Spring源码学习篇6 - DispatcherServlet 基本工作流程

于 2021-10-04 20:11:08 发布
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分类专栏： Spring源码学习 文章标签： spring
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配置DispatcherServlet

Servlet3.0之后, Servlet可以被动态监测到，web.xml不再是必须的，一般的servlet可以使用@WebServlet，除了添加注解，也可编程配置。WebApplicationInitializer的实现类在应用部署的时候会被执行，可以再WebApplicationInitializer.onStartup中配置Spring的DispatcherServlet：

public class MyWebApplicationInitializer implements WebApplicationInitializer {
    @Override
    public void onStartup(ServletContext servletContext) throws ServletException {
        AnnotationConfigWebApplicationContext webApplicationContext = new AnnotationConfigWebApplicationContext();
        webApplicationContext.register(AppConfig.class);

        DispatcherServlet dispatcherServlet = new DispatcherServlet(webApplicationContext);
        ServletRegistration.Dynamic registration = servletContext.addServlet("springApp", dispatcherServlet);
        registration.setLoadOnStartup(1);
        registration.addMapping("/app/*");
    }
}

WebApplicationInitializer会被执行的原因是，从servlet3.0之后，Servlet容器会根据SPI机制，在Spring-web模块下的MATE-INF/services/javax.servlet.ServletContainerInitializer文件中，找到ServletContainerInitializer具体实现类：

//文件:MATE-INF/services/javax.servlet.ServletContainerInitializer
org.springframework.web.SpringServletContainerInitializer

ServletContainerInitializer只有一个方法，第一个参数将会是HandlesTypes注解声明将会处理的类型；在Spring中实现类就是SpringServletContainerInitializer，HandlesTypes声明的类型是WebApplicationInitializer, 所以在应用初始化的时候WebApplicationInitializer的实现类将会被执行，可以在其中配置servlet、filter等等。

public interface ServletContainerInitializer {
    public void onStartup(Set<Class<?>> c, ServletContext ctx)
        throws ServletException; 
}

@HandlesTypes(WebApplicationInitializer.class)
public class SpringServletContainerInitializer implements ServletContainerInitializer {
		@Override
	public void onStartup(@Nullable Set<Class<?>> webAppInitializerClasses, ServletContext servletContext)
			throws ServletException {
		List<WebApplicationInitializer> initializers = Collections.emptyList();
		// ... 省略部分：
		// 主要内容是：将webAppInitializerClasses中非接口和非抽象类的实例化后，添加到initializers
		AnnotationAwareOrderComparator.sort(initializers);
		for (WebApplicationInitializer initializer : initializers) {
			initializer.onStartup(servletContext);
		}
	}
}

可以配置多个DispatcherServlet，每个Servlet有自己专属的Servlet WebApplicationContext, 它的parent将会是Root WebAppcalitionContext。通过这种方式，可以将公共的services等bean配置在ROOT中；

public class MyWebAppInitializer extends AbstractAnnotationConfigDispatcherServletInitializer {

    @Override
    protected Class<?>[] getRootConfigClasses() {
        return new Class<?>[] { RootConfig.class };
    }

    @Override
    protected Class<?>[] getServletConfigClasses() {
        return new Class<?>[] { App1Config.class };
    }

    @Override
    protected String[] getServletMappings() {
        return new String[] { "/app1/*" };
    }
}

DispatcherServlet 的初始化

init是Servlet生命周期的开始，对DispatcherServlet来说，配置webApplicationContext并初始Bean都在其中。org.springframework.web.servlet.FrameworkServlet#initWebApplicationContext中包含主要的两部分逻辑:

  1. org.springframework.web.servlet.FrameworkServlet#configureAndRefreshWebApplicationContext
  2. org.springframework.web.servlet.DispatcherServlet#initStrategies
初始化基础Bean

这部分，除了ApplicationContext上记录了servlet相关的信息之外，创建Bean的逻辑与之前了解到的没有什么不同，这里目前还不包含对Controller的处理。

wac.setServletContext(getServletContext());
wac.setServletConfig(getServletConfig());
wac.setNamespace(getNamespace());
wac.addApplicationListener(new SourceFilteringListener(wac, new ContextRefreshListener()));

