[Python知识库]SpringMvc源码分析-处理器适配器

适配器的作用我进行总结一下，可能不完全准确：
1）将http request中数据转成 java object，比如json 转成pojo，做为控制器方法入参
2）用于在适配器中执行控制器方法
3）创建ModelAndView

一、查找Adapter

/**
 * Return the HandlerAdapter for this handler object.
 * @param handler the handler object to find an adapter for
 * @throws ServletException if no HandlerAdapter can be found for the handler. This is a fatal error.
 */
protected HandlerAdapter getHandlerAdapter(Object handler) throws ServletException {
	if (this.handlerAdapters != null) {
	    //循环遍历
		for (HandlerAdapter adapter : this.handlerAdapters) {
			if (adapter.supports(handler)) {
				return adapter;
			}
		}
	}
	throw new ServletException("No adapter for handler [" + handler +
			"]: The DispatcherServlet configuration needs to include a HandlerAdapter that supports this handler");
}

对于查找不难理解，注册才是关键点

二、注册Adapter

2.1、初始化

private void initHandlerAdapters(ApplicationContext context) {
	this.handlerAdapters = null;

	if (this.detectAllHandlerAdapters) {//默认进入if分支
		// Find all HandlerAdapters in the ApplicationContext, including ancestor contexts.
		// 从IoC容器中获取Adapter
		Map<String, HandlerAdapter> matchingBeans =
				BeanFactoryUtils.beansOfTypeIncludingAncestors(context, HandlerAdapter.class, true, false);
		if (!matchingBeans.isEmpty()) {
			this.handlerAdapters = new ArrayList<>(matchingBeans.values());
			// We keep HandlerAdapters in sorted order.
			AnnotationAwareOrderComparator.sort(this.handlerAdapters);
		}
	}
	else {
		try {
			HandlerAdapter ha = context.getBean(HANDLER_ADAPTER_BEAN_NAME, HandlerAdapter.class);
			this.handlerAdapters = Collections.singletonList(ha);
		}
		catch (NoSuchBeanDefinitionException ex) {
			// Ignore, we'll add a default HandlerAdapter later.
		}
	}

	// Ensure we have at least some HandlerAdapters, by registering
	// default HandlerAdapters if no other adapters are found.
	if (this.handlerAdapters == null) {//获取默认的适配器
		this.handlerAdapters = getDefaultStrategies(context, HandlerAdapter.class);
		if (logger.isTraceEnabled()) {
			logger.trace("No HandlerAdapters declared for servlet '" + getServletName() +
					"': using default strategies from DispatcherServlet.properties");
		}
	}
}

通过上面初始化流程，可以得出：
1）方法返回前，会给handlerAdapters赋值，之后用于查找
2）如果在IoC容器中存在Adapter类型的bean则从IoC中获取，否则将注册默认的适配器。
那么现在就存在两个问题：
问题1：什么条件下IoC容器中会存在Adapter呢？
问题2：默认适配器都有哪些呢？

2.2、将Adapter注册到IoC容器中

问题1，本质上很简单，我们自己创建一个类并且实现接口Adapter，将该类的对象注入到IoC容器中即可，这样就不会使用默认的适配器了。那么springmvc是通过什么方式实现的呢？答案：

<mvc:default-servlet-handler/>
<mvc:annotation-driven/>

以上两个配置都将Adapter注入到IoC容器中，具体源码分析可以参考《SpringMvc源码分析–配置文件解析》

2.3、默认Adapter

在方法getDefaultStrategies是对默认Adapter进行注册，该方法核心内容是读取配置文件DispatcherServlet.properties，配置文件内容：

org.springframework.web.servlet.HandlerAdapter=org.springframework.web.servlet.mvc.HttpRequestHandlerAdapter,\
	org.springframework.web.servlet.mvc.SimpleControllerHandlerAdapter,\
	org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter,\
	org.springframework.web.servlet.function.support.HandlerFunctionAdapter

通过反射方式，将这些类进行注册

2.3、RequestMappingHandlerAdapter

在工程中通过@Controller进行定义的控制器方法，使用的适配器就是RequestMappingHandlerAdapter

	protected ModelAndView invokeHandlerMethod(HttpServletRequest request,
		HttpServletResponse response, HandlerMethod handlerMethod) throws Exception {

	ServletWebRequest webRequest = new ServletWebRequest(request, response);//封装 req和res对象
	try {
		WebDataBinderFactory binderFactory = getDataBinderFactory(handlerMethod);
		ModelFactory modelFactory = getModelFactory(handlerMethod, binderFactory);

		ServletInvocableHandlerMethod invocableMethod = createInvocableHandlerMethod(handlerMethod);//封Method对象，用于通过反射进行调用
		if (this.argumentResolvers != null) {
			invocableMethod.setHandlerMethodArgumentResolvers(this.argumentResolvers);
		}
		if (this.returnValueHandlers != null) {
			invocableMethod.setHandlerMethodReturnValueHandlers(this.returnValueHandlers);
		}
		invocableMethod.setDataBinderFactory(binderFactory);
		invocableMethod.setParameterNameDiscoverer(this.parameterNameDiscoverer);

		ModelAndViewContainer mavContainer = new ModelAndViewContainer(); //创建ModelAndView的容器
		mavContainer.addAllAttributes(RequestContextUtils.getInputFlashMap(request));
		modelFactory.initModel(webRequest, mavContainer, invocableMethod);
		mavContainer.setIgnoreDefaultModelOnRedirect(this.ignoreDefaultModelOnRedirect);

		AsyncWebRequest asyncWebRequest = WebAsyncUtils.createAsyncWebRequest(request, response);
		asyncWebRequest.setTimeout(this.asyncRequestTimeout);

		WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
		asyncManager.setTaskExecutor(this.taskExecutor);
		asyncManager.setAsyncWebRequest(asyncWebRequest);
		asyncManager.registerCallableInterceptors(this.callableInterceptors);
		asyncManager.registerDeferredResultInterceptors(this.deferredResultInterceptors);

		if (asyncManager.hasConcurrentResult()) {
			Object result = asyncManager.getConcurrentResult();
			mavContainer = (ModelAndViewContainer) asyncManager.getConcurrentResultContext()[0];
			asyncManager.clearConcurrentResult();
			LogFormatUtils.traceDebug(logger, traceOn -> {
				String formatted = LogFormatUtils.formatValue(result, !traceOn);
				return "Resume with async result [" + formatted + "]";
			});
			invocableMethod = invocableMethod.wrapConcurrentResult(result);
		}

		invocableMethod.invokeAndHandle(webRequest, mavContainer); //调用控制器方法
		if (asyncManager.isConcurrentHandlingStarted()) {
			return null;
		}

		return getModelAndView(mavContainer, modelFactory, webRequest); //返回ModelAndView对象
	}
	finally {
		webRequest.requestCompleted();
	}
}

三、总结

适配器的内容是最复杂的，每一点都值得仔细推敲分析，能力有限，这里只能简单介绍一下，希望可以起到抛砖引玉的作用。