一、springBoot自动配置原理

我们在springBoot的启动类中，点击@SpringBootApplication注解，进入到注解类中，可以看到以下注解。
@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)  //表示运行时生效
@Documented
@Inherited
@SpringBootConfiguration
@EnableAutoConfiguration  //自动配置注解
@ComponentScan(
    excludeFilters = {@Filter(
    type = FilterType.CUSTOM,
    classes = {TypeExcludeFilter.class}
), @Filter(
    type = FilterType.CUSTOM,
    classes = {AutoConfigurationExcludeFilter.class}
)}
)
public @interface SpringBootApplication {


SpringBoot启动的时候加载主配置类，开启了自动配置功能 ==@EnableAutoConfiguration==

进入到@EnableAutoConfiguration中，我们可以看到以下注解
@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import({AutoConfigurationImportSelector.class})//导入选择器
public @interface EnableAutoConfiguration {


我们需要注意的是@Import({AutoConfigurationImportSelector.class})注解，这个注解利用AutoConfigurationImportSelector选择器，来获取自动配置类。
进入到AutoConfigurationImportSelector.class中，查看一个selectImports方法
public String[] selectImports(AnnotationMetadata annotationMetadata) {
    if (!this.isEnabled(annotationMetadata)) {
        return NO_IMPORTS;
    } else {
        AutoConfigurationMetadata autoConfigurationMetadata     
                 = AutoConfigurationMetadataLoader.loadMetadata(this.beanClassLoader);
        AutoConfigurationImportSelector.AutoConfigurationEntry autoConfigurationEntry 
                    //这个方法就是下面列的方法
                = this.getAutoConfigurationEntry(autoConfigurationMetadata, annotationMetadata);
        return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations());
    }
}


final class AutoConfigurationMetadataLoader {
    protected static final String PATH 
                    = "META-INF/spring-autoconfigure-metadata.properties";

private AutoConfigurationMetadataLoader() {
}

static AutoConfigurationMetadata loadMetadata(ClassLoader classLoader) {
       //调用loadMetadata方法  传入classLoader 
         // 和META-INF/spring-autoconfigure-metadata.properties路径
     return loadMetadata(classLoader, "META-INF/spring-autoconfigure-metadata.properties");
}

static AutoConfigurationMetadata loadMetadata(ClassLoader classLoader, String path) {
    try {
        //读取spring-boot-autoconfigure-2.1.5.RELEASE.jar包中
        //spring-autoconfigure-metadata.properties的信息生成urls枚举对象
        Enumeration<URL> urls = classLoader != null ? classLoader.getResources(path) : ClassLoader.getSystemResources(path);
        Properties properties = new Properties();
        
         //解析urls枚举对象中的信息封装成properties对象并加载
        while(urls.hasMoreElements()) {
            properties.putAll(PropertiesLoaderUtils.loadProperties(new UrlResource((URL)urls.nextElement())));
        }
         //根据封装好的properties对象生成AutoConfigurationMetadata对象返回
        return loadMetadata(properties);
    } catch (IOException var4) {
        throw new IllegalArgumentException("Unable to load @ConditionalOnClass location [" + path + "]", var4);
    }
}

