SpringBoot(3)-MVC自動配置及自定義檢視控制器

1. 主要類：

WebMvcAutoConfiguration： MVC的自動配置類
EnableWebMvcConfiguration： 啟用WebMvcConfiguration的類
DelegatingWebMvcConfiguration： WebMvcConfiguration委託的代理類
WebMvcConfigurerComposite： “具體WebMvcConfiguration的委託類”
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WebMvcAutoConfigurationAdapter： MVC自動配置介面卡，其實現了WebMvcConfiguration
WebMvcConfigurer： MVC配置介面
WebMvcConfigurerAdapter： MVC配置介面卡（1.8後棄用，因為介面支援default預設實現）

2. 檢視解析器初始化：

 springboot啟動後會自動配置mvc的元件，即WebMvcAutoConfiguration，自動進行mvc的一些配置
 其內部靜態類WebMvcAutoConfigurationAdapter也是一個元件，通過@Import匯入EnableWebMvcConfiguration元件

	@Configuration(proxyBeanMethods = false)
	@Import(EnableWebMvcConfiguration.class)
	@EnableConfigurationProperties({ WebMvcProperties.class, ResourceProperties.class })
	@Order(0)
	public static class WebMvcAutoConfigurationAdapter implements WebMvcConfigurer {

 EnableWebMvcConfiguration繼承了DelegatingWebMvcConfiguration委託代理類

	@Configuration(proxyBeanMethods = false)
	public static class EnableWebMvcConfiguration extends DelegatingWebMvcConfiguration implements ResourceLoaderAware {

 代理類通過setConfigurers方法來初始化mvc配置的具體委託類
 @Autowired會從容器總把實現WebMvcConfigurer介面的類都注入到configurers
 然後將所有的mvc配置類新增到WebMvcConfigurerComposite

@Configuration(proxyBeanMethods = false)
public class DelegatingWebMvcConfiguration extends WebMvcConfigurationSupport {

	private final WebMvcConfigurerComposite configurers = new WebMvcConfigurerComposite();


	@Autowired(required = false)
	public void setConfigurers(List<WebMvcConfigurer> configurers) {
		if (!CollectionUtils.isEmpty(configurers)) {
			this.configurers.addWebMvcConfigurers(configurers);
		}
	}

//也實現了WebMvcConfigurer當並沒有@Configuration到容器中
class WebMvcConfigurerComposite implements WebMvcConfigurer {

	private final List<WebMvcConfigurer> delegates = new ArrayList<>();


	public void addWebMvcConfigurers(List<WebMvcConfigurer> configurers) {
		if (!CollectionUtils.isEmpty(configurers)) {
			this.delegates.addAll(configurers);
		}
	}

 dubug可以知道，預設使用WebMvcAutoConfigurationAdapter

 所以，要想自定義mvc的配置，只需要實現WebMvcConfigurer介面，並將該類放入容器中即可

@Configuration
public class MyConfigure implements WebMvcConfigurer {
    @Override
    public void addViewControllers(ViewControllerRegistry registry) {
        registry.addViewController("/testView").setViewName("success");
    }
}

3. MVC檢視功能配置：

 由上可知，mvc的配置，預設使用WebMvcAutoConfigurationAdapter來配置
 這個類在配置的時候，會往容器中新增三個檢視bean，主要關注ContentNegotiatingViewResolver

 注意其該配置resolver.setOrder(Ordered.HIGHEST_PRECEDENCE);設定最高階的Order，後續會提到

	@Configuration(proxyBeanMethods = false)
	@Import(EnableWebMvcConfiguration.class)
	@EnableConfigurationProperties({ WebMvcProperties.class, ResourceProperties.class })
	@Order(0)
	public static class WebMvcAutoConfigurationAdapter implements WebMvcConfigurer {

		... ... ...
		
		@Bean
		@ConditionalOnMissingBean
		public InternalResourceViewResolver defaultViewResolver() {
			InternalResourceViewResolver resolver = new InternalResourceViewResolver();
			resolver.setPrefix(this.mvcProperties.getView().getPrefix());
			resolver.setSuffix(this.mvcProperties.getView().getSuffix());
			return resolver;
		}

		@Bean
		@ConditionalOnBean(View.class)
		@ConditionalOnMissingBean
		public BeanNameViewResolver beanNameViewResolver() {
			BeanNameViewResolver resolver = new BeanNameViewResolver();
			resolver.setOrder(Ordered.LOWEST_PRECEDENCE - 10);
			return resolver;
		}

		@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;
		}
		... ... ...

 ContentNegotiatingViewResolver這個檢視解析器主要是用來組合所有檢視解析器的
 在其內部實現方法initServletContext可以看到，將所有檢視解析器（除本類）新增到本類的中viewResolvers

 可見，想要自定義檢視解析器，可以實現ViewResolver介面，然後加入容器即可，本文不論述

4. MVC解析檢視過程：

 前端控制器（DispatcherServlet），在第一次呼叫檢視解析器之前，會通過initViewResolvers方法先初始化檢視解析器
 該方法中，獲取容器內所有檢視解析器，即容器中實現ViewResolver介面的類
 然後所有檢視解析器根據Order排序，上文提及ContentNegotiatingViewResolver設定了最高階別Order，所以它會排在第一個

 當初始化檢視解析器後，前端控制器通過resolveViewName方法遍歷所有檢視解析器來解析，能解析立即返回View
 ContentNegotiatingViewResolver排在第一個，所以會最先呼叫

 其內部方法通過getCandidateViews來解析出所有檢視，最後處理返回一個bestView給前端控制器

 在getCandidateViews方法中，this.viewResolvers就是之前ContentNegotiatingViewResolver組合的所有檢視解析器
 呼叫各檢視解析器的實現方法resolveViewName來解析檢視
 有興趣的可以研究一些這個方法，我就算了

 詳細的MVC執行過程可參考：SpringBoot(2)-MVC執行過程