// The wac environment's #initPropertySources will be called in any case when the context
// is refreshed; do it eagerly here to ensure servlet property sources are in place for
// use in any post-processing or initialization that occurs below prior to #refresh
ConfigurableEnvironment env = wac.getEnvironment();
if (env instanceof ConfigurableWebEnvironment) {
	((ConfigurableWebEnvironment) env).initPropertySources(getServletContext(), getServletConfig());
}

postProcessWebApplicationContext(wac);
applyInitializers(wac);
wac.refresh();
Servlet处理请求用的相关Bean
protected void initStrategies(ApplicationContext context) {
	initMultipartResolver(context);
	initLocaleResolver(context);
	initThemeResolver(context);
	initHandlerMappings(context);
	initHandlerAdapters(context);
	initHandlerExceptionResolvers(context);
	initRequestToViewNameTranslator(context);
	initViewResolvers(context);
	initFlashMapManager(context);
}

这里先主要关心Controller 以及RestContoller的工作流程，这里HandlerMapping是非常重要的，如果没有配置自己的HandlerMapping的话，将会从DispatcherServlet.properties读取默认的实现类：

org.springframework.web.servlet.handler.BeanNameUrlHandlerMapping
org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerMapping
org.springframework.web.servlet.function.support.RouterFunctionMapping

读取到这些类并实例化为Spring中的Bean，这些类都实现了afterPropertiesSet。以RequestMappingHandlerMapping为例：

// AbstractHandlerMethodMapping.java
public void afterPropertiesSet() {
	initHandlerMethods();
}

/**
 * Scan beans in the ApplicationContext, detect and register handler methods.
 * @see #getCandidateBeanNames()
 * @see #processCandidateBean
 * @see #handlerMethodsInitialized
 */
protected void initHandlerMethods() {
	for (String beanName : getCandidateBeanNames()) {
		if (!beanName.startsWith(SCOPED_TARGET_NAME_PREFIX)) {
			processCandidateBean(beanName);
		}
	}
	handlerMethodsInitialized(getHandlerMethods());
}

protected void processCandidateBean(String beanName) {
	Class<?> beanType = null;
	try {
		beanType = obtainApplicationContext().getType(beanName);
	}
	catch (Throwable ex) {
		// An unresolvable bean type, probably from a lazy bean - let's ignore it.
		if (logger.isTraceEnabled()) {
			logger.trace("Could not resolve type for bean '" + beanName + "'", ex);
		}
	}
	if (beanType != null && isHandler(beanType)) {
		detectHandlerMethods(beanName);
	}
}

@Override
protected boolean isHandler(Class<?> beanType) {
	return (AnnotatedElementUtils.hasAnnotation(beanType, Controller.class) ||
			AnnotatedElementUtils.hasAnnotation(beanType, RequestMapping.class));
}

可以看会对所有的Bean进行处理，如果Bean含有Controller或者RequestMapping注解，那么对所有的 RequestMapping注解的方法进行处理：

  1. 根据@RequestMapping注解，构造RequestMappingInfo
    protected RequestMappingInfo createRequestMappingInfo(
	RequestMapping requestMapping, @Nullable RequestCondition<?> customCondition) {

RequestMappingInfo.Builder builder = RequestMappingInfo
		.paths(resolveEmbeddedValuesInPatterns(requestMapping.path()))
		.methods(requestMapping.method())
		.params(requestMapping.params())
		.headers(requestMapping.headers())
		.consumes(requestMapping.consumes())
		.produces(requestMapping.produces())
		.mappingName(requestMapping.name());
if (customCondition != null) {
	builder.customCondition(customCondition);
}
return builder.options(this.config).build();
}
  2. 构造HandlerMethod，它主要包含两个重要属性：bean 以及对应的含有@RequestMapping的Method
  3. 获取DirectPath, 并记录DirectPath与RequestMappingInfo的关联
  4. 记录方法名，与HandlerMethod对象的关联
  5. 记录 handlerMethod 与 CorsConfiguration配置对象的关联
  6. 记录RequestMappingInfo 与 MappingRegistratio对象的关联
    //MappingRegistration
public MappingRegistration(T mapping, HandlerMethod handlerMethod,
	@Nullable Set<String> directPaths, @Nullable String mappingName, boolean corsConfig) {