protected AutoConfigurationImportSelector.AutoConfigurationEntry getAutoConfigurationEntry(AutoConfigurationMetadata autoConfigurationMetadata, AnnotationMetadata annotationMetadata) {
    if (!this.isEnabled(annotationMetadata)) {
        return EMPTY_ENTRY;
    } else {
         //将注解元信息封装成注解属性对象
        AnnotationAttributes attributes = this.getAttributes(annotationMetadata);
        //最主要的方法this.getCandidateConfigurations
        //获取到配置类的全路径字符串集合
        List<String> configurations = this.getCandidateConfigurations(annotationMetadata, attributes);
        configurations = this.removeDuplicates(configurations);
        Set<String> exclusions = this.getExclusions(annotationMetadata, attributes);
        this.checkExcludedClasses(configurations, exclusions);
        configurations.removeAll(exclusions);
        configurations = this.filter(configurations, autoConfigurationMetadata);
        this.fireAutoConfigurationImportEvents(configurations, exclusions);
        return new AutoConfigurationImportSelector.AutoConfigurationEntry(configurations, exclusions);
    }
//在getCandidateConfigurations方法中调用loadFactoryNames方法
protected List<String> getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) {
                                //调用loadFactoryNames方法
    List<String> configurations = SpringFactoriesLoader.loadFactoryNames(this.getSpringFactoriesLoaderFactoryClass(), this.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;
}
//在loadFactoryNames方法中调用loadSpringFactories方法
private static Map<String, List<String>> loadSpringFactories(@Nullable ClassLoader classLoader) {
    MultiValueMap<String, String> result = (MultiValueMap)cache.get(classLoader);
    if (result != null) {
        return result;
    } else {
        try {
            //在低版本springBoot中例如1.5XX等等  和2.X版本是不一样的
                //--低版本我们使用的是常量  常量的值为META-INF/spring.factories
                //--高版本我们直接使用值
             //classLoader.getResources("META-INF/spring.factories")
                // --扫描所有jar包类路径下  META-INF/spring.factories
                //--把扫描到的这些文件的内容包装成properties对象
                //--从properties中获取到EnableAutoConfiguration.class类（类名）对应的值，然后把他们添加在容器中
            //ClassLoader.getSystemResources("META-INF/spring.factories")
            //扫描文件  得到所有的url
            Enumeration<URL> urls = classLoader != null ? classLoader.getResources("META-INF/spring.factories") : ClassLoader.getSystemResources("META-INF/spring.factories");
            LinkedMultiValueMap result = new LinkedMultiValueMap();
            //遍历每一个url
            while(urls.hasMoreElements()) {
                URL url = (URL)urls.nextElement();
                UrlResource resource = new UrlResource(url);
                //最终将这些文件包装成properties文件
                Properties properties = PropertiesLoaderUtils.loadProperties(resource);
                //迭代这些文件
                Iterator var6 = properties.entrySet().iterator();

            while(var6.hasNext()) {
                Entry<?, ?> entry = (Entry)var6.next();
                String factoryTypeName = ((String)entry.getKey()).trim();
                String[] var9 = StringUtils.commaDelimitedListToStringArray((String)entry.getValue());
                int var10 = var9.length;

                for(int var11 = 0; var11 < var10; ++var11) {
                    //获取到每一个文件的名字
                    String factoryImplementationName = var9[var11];
                    result.add(factoryTypeName, factoryImplementationName.trim());
                }
            }
        }
        //添加到缓存中
        cache.put(classLoader, result);
        return result;
    } catch (IOException var13) {
        throw new IllegalArgumentException("Unable to load factories from location [META-INF/spring.factories]", var13);
    }
}

}

 在我们spring-boot-autoconfigure-2.2.2.RELEASE.jar中
  META-INF/spring-autoconfigure-metadata.properties 指定了自动配置匹配到
  例如：
      #Fri Dec 06 06:02:56 GMT 2019
    org.springframework.boot.autoconfigure.data.jpa.JpaRepositoriesAutoConfiguration=
    org.springframework.boot.autoconfigure.web.client.RestTemplateAutoConfiguration.AutoConfigureAfter=org.springframework.boot.autoconfigure.http.HttpMessageConvertersAutoConfiguration
如果有的话  那么后面就会跟上相应的类的全限定名

META-INF/spring.factories  
    记录了所有生效的自动化配置并且添加到了容器中
    每一个这样的  xxxAutoConfiguration类都是容器中的一个组件，
    都加入到容器中；用他们来做自动配置； 
例如：
# 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

