SSM-SpringMVC-17：SpringMVC中深度剖析HandlerAdapter处理器适配器底层

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 先放一张图

很熟悉啊，之前就看过，我们之前已经把handlerMapping剖了个底朝天，顺着上次的进度，继续跟，把HandlerAdapter处理器适配器剖一下

它同样是由中央调度器DispatcherServlet调度到的，所以，还是这个类中找doDispatch(request,response)这个方法，我把有用的代码，扒下来

protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
        HttpServletRequest processedRequest = request;
        HandlerExecutionChain mappedHandler = null;
        boolean multipartRequestParsed = false;
        WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);

        try {
            try {
                ModelAndView err = null;
                Exception dispatchException = null;

                try {
                    processedRequest = this.checkMultipart(request);
                    multipartRequestParsed = processedRequest != request;
                    mappedHandler = this.getHandler(processedRequest);
                    if(mappedHandler == null || mappedHandler.getHandler() == null) {
                        this.noHandlerFound(processedRequest, response);
                        return;
                    }

//上面的是我说的HandleMapping的执行 流程

//所以从后面的第一行开始看



                    HandlerAdapter ex = this.getHandlerAdapter(mappedHandler.getHandler());

HandlerExecutionChain mappedHandler这个变量，是处理器执行链，它的getHandler方法，就是获取它里面的处理器
它外面的this.getHandlerAdpter
点过去追踪

    protected HandlerAdapter getHandlerAdapter(Object handler) throws ServletException {
//迭代器
        Iterator var2 = this.handlerAdapters.iterator();
//一会要返回去的适配器对象
        HandlerAdapter ha;
        do {
//如果没有下一个的话。就抛异常
            if(!var2.hasNext()) {
                throw new ServletException("No adapter for handler [" + handler + "]: The DispatcherServlet configuration needs to include a HandlerAdapter that supports this handler");
            }
//将此强转成为处理器适配器
            ha = (HandlerAdapter)var2.next();
            if(this.logger.isTraceEnabled()) {
                this.logger.trace("Testing handler adapter [" + ha + "]");
            }
        } while(!ha.supports(handler));
//这儿有个适配器模式
        return ha;
    }

这是下载源码后的最直观的源码

    protected HandlerAdapter getHandlerAdapter(Object handler) throws ServletException {
        for (HandlerAdapter ha : this.handlerAdapters) {
            if (logger.isTraceEnabled()) {
                logger.trace("Testing handler adapter [" + ha + "]");
            }
            if (ha.supports(handler)) {
                return ha;
            }
        }
        throw new ServletException("No adapter for handler [" + handler +
                "]: The DispatcherServlet configuration needs to include a HandlerAdapter that supports this handler");
    }

看ha.sipports(handler)这个方法

找到HandlerAdapter接口

public interface HandlerAdapter {

    boolean supports(Object handler);

    ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception;

    long getLastModified(HttpServletRequest request, Object handler);

}

再来看一下自己写的控制器实现了Controller。这样就看DispatcherServlet属性中的HandlerApater谁支持Controller类型的处理器了。在运行的过程中发现SimpleControllerHandlerAdapter是支持Controller类型的控制器的。
来看一下SimpleControllerHandlerAdapter的代码

找它的实现类：Ctrl+H      SimpleControllerHandlerAdapter

public class SimpleControllerHandlerAdapter implements HandlerAdapter {  
  
    public boolean supports(Object handler) {  
        return (handler instanceof Controller);  
    }  
  
    public ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler)  
            throws Exception {  
  
        return ((Controller) handler).handleRequest(request, response);  
    }  
  
    public long getLastModified(HttpServletRequest request, Object handler) {  
        if (handler instanceof LastModified) {  
            return ((LastModified) handler).getLastModified(request);  
        }  
        return -1L;  
    }  
  
}  

它support来判断是否是处理器

拿到处理器在中央调度器一处调用了handle方法

    protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
        HttpServletRequest processedRequest = request;
        HandlerExecutionChain mappedHandler = null;
        boolean multipartRequestParsed = false;

        WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);

        try {
            ModelAndView mv = null;
            Exception dispatchException = null;

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

                // Determine handler for the current request.
                mappedHandler = getHandler(processedRequest);
                if (mappedHandler == null || mappedHandler.getHandler() == 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 = "GET".equals(method);
                if (isGet || "HEAD".equals(method)) {
                    long lastModified = ha.getLastModified(request, mappedHandler.getHandler());
                    if (logger.isDebugEnabled()) {
                        logger.debug("Last-Modified value for [" + getRequestUri(request) + "] is: " + lastModified);
                    }
                    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());

//就是上面这处

看看handle方法,因为被执行调用到，它返回ModelAndView

里面return走的handleRequest是Controller接口的，找它的实现类

AbstractController抽象类，它有实现handleRequest

    @Override
    public ModelAndView handleRequest(HttpServletRequest request, HttpServletResponse response)
            throws Exception {

        // Delegate to WebContentGenerator for checking and preparing.
        checkRequest(request);
        prepareResponse(response);

        // Execute handleRequestInternal in synchronized block if required.
        if (this.synchronizeOnSession) {
            HttpSession session = request.getSession(false);
            if (session != null) {
                Object mutex = WebUtils.getSessionMutex(session);
                synchronized (mutex) {
                    return handleRequestInternal(request, response);
                }
            }
        }

        return handleRequestInternal(request, response);
    }

看到最后面的那个返回值了吗？是ModelAndView对象，它调用了handleRequestInternal(request,respone)这个方法

还记得AbstractController的实现类吗？不是有种处理器的定义方法就是继承AbstractController

package cn.dawn.day04abstractController;

import org.springframework.web.servlet.ModelAndView;
import org.springframework.web.servlet.mvc.AbstractController;
import org.springframework.web.servlet.mvc.Controller;

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

/**
 * Created by Dawn on 2018/3/19.
 */
//处理器
public class FirstController extends AbstractController {

    protected ModelAndView handleRequestInternal(HttpServletRequest httpServletRequest, HttpServletResponse httpServletResponse) throws Exception {
        ModelAndView me=new ModelAndView();
        me.setViewName("index");
        return me;
    }
}

它的实现类默认要求必须实现handleRequestInternal（request,response）这个方法

所以，理清了吧，再一步步return回去，最后回到了中央调度器

结论，中央处理器DispatcherServlet通过处理器执行链找到handlerAdapter处理器适配器，后者调度了它的处理器，通过处理器处理，返回一个ModelAndView模型和视图对象，一路返回，到中央调度器