SpringMvc源码分析4.1——HandlerAdapter

一. 准备

接着上一篇，我们暴露的接口:

@RestController
public class IndexController {
    @GetMapping("/hello")
    public String hello(String name) {
        return "hello: " + name;
    }
}

bebug到DispatcherServlet#doDispatch：

protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
        // 1. 关注点一
				mappedHandler = getHandler(processedRequest);
				
				// 2. todo待分析，关注点二
				HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

			  // 3. todo待分析，关注点三
				mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
  
  			// 4. todo待分析，关注点四
				applyDefaultViewName(processedRequest, mv);
			
  			// 5. todo待分析，关注点五
 			  processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
	
	}

二. 分析

1. 关注几个属性值

我们先看一下DispatcherServlet内部几个属性此时的值：

/** List of HandlerMappings used by this servlet. */
	@Nullable
	private List<HandlerMapping> handlerMappings;

	/** List of HandlerAdapters used by this servlet. */
	@Nullable
	private List<HandlerAdapter> handlerAdapters;

	/** List of HandlerExceptionResolvers used by this servlet. */
	@Nullable
	private List<HandlerExceptionResolver> handlerExceptionResolvers;

	/** RequestToViewNameTranslator used by this servlet. */
	@Nullable
	private RequestToViewNameTranslator viewNameTranslator;
	
/** List of ViewResolvers used by this servlet. */
	@Nullable
	private List<ViewResolver> viewResolvers;

• handlerMappings

• handlerAdapters

  

• handlerExceptionResolvers

• viewNameTranslator

• viewResolvers

2. 关注点二源码

HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

这里主要是根据获取到的handler找到匹配的HandlerAdapter,源码如下：

protected HandlerAdapter getHandlerAdapter(Object handler) throws ServletException {
		if (this.handlerAdapters != null) {
			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");
	}

首先我们再次看一下HandlerAdapter结构图：

public interface HandlerAdapter {
	// 判断是否支持给定的handler
	boolean supports(Object handler);

  // 用给定的handler处理请求
	@Nullable
	ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception;

	long getLastModified(HttpServletRequest request, Object handler);

}

比如，我们测试的项目，注入的HandlerAdapter为：

HandlerAdapter类型	支持的handler
HttpRequestHandlerAdapter	HttpRequestHandler
SimpleControllerHandlerAdapter	org.springframework.web.servlet.mvc.Controller实例
RequestMappingHandlerAdapter	handler instanceof HandlerMethod

从SpringMvc源码分析3——HandlerMapping可以知道，handler类型为HandlerMethod,所以，这一步getHandlerAdapter得到的HandlerAdapter为RequestMappingHandlerAdapter。

关注点二就分析到这，那么大家有没有疑问，DispatcherServlet里面的这些属性，如handlerAdapters是什么时候初始化的呢？

三. handlerAdapters等的初始化

既然DispatcherServlet是Servlet，那么就肯定会执行它的init()方法；

观察DispatcherServlet，发现里面有一个DispatcherServlet#initStrategies方法,如下：

protected void initStrategies(ApplicationContext context) {
		initMultipartResolver(context);
		initLocaleResolver(context);
		initThemeResolver(context);
		initHandlerMappings(context);
		initHandlerAdapters(context);
		initHandlerExceptionResolvers(context);
		initRequestToViewNameTranslator(context);
		initViewResolvers(context);
		initFlashMapManager(context);
	}

这里面包含了一系列属性的初始化。那么直接在这个方法内debug，然后启动springboot应用，可以看到调用栈：

我们可以看到Servlet的init()调用过程。

然后来看下， initHandlerAdapters(context)源码:

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

   //  detectAllHandlerAdapters 默认为true
   if (this.detectAllHandlerAdapters) {
      // Find all HandlerAdapters in the ApplicationContext, including ancestor contexts.
      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) {
      this.handlerAdapters = getDefaultStrategies(context, HandlerAdapter.class);
      if (logger.isTraceEnabled()) {
         logger.trace("No HandlerAdapters declared for servlet '" + getServletName() +
               "': using default strategies from DispatcherServlet.properties");
      }
   }
}

初始化handlerAdapters分了这几种情况：

1. detectAllHandlerAdapter为true(默认就为true)，会从应用上下文中找到所有的HandlerAdapters,然后做一个排序。

2. detectAllHandlerAdapter为false，就会找到名称为handlerAdapter的handlerAdapter。

3. 最后，如果handlerAdapters仍然为空，则会走默认的策略。这默认的实现可以在DispatcherServlet.properties查看到，如下：

   # Default implementation classes for DispatcherServlet's strategy interfaces.
   # Used as fallback when no matching beans are found in the DispatcherServlet context.
   # Not meant to be customized by application developers.
   
   org.springframework.web.servlet.LocaleResolver=org.springframework.web.servlet.i18n.AcceptHeaderLocaleResolver
   
   org.springframework.web.servlet.ThemeResolver=org.springframework.web.servlet.theme.FixedThemeResolver
   
   org.springframework.web.servlet.HandlerMapping=org.springframework.web.servlet.handler.BeanNameUrlHandlerMapping,\
   	org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerMapping
   
   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.HandlerExceptionResolver=org.springframework.web.servlet.mvc.method.annotation.ExceptionHandlerExceptionResolver,\
   	org.springframework.web.servlet.mvc.annotation.ResponseStatusExceptionResolver,\
   	org.springframework.web.servlet.mvc.support.DefaultHandlerExceptionResolver
   
   org.springframework.web.servlet.RequestToViewNameTranslator=org.springframework.web.servlet.view.DefaultRequestToViewNameTranslator
   
   org.springframework.web.servlet.ViewResolver=org.springframework.web.servlet.view.InternalResourceViewResolver
   
   org.springframework.web.servlet.FlashMapManager=org.springframework.web.servlet.support.SessionFlashMapManager