以HttpEncodingAutoConfiguration为例：
 //表示这是一个配置类，以前编写的配置文件一样，也可以给容器中添加组件
@Configuration(
    //为了提高性能，如果你的 @Bean 方法之间没有调用关系
    //可以把 proxyBeanMethods 设置为 false 配置类不会被代理了
    proxyBeanMethods = false
)
//指定与自动配置类绑定的Properties属性文件
//、所有在配置文件中能配置的属性都是在xxxxProperties类中封装着；
//配置文件能配置什么就可以参照某个功能对应的这个属性类
@EnableConfigurationProperties({HttpProperties.class})
//判断是不是web环境
@ConditionalOnWebApplication(
    type = Type.SERVLET
)
//判断当前项目有没有这个类
@ConditionalOnClass({
    //是项目过滤编码的类
CharacterEncodingFilter.class
})

//判断配置文件中是否存在某个配置
@ConditionalOnProperty(
    --全缀为spring.http.encoding.enabled
    //前缀为spring.http.encoding
    prefix = "spring.http.encoding",
    //值为enabled
    value = {"enabled"},
    //如果没有  判断也成功
    matchIfMissing = true
)
public class HttpEncodingAutoConfiguration {
    
    private final Encoding properties;
    //只有一个有参的构造方法，只有一个有参构造器的情况下，参数的值就会从容器中拿
    public HttpEncodingAutoConfiguration(HttpProperties properties) {
        this.properties = properties.getEncoding();
    }
    //给容器中添加一个组件，这个组件的某些值需要从properties中获取
    @Bean
    //判断容器中有没有这个bean  如果有的话  就不创建新的bean
    @ConditionalOnMissingBean
    public CharacterEncodingFilter characterEncodingFilter() {
    CharacterEncodingFilter filter = new OrderedCharacterEncodingFilter();
    filter.setEncoding(this.properties.getCharset().name());
    filter.setForceRequestEncoding(this.properties.shouldForce(org.springframework.boot.autoconfigure.http.HttpProperties.Encoding.Type.REQUEST));
    filter.setForceResponseEncoding(this.properties.shouldForce(org.springframework.boot.autoconfigure.http.HttpProperties.Encoding.Type.RESPONSE));
    return filter;
}
    //定制器
    @Bean
public HttpEncodingAutoConfiguration.LocaleCharsetMappingsCustomizer localeCharsetMappingsCustomizer() {
    return new HttpEncodingAutoConfiguration.LocaleCharsetMappingsCustomizer(this.properties);
}
    

//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by Fernflower decompiler)
//指定的配置类
package org.springframework.boot.autoconfigure.http;

import java.nio.charset.Charset;
import java.nio.charset.StandardCharsets;
import java.util.Locale;
import java.util.Map;
import org.springframework.boot.context.properties.ConfigurationProperties;
//从配置文件中获取指定的值和bean的属性进行绑定
@ConfigurationProperties(
    prefix = "spring.http" //前缀为spring.http的
)
public class HttpProperties {
    private boolean logRequestDetails;
    private final HttpProperties.Encoding encoding = new HttpProperties.Encoding();

public HttpProperties() {
}

public boolean isLogRequestDetails() {
    return this.logRequestDetails;
}

public void setLogRequestDetails(boolean logRequestDetails) {
    this.logRequestDetails = logRequestDetails;
}

public HttpProperties.Encoding getEncoding() {
    return this.encoding;
}
//这些是与配置文件绑定的属性  属性有什么  配置文件里就可以配置什么
public static class Encoding {
    public static final Charset DEFAULT_CHARSET;
    private Charset charset;
    private Boolean force;
    private Boolean forceRequest;
    private Boolean forceResponse;
    private Map<Locale, Charset> mapping;

    public Encoding() {
        this.charset = DEFAULT_CHARSET;
    }

    public Charset getCharset() {
        return this.charset;
    }

    public void setCharset(Charset charset) {
        this.charset = charset;
    }

    public boolean isForce() {
        return Boolean.TRUE.equals(this.force);
    }

    public void setForce(boolean force) {
        this.force = force;
    }

    public boolean isForceRequest() {
        return Boolean.TRUE.equals(this.forceRequest);
    }

