Spring的自动配置原理

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2022-04-26 / 0 评论 / 33 阅读 / 正在检测是否收录...

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
@ConditionalOnJndiJNDI存在指定项
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