5.10 MVC自动配置原理

Spring MVC Auto-configuration

Spring Boot provides auto-configuration for Spring MVC that works well with most applications.

The auto-configuration adds the following features on top of Spring’s defaults:

Inclusion of ContentNegotiatingViewResolver and BeanNameViewResolver beans.
- 视图解析器
Support for serving static resources, including support for WebJars (covered later in this document)).
- 支持静态资源文件夹的路径，以及webjars
Automatic registration of Converter, GenericConverter, and Formatter beans.
- Converter转换器：网页提交数据到后台自动封装成为对象，比如把"1"字符串自动转换为int类型
- Formatter格式化器：比如页面返回一个2019-8-10，可以自动格式化为Date对象
Support for HttpMessageConverters (covered later in this document).
- SpringMVC用来转换Http请求和响应的的
- 比如我们要把一个User对象转换为JSON字符串
Automatic registration of MessageCodesResolver (covered later in this document).
- 定义错误代码生成规则
Static index.html support.
- 首页定制
Custom Favicon support (covered later in this document).
- 图标定制
Automatic use of a ConfigurableWebBindingInitializer bean (covered later in this document).
- /初始化数据绑定器：把请求数据绑定到JavaBean中

If you want to keep those Spring Boot MVC customizations and make more MVC customizations (interceptors, formatters, view controllers, and other features), you can add your own @Configuration class of type WebMvcConfigurer but without@EnableWebMvc.

If you want to provide custom instances of RequestMappingHandlerMapping, RequestMappingHandlerAdapter, or ExceptionHandlerExceptionResolver, and still keep the Spring Boot MVC customizations, you can declare a bean of type WebMvcRegistrations and use it to provide custom instances of those components.

If you want to take complete control of Spring MVC, you can add your own @Configuration annotated with @EnableWebMvc, or alternatively add your own @Configuration-annotated DelegatingWebMvcConfiguration as described in the Javadoc of @EnableWebMvc.

ViewResolver 视图解析器

进入WebMvcAutoConfiguration，这是webmvc的配置类，有viewResolver的相关配置

@Bean
@ConditionalOnBean(ViewResolver.class)
@ConditionalOnMissingBean(name = "viewResolver", value = ContentNegotiatingViewResolver.class)
public ContentNegotiatingViewResolver viewResolver(BeanFactory beanFactory) {
   ContentNegotiatingViewResolver resolver = new ContentNegotiatingViewResolver();
   resolver.setContentNegotiationManager(beanFactory.getBean(ContentNegotiationManager.class));
   // ContentNegotiatingViewResolver uses all the other view resolvers to locate
   // a view so it should have a high precedence
   // ContentNegotiatingViewResolver使用所有其他视图解析器来定位视图，因此它应该具有较高的优先
   resolver.setOrder(Ordered.HIGHEST_PRECEDENCE);
   return resolver;
}

ContentNegotiatingViewResolver获得最高优先级

进入ContentNegotiatingViewResolver类查看

public class ContentNegotiatingViewResolver extends WebApplicationObjectSupport
      implements ViewResolver, Ordered, InitializingBean {}

可知其实现了ViewResolver接口，再看看这个接口吧

public interface ViewResolver {
   /**
    * Resolve the given view by name.
    */
   @Nullable
   View resolveViewName(String viewName, Locale locale) throws Exception;
}

此接口的核心方法是resolveViewName，使用我去看看ContentNegotiatingViewResolver类如何实现这个方法

查看ContentNegotiatingViewResolver类的resolveViewName方法

看看如何获取候选视图的，看看getCandidateViews方法

是从viewResolvers属性拿到的，看看这个属性如何初始化

把所有的视图解析器从BeanFactory找出来，放进viewResolvers属性

我们自己写一个ViewResolver，注册成bean应该就可以了

// 扩展webmvc配置
@Configuration
public class MyMvcConfig implements WebMvcConfigurer {
    // 将自定义的视图解析器装配
    @Bean
    public ViewResolver myViewResolver() {
        return new MyViewResolver();
    }
    // 自定义一个视图解析器
    public static class MyViewResolver implements ViewResolver {
        @Override
        public View resolveViewName(String viewName, Locale locale) throws Exception {
            return null;
        }
    }
}

