个人博客地址：https://www.liujunit.com

SpringBoot自动配置

自动配置的原理

1. SpringBoot在启动的时候加载自动配置，开启自动配置的注解@EnableAutoConfigration

2. @EnableAutoConfigration的作用

   • 通过AutoConfigurationImportSelector给容器导入一些组件

   • 可以查看selectImports()方法

   • List configurations = getCandidateConfigurations(annotationMetadata, attributes);

     查看候选的配置

 查看源码可以看到这里将类路径下的META-INF\spring.factories下的EnableAutoConfiguration的所有的值加入到了容器中

​ 这里的每一个的XXXAutoConfigration类都是容器的一个组件，用他们来做自动配置。

3. 每一自动配置类进行自动配置功能

4. 以HttpEncodingAutoConfigration(HTTP自动编码配置类)为例

   @Configuration//表示这是一个配置类，跟以前编写的配置文件一样，也可以给容器中添加组件
   @EnableConfigurationProperties(HttpProperties.class) //启动EnableConfigurationProperties功能，将配置文件中的配置与HttpProperties类绑定起来，同时加载到ioc容器当中去
   @ConditionalOnWebApplication(type = ConditionalOnWebApplication.Type.SERVLET)//Spring底层的注解Conditional，判断是不是Web应用程序，如果是的话生效
   @ConditionalOnClass(CharacterEncodingFilter.class)//判断当前的项目中有没有这个类，SpringMVC的进行乱码解决的过滤器
   @ConditionalOnProperty(prefix = "spring.http.encoding", value = "enabled", matchIfMissing = true)//判断配置文件中是否有这个配置，spring.http.encoding.enabled如果不存在判断也是成立的，spring.http.encoding.enabled=true即使不配置也是默认生效的。
   public class HttpEncodingAutoConfiguration {//和springboot的配置文件映射private final HttpProperties.Encoding properties;//只有一个有参构造器，参数的值就会从容器中获取public HttpEncodingAutoConfiguration(HttpProperties properties) {this.properties = properties.getEncoding();}@Bean//给容器中添加一个组件，这个组件的值是从properties中获取的@ConditionalOnMissingBean//容器中如果没有组件 添加组件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;}...省略}
   

   一旦这个配置类生效，就会将properties配置文件中的值配置到对应组件当中去。

5. properties配置文件中的所有配置都有对应的XXXProperties配置类，可以配置哪些属性，可以从对应的类中去查看

   @ConfigurationProperties(prefix = "spring.http")//从配置文件中获取值和对应的属性进行绑定
   public class HttpProperties {/*** Whether logging of (potentially sensitive) request details at DEBUG and TRACE level* is allowed.*/private boolean logRequestDetails;/*** HTTP encoding properties.*/private final Encoding encoding = new Encoding();public boolean isLogRequestDetails() {return this.logRequestDetails;}public void setLogRequestDetails(boolean logRequestDetails) {this.logRequestDetails = logRequestDetails;}public Encoding getEncoding() {return this.encoding;}/*** Configuration properties for http encoding.*/public static class Encoding {//默认配置编码是UTF-8public static final Charset DEFAULT_CHARSET = StandardCharsets.UTF_8;/*** Charset of HTTP requests and responses. Added to the "Content-Type" header if* not set explicitly.*/private Charset charset = DEFAULT_CHARSET;/*** Whether to force the encoding to the configured charset on HTTP requests and* responses.是否在http请求和响应上进行强制编码*/private Boolean force;/*** Whether to force the encoding to the configured charset on HTTP requests. 如果没有设定强制编码force，默认是true* Defaults to true when "force" has not been specified.*/private Boolean forceRequest;/*** Whether to force the encoding to the configured charset on HTTP responses.*/private Boolean forceResponse;/*** Locale in which to encode mapping.*/private Map<Locale, Charset> mapping;...省略}}
   

总结：

1. SpringBoot启动的时候会加载大量的自动配置类
2. 我们可以查看我们需要的配置类是否已经写好了
3. 可以查看配置类中配置了哪些组件，只要我们要用的组件有就不需要再来配置
4. 我们可以在properties中配置这些属性，从对应的XXXProperties类中查看有哪些属性可以配置

官方参考配置

Spring的原生注解@Conditional

SpringBoot中的只要有@Conditional扩展的地方说明必须条件满足，才能给容器中添加组件，配置里的能容才会生效

自动配置必须是一定的条件才能生效，可以通过properties设置debug=true，让控制台打印查看生效的配置类。

============================
CONDITIONS EVALUATION REPORT
============================Positive matches://生效的配置类
-----------------CodecsAutoConfiguration matched:- @ConditionalOnClass found required class 'org.springframework.http.codec.CodecConfigurer' (OnClassCondition)CodecsAutoConfiguration.JacksonCodecConfiguration matched:- @ConditionalOnClass found required class 'com.fasterxml.jackson.databind.ObjectMapper' (OnClassCondition)CodecsAutoConfiguration.JacksonCodecConfiguration#jacksonCodecCustomizer matched:- @ConditionalOnBean (types: com.fasterxml.jackson.databind.ObjectMapper; SearchStrategy: all) found bean 'jacksonObjectMapper' (OnBeanCondition)DispatcherServletAutoConfiguration matched:- @ConditionalOnClass found required class 'org.springframework.web.servlet.DispatcherServlet' (OnClassCondition)- found 'session' scope (OnWebApplicationCondition)...省略Negative matches://没有生效的配置类
-----------------ActiveMQAutoConfiguration:Did not match:- @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)ArtemisAutoConfiguration:Did not match:- @ConditionalOnClass did not find required class 'javax.jms.ConnectionFactory' (OnClassCondition)...省略

后记

加油吧！