    public void setForceRequest(boolean forceRequest) {
        this.forceRequest = forceRequest;
    }

    public boolean isForceResponse() {
        return Boolean.TRUE.equals(this.forceResponse);
    }

    public void setForceResponse(boolean forceResponse) {
        this.forceResponse = forceResponse;
    }

    public Map<Locale, Charset> getMapping() {
        return this.mapping;
    }

    public void setMapping(Map<Locale, Charset> mapping) {
        this.mapping = mapping;
    }

    public boolean shouldForce(HttpProperties.Encoding.Type type) {
        Boolean force = type != HttpProperties.Encoding.Type.REQUEST ? this.forceResponse : this.forceRequest;
        if (force == null) {
            force = this.force;
        }

        if (force == null) {
            force = type == HttpProperties.Encoding.Type.REQUEST;
        }

        return force.booleanValue();
    }

    static {
        DEFAULT_CHARSET = StandardCharsets.UTF_8;
    }

    public static enum Type {
        REQUEST,
        RESPONSE;

        private Type() {
        }
    }
}

}

1）、SpringBoot启动会加载大量的自动配置类
2）、我们看我们需要的功能有没有SpringBoot默认写好的自动配置类；
3）、我们再来看这个自动配置类中到底配置了哪些组件；（只要我们要用的组件						    有，我们就不需要再来配置了）
4）、给容器中自动配置类添加组件的时候，会从properties类中获取某些属性。我们就可以在配置文件中指定这些属性的值；

xxxxAutoConfigurartion：自动配置类；自动配置类关联着它的Properties属性文件；
xxxxProperties:封装配置文件中相关属性；
我们可以在application.properties中或者application.yml
             添加上debug=true         debug：true 
这样在项目启动的时候，就可以看到生效的一些自动配置
例如：
       Positive matches:
-----------------

   AopAutoConfiguration matched:
      - @ConditionalOnProperty (spring.aop.auto=true) matched (OnPropertyCondition)

   AopAutoConfiguration.ClassProxyingConfiguration matched:
      - @ConditionalOnMissingClass did not find unwanted class 'org.aspectj.weaver.Advice' (OnClassCondition)
      - @ConditionalOnProperty (spring.aop.proxy-target-class=true) matched (OnPropertyCondition)
...................

             
 没有生效的：
 
              
Negative matches:
-----------------

   ActiveMQAutoConfiguration:
      Did not match:
         - @ConditionalOnClass did not find required class 'javax.jms.ConnectionFactory' (OnClassCondition)

   AopAutoConfiguration.AspectJAutoProxyingConfiguration:
      Did not match:
         - @ConditionalOnClass did not find required class 'org.aspectj.weaver.Advice' (OnClassCondition)
.........................             
                                                           

@Conditional派生注解（Spring注解版原生的@Conditional作用）
作用：必须是@Conditional指定的条件成立，才给容器中添加组件，配置配里面的所有内容才生效；
@Conditional扩展注解 作用（判断是否满足当前指定条件）
@ConditionalOnJava 系统的java版本是否符合要求
@ConditionalOnBean 容器中存在指定Bean；
@ConditionalOnMissingBean 容器中不存在指定Bean；
@ConditionalOnExpression 满足SpEL表达式指定
@ConditionalOnClass 系统中有指定的类
@ConditionalOnMissingClass 系统中没有指定的类
@ConditionalOnSingleCandidate 容器中只有一个指定的Bean，或者这个Bean是首选Bean
@ConditionalOnProperty 系统中指定的属性是否有指定的值
@ConditionalOnResource 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
@ConditionalOnJndi JNDI存在指定项

来源：CSDN

作者：weixin_45342197

链接：https://blog.csdn.net/weixin_45342197/article/details/103753848