spring的自动配置原理

springboot配置文件的装配过程

1、springboot在启动的时候会加载主配置类，开启了@EnableAutoConfiguration。

2、@EnableAutoConfiguration的作用：

  • 利用AutoConfigurationImportSelector给容器导入一些组件。
  • 查看selectImports方法的内容，返回一个AutoConfigurationEntry
AutoConfigurationEntry autoConfigurationEntry = getAutoConfigurationEntry(autoConfigurationMetadata,
      annotationMetadata);
------
List<String> configurations = getCandidateConfigurations(annotationMetadata, attributes);
------
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;
    }


  • 可以看到SpringFactoriesLoader.loadFactoryNames，继续看又调用了loadSpringFactories方法，获取META-INF/spring.factories资源文件
public static List<String> loadFactoryNames(Class<?> factoryType, @Nullable ClassLoader classLoader) {
        String factoryTypeName = factoryType.getName();
        return loadSpringFactories(classLoader).getOrDefault(factoryTypeName, 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 {
            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();
                UrlResource resource = new UrlResource(url);
                Properties properties = PropertiesLoaderUtils.loadProperties(resource);
                for (Map.Entry<?, ?> entry : properties.entrySet()) {
                    String factoryTypeName = ((String) entry.getKey()).trim();
                    for (String factoryImplementationName : StringUtils.commaDelimitedListToStringArray((String) entry.getValue())) {
                        result.add(factoryTypeName, factoryImplementationName.trim());
                    }
                }
            }
            cache.put(classLoader, result);
            return result;
        }
        catch (IOException ex) {
            throw new IllegalArgumentException("Unable to load factories from location [" +
                    FACTORIES_RESOURCE_LOCATION + "]", ex);
        }
    }


总结：将类路径下 META-INF/spring.factories 里面配置的所有EnableAutoConfiguration的值加入到了容器中；每一个xxxAutoConfiguration类都是容器中的一个组件，最后都加入到容器中，用来做自动配置，每一个自动配置类都可以进行自动配置功能

使用HttpEncodingAutoConfiguration来解释自动装配原理

/*
表名这是一个配置类，
*/
@Configuration(proxyBeanMethods = false)
/*
启动指定类的ConfigurationProperties功能,进入HttpProperties查看，将配置文件中对应的值和HttpProperties绑定起来，并把HttpProperties加入到ioc容器中
*/
@EnableConfigurationProperties(HttpProperties.class)
/*
spring底层@Confitional注解，根据不同的条件判断，如果满足指定的条件，整个配置类里面的配置就会生效
此时表示判断当前应用是否是web应用，如果是，那么配置类生效
*/
@ConditionalOnWebApplication(type = ConditionalOnWebApplication.Type.SERVLET)
/*
判断当前项目由没有这个类CharacterEncodingFilter，springmvc中进行乱码解决的过滤器
*/
@ConditionalOnClass(CharacterEncodingFilter.class)
/*
判断配置文件中是否存在某个配置：spring.http.encoding.enabled
如果不存在，判断也是成立的，
即使我们配置文件中不配置spring.http.encoding.enabled=true，也是默认生效的
*/
@ConditionalOnProperty(prefix = "spring.http.encoding", value = "enabled", matchIfMissing = true)
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
    @ConditionalOnMissingBean//判断容器中是否有此组件
    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;
        }
    }
}


根据当前不同的条件判断，决定这个配置类是否生效！

总结：

1、springboot启动会加载大量的自动配置类

2、查看需要的功能有没有在springboot默认写好的自动配置类中华

3、查看这个自动配置类到底配置了哪些组件

4、给容器中自动配置类添加组件的时候，会从properties类中获取属性

@Conditional：自动配置类在一定条件下才能生效

@Conditional扩展注解

作用

@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

@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

@ConditionOnWebApplication

当前是web环境

@ConditionalOnNotWebApplication

当前不是web环境

@ConditionalOnJndi

JNDI存在指定项