SpringMVC之源码分析--HandlerAdapter(二)

概述

本章我们主要分析Spring处理HandlerAdapter组件的处理流程以及其接口源码。概括来说,Spring使用HandlerAdapter组件分为两步,首先是注册组件,其次是处理用户请求,以下针对这两个过程进行详细的分析。

本系列文章是基于Spring5.0.5RELEASE。

注册HandlerAdapter

一般情况下,在使用Spring MVC时,我们会配置在应用启动时加载和初始化Spring MVC组件,也就是在部署描述文件中配置<load-on-startup>1</load-on-startup>,启动过程会最终调用到DispatcherServlet的initStrategies(context)方法,此方法即为初始化九大组件的入口,当然也包括我们今天说要分析的HandlerAdapter,源码如下:

/**
 * 初始化策略对象
 */
protected void initStrategies(ApplicationContext context) {
    initMultipartResolver(context);
    initLocaleResolver(context);
    initThemeResolver(context);
    initHandlerMappings(context);
    // 初始化处理器适配器HandlerAdapter
    initHandlerAdapters(context);
    initHandlerExceptionResolvers(context);
    initRequestToViewNameTranslator(context);
    initViewResolvers(context);
    initFlashMapManager(context);
}

private void initHandlerAdapters(ApplicationContext context) {
    this.handlerAdapters = null;
    // 在部署描述文件中可控制该参数
    if (this.detectAllHandlerAdapters) {
        // 从应用上下文中查找HandlerAdapter
        Map<String, HandlerAdapter> matchingBeans =
                BeanFactoryUtils.beansOfTypeIncludingAncestors(context, HandlerAdapter.class, true, false);
        if (!matchingBeans.isEmpty()) {
            this.handlerAdapters = new ArrayList<>(matchingBeans.values());
            // 对使用的HandlerAdapter进行排序,spring提供的只有RequestMappingHandlerAdapter实现了Ordered接口,其他都不具备排序功能
            AnnotationAwareOrderComparator.sort(this.handlerAdapters);
        }
    }
    else {
        try {
            // 如果在部署描述文件中配置了detectAllHandlerAdapters=false,此时spring会加载名称为handlerAdapter的bean为处理器适配器
            HandlerAdapter ha = context.getBean(HANDLER_ADAPTER_BEAN_NAME, HandlerAdapter.class);
            // 转化为集合赋给handlerAdapters属性
            this.handlerAdapters = Collections.singletonList(ha);
        }
        catch (NoSuchBeanDefinitionException ex) {
            // Ignore, we'll add a default HandlerAdapter later.
        }
    }

    // 如果未配置HandlerAdapter,注册默认的处理器适配器,即从DispatcherServlet.properties中获取的HttpRequestHandlerAdapter、SimpleControllerHandlerAdapter和ReqeustMappingHandlerAdapter
    if (this.handlerAdapters == null) {
        this.handlerAdapters = getDefaultStrategies(context, HandlerAdapter.class);
        if (logger.isDebugEnabled()) {
            logger.debug("No HandlerAdapters found in servlet '" + getServletName() + "': using default");
        }
    }
}

以上就是Spring MVC对HandlerAdapter组件的注册过程。

处理请求

应用在启动时完成了HandlerAdapter的注册,即具备了处理用户请求的能力,那么在用户发起请求时,请求会有DispatcherSerlvlet所拦截,最终调用其doDispatch方法进行处理,源码如下:

/**
 * 处理请求分发给handler
 */
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 {
            // 附件上传有关,后续分析multipartResolver时再详细分析
            processedRequest = checkMultipart(request);
            multipartRequestParsed = (processedRequest != request);

            // 获取请求处理的HandlerExecutionChain对象,该对象组装了我们的handler和相关拦截器
            mappedHandler = getHandler(processedRequest);
            if (mappedHandler == null) {
                noHandlerFound(processedRequest, response);
                return;
            }

            // 获取请求处理的处理器适配器,在getHandlerAdapter方法中进行适配策略的判断
            // 参加下面getHandlerAdapter的方法详解
            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;
                }
            }
            // 拦截器处理用户请求,即执行请求相关的拦截器方法
            if (!mappedHandler.applyPreHandle(processedRequest, response)) {
                return;
            }

            // 调用handler处理方法,由此,通过适配器模式就调用到了我们使用的handler的处理方法
            mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

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

            applyDefaultViewName(processedRequest, mv);
            mappedHandler.applyPostHandle(processedRequest, response, mv);
        }
        catch (Exception ex) {
            dispatchException = ex;
        }
        catch (Throwable err) {
            // As of 4.3, we're processing Errors thrown from handler methods as well,
            // making them available for @ExceptionHandler methods and other scenarios.
            dispatchException = new NestedServletException("Handler dispatch failed", err);
        }
        processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
    }
    catch (Exception ex) {
        triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
    }
    catch (Throwable err) {
        triggerAfterCompletion(processedRequest, response, mappedHandler,
                new NestedServletException("Handler processing failed", err));
    }
    finally {
        if (asyncManager.isConcurrentHandlingStarted()) {
            // Instead of postHandle and afterCompletion
            if (mappedHandler != null) {
                mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
            }
        }
        else {
            // Clean up any resources used by a multipart request.
            if (multipartRequestParsed) {
                cleanupMultipart(processedRequest);
            }
        }
    }
}

/**
 * 返回handler对象的处理器适配器
 */
protected HandlerAdapter getHandlerAdapter(Object handler) throws ServletException {
    if (this.handlerAdapters != null) {
        // 迭代处理器适配器策略,判断handler是否适配成功
        for (HandlerAdapter ha : this.handlerAdapters) {
            if (logger.isTraceEnabled()) {
                logger.trace("Testing handler adapter [" + ha + "]");
            }
            // 进行适配策略的判断
            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");
}

以上就是HandlerAdatper处理用户请求源码分析。

接口分析

通过前面两部分,我们分析了Spring MVC对HandlerAdapter组件的使用,包括注册和处理请求过程,接下来我们看一下给接口的定义,源码如下:

package org.springframework.web.servlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import org.springframework.lang.Nullable;

public interface HandlerAdapter {
    
    /**
     * 判断适配器是否适配handler,适配策略由子类实现
     */
    boolean supports(Object handler);

    /*
     * 使用适配的handler执行用户请求
     */
    @Nullable
    ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception;

    /**
     * 返回资源的最后修改时间,如果handler实现类不支持可以返回-1
     */
    long getLastModified(HttpServletRequest request, Object handler);

}

以上是HandlerAdapter接口的源码分析,如需自定义HandlerAdapter,只需要实现该接口,在supports方法中定义适配策略,并实现handle方法进行调用即可。

总结

本文主要分析了Spring MVC使用HandlerAdapter组件处理用户请求的过程,从过程来看,用户可干预的也就是实现HanderApater接口,自定义处理器适配器。

接下来的几章将分析Spring MVC提供的HandlerAdapter适配策略,希望本节对大家能有帮助,谢谢。

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SpringMVC之源码分析--HandlerAdapter(二)

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