SpringMvc-HandlerAdapter

1、HandlerAdapter的作用

HandlerAdapter 字面上的意思就是处理适配器，它的作用用一句话概括就是调用具体的方法对用户发来的请求来进行处理。当 handlerMapping 获取到执行请求的 controller 时，DispatcherServlte 会根据 controller 对应的 controller 类型来调用相应的 HandlerAdapter 来进行处理。

2、HandlerAdapter的注册

DispatcherServlte 会根据配置文件信息注册 HandlerAdapter，如果在配置文件中没有配置，那么 DispatcherServlte 会获取 HandlerAdapter 的默认配置，如果是读取默认配置的话，DispatcherServlte 会读取 DispatcherServlte.properties 文件, 该文件中配置了三种 HandlerAdapter：HttpRequestHandlerAdapter，SimpleControllerHandlerAdapter 和 AnnotationMethodHandlerAdapter。DispatcherServlte 会将这三个 HandlerAdapter 对象存储到它的 handlerAdapters 这个集合属性中，这样就完成了 HandlerAdapter 的注册。

3、HandlerAdapter的执行

HandlerAdapter接口

/*
 * Copyright 2002-2018 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.web.servlet;

import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import org.springframework.lang.Nullable;

/**
 * MVC framework SPI, allowing parameterization of the core MVC workflow.
 *
 * <p>Interface that must be implemented for each handler type to handle a request.
 * This interface is used to allow the {@link DispatcherServlet} to be indefinitely
 * extensible. The {@code DispatcherServlet} accesses all installed handlers through
 * this interface, meaning that it does not contain code specific to any handler type.
 *
 * <p>Note that a handler can be of type {@code Object}. This is to enable
 * handlers from other frameworks to be integrated with this framework without
 * custom coding, as well as to allow for annotation-driven handler objects that
 * do not obey any specific Java interface.
 *
 * <p>This interface is not intended for application developers. It is available
 * to handlers who want to develop their own web workflow.
 *
 * <p>Note: {@code HandlerAdapter} implementors may implement the {@link
 * org.springframework.core.Ordered} interface to be able to specify a sorting
 * order (and thus a priority) for getting applied by the {@code DispatcherServlet}.
 * Non-Ordered instances get treated as lowest priority.
 *
 * @author Rod Johnson
 * @author Juergen Hoeller
 * @see org.springframework.web.servlet.mvc.SimpleControllerHandlerAdapter
 * @see org.springframework.web.servlet.handler.SimpleServletHandlerAdapter
 */
public interface HandlerAdapter {

	/**
	 * 是否支持该处理器
	 *
	 * Given a handler instance, return whether or not this {@code HandlerAdapter}
	 * can support it. Typical HandlerAdapters will base the decision on the handler
	 * type. HandlerAdapters will usually only support one handler type each.
	 * <p>A typical implementation:
	 * <p>{@code
	 * return (handler instanceof MyHandler);
	 * }
	 * @param handler the handler object to check
	 * @return whether or not this object can use the given handler
	 */
	boolean supports(Object handler);

	/**
	 * 执行处理器，返回ModelAndView结果
	 *
	 * Use the given handler to handle this request.
	 * The workflow that is required may vary widely.
	 * @param request current HTTP request
	 * @param response current HTTP response
	 * @param handler the handler to use. This object must have previously been passed
	 * to the {@code supports} method of this interface, which must have
	 * returned {@code true}.
	 * @throws Exception in case of errors
	 * @return a ModelAndView object with the name of the view and the required
	 * model data, or {@code null} if the request has been handled directly
	 */
	@Nullable
	ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception;

	/**
	 * 返回请求的最新更新时间，如果不支持该操作，则返回-1即可
	 *
	 * Same contract as for HttpServlet's {@code getLastModified} method.
	 * Can simply return -1 if there's no support in the handler class.
	 * @param request current HTTP request
	 * @param handler the handler to use
	 * @return the lastModified value for the given handler
	 * see javax.servlet.http.HttpServlet#getLastModified
	 * @see org.springframework.web.servlet.mvc.LastModified#getLastModified
	 */
	long getLastModified(HttpServletRequest request, Object handler);

}

DispatcherServlte 会根据 handlerMapping 传过来的 controller 与已经注册好了的 HandlerAdapter 一一匹配，看哪一种 HandlerAdapter 是支持该 controller 类型的，这调用的是supports方法，如果支持的话就会返回true.如果找到了其中一种 HandlerAdapter 是支持传过来的 controller 类型，那么该 HandlerAdapter 会调用自己的 handle 方法，handle 方法运用 java 的反射机制执行 controller 的具体方法来获得 ModelAndView, 例如 SimpleControllerHandlerAdapter 是支持实现了 controller 接口的控制器，如果自己写的控制器实现了 controller 接口，那么 SimpleControllerHandlerAdapter 就会去执行自己写控制器中的具体方法来完成请求。

今天再来看源码，发现处理器根本就不只有 Controller 这一种。还有 HttpRequestHandler，Servlet 等处理器。下面来介绍一下几种适配器对应的处理器以及这些处理器的作用

AnnotationMethodHandlerAdapter 主要是适配注解类处理器，注解类处理器就是我们经常使用的 @Controller 的这类处理器
HttpRequestHandlerAdapter 主要是适配静态资源处理器，静态资源处理器就是实现了 HttpRequestHandler 接口的处理器，这类处理器的作用是处理通过 SpringMVC 来访问的静态资源的请求
SimpleControllerHandlerAdapter 是 Controller 处理适配器，适配实现了 Controller 接口或 Controller 接口子类的处理器，比如我们经常自己写的 Controller 来继承 MultiActionController.
SimpleServletHandlerAdapter 是 Servlet 处理适配器, 适配实现了 Servlet 接口或 Servlet 的子类的处理器，我们不仅可以在 web.xml 里面配置 Servlet，其实也可以用 SpringMVC 来配置 Servlet，不过这个适配器很少用到，而且 SpringMVC 默认的适配器没有他，默认的是前面的三种。