Spring Boot 对 Spring MVC 的自动配置

Inclusion of ContentNegotiatingViewResolver and BeanNameViewResolver beans.

自动配置了视图解析器。
在 WebMvcAutoConfiguration 里直接搜索这个类。

@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
    resolver.setOrder(Ordered.HIGHEST_PRECEDENCE);
    return resolver;
}

进入 ContentNegotiatingViewResolver 类，找到初始化方法：

protected void initServletContext(ServletContext servletContext) {
    Collection<ViewResolver> matchingBeans = BeanFactoryUtils.beansOfTypeIncludingAncestors(this.obtainApplicationContext(), ViewResolver.class).values();
    ViewResolver viewResolver;
    if (this.viewResolvers == null) {
        this.viewResolvers = new ArrayList(matchingBeans.size());
        Iterator var3 = matchingBeans.iterator();

        while(var3.hasNext()) {
            viewResolver = (ViewResolver)var3.next();
            if (this != viewResolver) {
                this.viewResolvers.add(viewResolver);
            }
        }
    } else {
        for(int i = 0; i < this.viewResolvers.size(); ++i) {
            viewResolver = (ViewResolver)this.viewResolvers.get(i);
            if (!matchingBeans.contains(viewResolver)) {
                String name = viewResolver.getClass().getName() + i;
                this.obtainApplicationContext().getAutowireCapableBeanFactory().initializeBean(viewResolver, name);
            }
        }
    }

    AnnotationAwareOrderComparator.sort(this.viewResolvers);
    this.cnmFactoryBean.setServletContext(servletContext);
}

发现它把项目里所有的视图解析器都组合到一起。
那么我们就可以写自己的视图解析器，ContentNegotiatingViewResolver 会自动组合起来。在解析视图的时候，会遍历视图解析器进行解析，直到找到合适的视图解析器。

Support for serving static resources, including support for WebJars (covered later in this document)).
Static index.html support.
Custom Favicon support (covered later in this document).

这三个是处理静态资源的。原理呢，就是通过一个静态资源映射。

Automatic registration of Converter, GenericConverter, and Formatter beans.

自动注册了转换器、格式化器。比如，日期格式化，封装对象等等。

@Override
public void addFormatters(FormatterRegistry registry) {
    for (Converter<?, ?> converter : getBeansOfType(Converter.class)) {
        registry.addConverter(converter);
    }
    for (GenericConverter converter : getBeansOfType(GenericConverter.class)) {
        registry.addConverter(converter);
    }
    for (Formatter<?> formatter : getBeansOfType(Formatter.class)) {
        registry.addFormatter(formatter);
    }
}

Support for HttpMessageConverters (covered later in this document).

httpMessage 转换器。在原生版本的 Spring mvc，讲求的是一个 viewModel，交给视图解析器去解析，现在使用转换器，把数据转成 json。

进入这个类一看：

public HttpMessageConverters(boolean addDefaultConverters,
        Collection<HttpMessageConverter<?>> converters) {
    List<HttpMessageConverter<?>> combined = getCombinedConverters(converters,
            addDefaultConverters ? getDefaultConverters() : Collections.emptyList());
    combined = postProcessConverters(combined);
    this.converters = Collections.unmodifiableList(combined);
}

它也把容器中所有的 HttpMessageConverter 给组合到一起，到处都是这种思想，这样方便我们定义自己的转换器。

Automatic registration of MessageCodesResolver (covered later in this document).

错误代码装换器。

Automatic use of a ConfigurableWebBindingInitializer bean (covered later in this document).

@Override
protected ConfigurableWebBindingInitializer getConfigurableWebBindingInitializer() {
    try {
        return this.beanFactory.getBean(ConfigurableWebBindingInitializer.class);
    }
    catch (NoSuchBeanDefinitionException ex) {
        return super.getConfigurableWebBindingInitializer();
    }
}

从容器中拿，拿不到，就调用父类的方法。就是这样的写法，才使得 Spring Boot 可以自动配置，同时支持自定义。

public void initBinder(WebDataBinder binder) {
    binder.setAutoGrowNestedPaths(this.autoGrowNestedPaths);
    if (this.directFieldAccess) {
        binder.initDirectFieldAccess();
    }

    if (this.messageCodesResolver != null) {
        binder.setMessageCodesResolver(this.messageCodesResolver);
    }

    if (this.bindingErrorProcessor != null) {
        binder.setBindingErrorProcessor(this.bindingErrorProcessor);
    }

    if (this.validator != null && binder.getTarget() != null && this.validator.supports(binder.getTarget().getClass())) {
        binder.setValidator(this.validator);
    }

    if (this.conversionService != null) {
        binder.setConversionService(this.conversionService);
    }

    if (this.propertyEditorRegistrars != null) {
        PropertyEditorRegistrar[] var2 = this.propertyEditorRegistrars;
        int var3 = var2.length;

        for(int var4 = 0; var4 < var3; ++var4) {
            PropertyEditorRegistrar propertyEditorRegistrar = var2[var4];
            propertyEditorRegistrar.registerCustomEditors(binder);
        }
    }

}

数据绑定器，就是把前端传来的数据，转换、绑定到我们控制层方法设置的参数上。

If you want to keep Spring Boot MVC features and you want to add additional MVC configuration (interceptors, formatters, view controllers, and other features), you can add your own @Configuration class of type WebMvcConfigurer but without @EnableWebMvc. If you wish to provide custom instances of RequestMappingHandlerMapping, RequestMappingHandlerAdapter, or ExceptionHandlerExceptionResolver, you can declare a WebMvcRegistrationsAdapter instance to provide such components.

想使用现有特性，也想加一些其他的功能，可以自定义 WebMvcRegistrationsAdapter 配置类，但是要取消 @EnableWebMvc 注解。
如果加了 @EnableWebMvc 注解，Spring Boot 自动配置的部分就失效了。

@EnableWebMvc的核心, 导入一个DelegatingWebMvcConfiguration类。

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

2)、这个配置类继承了 WebMvcConfigurationSupport

@Configuration
public class DelegatingWebMvcConfiguration extends WebMvcConfigurationSupport {

3)、而 Spring Boot 自动配置要生效，需要没有 WebMvcConfigurationSupport 这个类，所以自动配置就不会生效了。

@Configuration
@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 {

4)、@EnableWebMvc 将 WebMvcConfigurationSupport 组件导入进来;
5)、导入的 WebMvcConfigurationSupport 只是 SpringMVC 最基本的功能;

总结：
1、在自动配置组件的时候，会先判断是否已经存在对应的组件，如果存在，则加载，不存在，则生成默认的。如果允许多个组件，比如视图解析器，那就组合到一起。我们可以定义自己的组件，Spring Boot 帮助我们进行扩展配置。
2)、在SpringBoot中会有非常多的xxxConfigurer帮助我们进行扩展配置。
3)、在SpringBoot中会有很多的xxxCustomizer帮助我们进行定制配置。

转载于:https://www.jianshu.com/p/2418f71011c0

Spring Boot 对 Spring MVC 的自动配置

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