Assert.notNull(mapping, "Mapping must not be null");
Assert.notNull(handlerMethod, "HandlerMethod must not be null");
this.mapping = mapping;
this.handlerMethod = handlerMethod;
this.directPaths = (directPaths != null ? directPaths : Collections.emptySet());
this.mappingName = mappingName;
this.corsConfig = corsConfig;
}

处理请求

当客户端发起请求的时候，如果满足DispatcherServlet的处理路径，那么经过doService,交由doDispatch进行处理请求，主要处理逻辑如下：

processedRequest = checkMultipart(request);
multipartRequestParsed = (processedRequest != request);

// Determine handler for the current request.
mappedHandler = getHandler(processedRequest);
if (mappedHandler == null) {
	noHandlerFound(processedRequest, response);
	return;
}

// Determine handler adapter for the current request.
HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

// Process last-modified header, if supported by the handler.
String method = request.getMethod();
boolean isGet = HttpMethod.GET.matches(method);
if (isGet || HttpMethod.HEAD.matches(method)) {
	long lastModified = ha.getLastModified(request, mappedHandler.getHandler());
	if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {
		return;
	}
}

if (!mappedHandler.applyPreHandle(processedRequest, response)) {
	return;
}

// Actually invoke the handler.
mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

if (asyncManager.isConcurrentHandlingStarted()) {
	return;
}

applyDefaultViewName(processedRequest, mv);
mappedHandler.applyPostHandle(processedRequest, response, mv);
大致的处理流程如下

第一步是根据请求，获取对应的HandlerMethod

protected HandlerMethod getHandlerInternal(HttpServletRequest request) throws Exception {
	String lookupPath = initLookupPath(request);
	this.mappingRegistry.acquireReadLock();
	try {
		HandlerMethod handlerMethod = lookupHandlerMethod(lookupPath, request);
		return (handlerMethod != null ? handlerMethod.createWithResolvedBean() : null);
	}
	finally {
		this.mappingRegistry.releaseReadLock();
	}
}

基本流程是获取到请求的path, 然后得到RequestMappingInfo，最后取得HandlerMethod

第二步，是构造HandlerExecutionChain，主要是添加合适的拦截器

protected HandlerExecutionChain getHandlerExecutionChain(Object handler, HttpServletRequest request) {
	HandlerExecutionChain chain = (handler instanceof HandlerExecutionChain ?
			(HandlerExecutionChain) handler : new HandlerExecutionChain(handler));

	for (HandlerInterceptor interceptor : this.adaptedInterceptors) {
		if (interceptor instanceof MappedInterceptor) {
			MappedInterceptor mappedInterceptor = (MappedInterceptor) interceptor;
			if (mappedInterceptor.matches(request)) {
				chain.addInterceptor(mappedInterceptor.getInterceptor());
			}
		}
		else {
			chain.addInterceptor(interceptor);
		}
	}
	return chain;
}

第三步，处理CorsConfiguration

protected HandlerExecutionChain getCorsHandlerExecutionChain(HttpServletRequest request,
		HandlerExecutionChain chain, @Nullable CorsConfiguration config) {

	if (CorsUtils.isPreFlightRequest(request)) {
		HandlerInterceptor[] interceptors = chain.getInterceptors();
		return new HandlerExecutionChain(new PreFlightHandler(config), interceptors);
	}
	else {
		chain.addInterceptor(0, new CorsInterceptor(config));
		return chain;
	}
}

第四步，确定HandlerAdapter，各个adapter是否可以处理当前handler的判断也比较简单，比如AbstractHandlerMethodAdapter 基本只要handler是HandlerMethod实例可以。

// AbstractHandlerMethodAdapter.java
@Override
public final boolean supports(Object handler) {
	return (handler instanceof HandlerMethod && supportsInternal((HandlerMethod) handler));
}

第五步，优先调用各拦截器的preHandle方法，如果有个拦截器返回false,那么结束处理

第六步，通过HandlerAdapter的实例调用具体的bean，像RequestMappingHandlerAdapter.handleInternal,并得到ModelAndView

第七步，调用各拦截器的postHandle方法

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