodingAutoConfiguration（http编码自动配置）类，一旦那些@Conditional的扩展条件注解都满足，那么这个配置类就会生效，一旦这个配置生效，就会通过@Bean注解给容器添加各种不同的组件。到此，自动配置，就是这么回事了。
总结一下：
Spirng Boot项目在启动时，会通过上面讲述的方法给我们的项目加入大量的自动配置类，每个配置类里面都会有多个组件，然后再看看这些组件是否都包含了我们功能需要用到的，如果有没包括的，我们就可以自己在主配置文件中去定义，因为给容器中添加组件的时候，会从properties中取某些属性，而properties是直接映射到我们的注配置文件的。上面有提到过。
好了，下面再说说@Conditional条件注解，当设置的条件都满足时，某些配置才会生效，或者是某个bean才会被注入到容器中。@Conditional是Spring底层注解，会有很多的扩展注解，常用的有如下：
| @Conditional扩展注解 | 作用（判断是否满足当前指定条件） |
| @ConditionalOnBean | 系统中存在指定bean |
| @ConditionalOnMissingBean | 系统中不存在指定bean |
| @ConditionalOnClass | 系统中有指定类 |
| @ConditionalOnMissingClass | 系统中不存在指定类 |
| @ConditonalOnResource | 类路径下是否存在指定资源文件 |
| @ConditionalOnWebApplication | 当前是web环境 |
| @ConditionalOnNotWebApplication | 当前不是web环境 |
那么满足条件的自动配置类就生效，生效的配置类中的组件就会注入进容器中，那么我们怎么知道哪些配置类是生效的呢？因为上面也看到了，有那么多的xxxAutoConfiguration类，总不能一个个点进去看吧。
先点进去看，如果有报红色的应该就是没有生效的。如下：
这就是上面提到的添加的条件注解没有满足，那么我们怎么能快速判断哪些配置类是生效的呢？
很简单，在配置文件中添加如下一行配置即可，开启debug模式：
然后启动项目，控制台会打印出如下记录，生效的和未生效的配置类一目了然：
============================
CONDITIONS EVALUATION REPORT
============================
Positive matches: // 生效
-----------------
CodecsAutoConfiguration matched: // 匹配上
- @ConditionalOnClass found required class 'org.springframework.http.codec.CodecConfigurer' (OnClassCondition)
CodecsAutoConfiguration.JacksonCodecConfiguration matched:
- @ConditionalOnClass found required class 'com.fasterxml.jackson.databind.ObjectMapper' (OnClassCondition)
Negative matches: // 未生效
-----------------
ActiveMQAutoConfiguration:
Did not match: // 未匹配上
- @ConditionalOnClass did not find required class 'javax.jms.ConnectionFactory' (OnClassCondition)
AopAutoConfiguration:
Did not match:
- @ConditionalOnClass did not find required class 'org.aspectj.lang.annotation.Aspect' (OnClassCondition)
到此，Spring Boot自动配置原理啃到这儿，后续再继续啃