调试看看

果然看到了自定义的ViewResolver，并且ContentNegotiatingViewResolver是优先级最高的。

FormattingConverter 格式转换器

在WebMvcAutoConfiguration类里，可以找到格式转换的方法。

可以发现这些方法去WebMvcProperties类拿到了相应的属性值，熟悉的properties类，也就是说这些值是可以直接配置的，prefix为spring.mvc.format
再看一眼WebMvcProperties类的结构吧，format是其中一个静态内部类

修改SpringBoot的默认配置

@Override
public void addViewControllers(ViewControllerRegistry registry) {
    registry.addViewController("/ttttt").setViewName("test");
}

跳转成功

扩展原理

WebMvcAutoConfiguration类 是 SpringMVC的自动配置类，里面有一个静态内部类WebMvcAutoConfigurationAdapter
这个类上有一个注解，在做其他自动配置时会导入：@Import(EnableWebMvcConfiguration.class)，点进EnableWebMvcConfiguration类看一下，它继承了DelegatingWebMvcConfiguration类

可以看到EnableWebMvcConfiguration类也是WebMvcAutoConfiguration类 的静态内部类

看看DelegatingWebMvcConfiguration类

只有一个属性configurers，类型是WebMvcConfigurerComposite，Composite是拼合的意思，我暂时将其理解为一些WebMvcConfigurer的集合，再点WebMvcConfigurerComposite类看看

也只有一个属性，delegates，类型是List<WebMvcConfigurer>，和我们刚才的猜测基本一致。

delegate意为委托

SpringBoot：最佳英语学习工具

回到DelegatingWebMvcConfiguration类，找一个方法参考下

@Override
protected void addViewControllers(ViewControllerRegistry registry) {
   this.configurers.addViewControllers(registry);
}

通过上一步已经知道configurers是WebMvcConfigurerComposite类，点开这个方法

@Override
public void addViewControllers(ViewControllerRegistry registry) {
   for (WebMvcConfigurer delegate : this.delegates) {
      delegate.addViewControllers(registry);
   }
}

将所有的WebMvcConfigurer相关配置来一起调用！包括我们自己配置的和Spring给我们配置的

全面接管SpringMVC

If you want to take complete control of Spring MVC, you can add your own @Configuration annotated with @EnableWebMvc, or alternatively add your own @Configuration-annotated DelegatingWebMvcConfiguration as described in the Javadoc of @EnableWebMvc.

使用@EnableWebMvc可以让SpringBoot的webmvc配置失效

看看@EnableWebMvc注解如何让自动配置失效

@Import(DelegatingWebMvcConfiguration.class)
public @interface EnableWebMvc {
}

导入了DelegatingWebMvcConfiguration类，这个类在扩展原理第三步出现了，此类继承了WebMvcConfigurationSupport类

/**
 * This is the main class providing the configuration behind the MVC Java config.
 */
public class WebMvcConfigurationSupport implements ApplicationContextAware, ServletContextAware {}

这是Java方式MVC配置的主类

回到WebMvcAutoConfiguration类

@Configuration(proxyBeanMethods = false)
@ConditionalOnWebApplication(type = Type.SERVLET)
@ConditionalOnClass({ Servlet.class, DispatcherServlet.class, WebMvcConfigurer.class })
@ConditionalOnMissingBean(WebMvcConfigurationSupport.class)
@AutoConfigureOrder(Ordered.HIGHEST_PRECEDENCE + 10)
@AutoConfigureAfter({ DispatcherServletAutoConfiguration.class, TaskExecutionAutoConfiguration.class,
      ValidationAutoConfiguration.class })
public class WebMvcAutoConfiguration {}

注意看第四行--->@ConditionalOnMissingBean(WebMvcConfigurationSupport.class)

这是一个条件判断，没有WebMvcConfigurationSupport类才能让本类生效

一旦使用@EnableWebMvc，就会使其生效，从而让SpringBoot有关webMVC的自动配置失效

SpringBoot---初探MVC自动配置原理