DispatcherServlet处理请求过程

前言

在DispatcherServlet初始化过程中，我们已经了解了DispatcherServlet的继承关系和初始化过程，现在我们继续学习下DispatcherServlet处理请求的整个过程，本篇文章我们不会讨论细节，特别是DispatcherServlet的处理细节，因为整个处理过程十分复杂，细节部分会留在后续的文章中分析。

Servlet

Servlet接口规定的请求的入口方法，即

public void service(ServletRequest req, ServletResponse res)
    throws ServletException, IOException;

GenericServlet

GenericServlet并没有具体实现service方法

public abstract void service(ServletRequest req, ServletResponse res)
    throws ServletException, IOException;

HttpServlet

HttpServlet是用HTTP协议实现的Servlet的基类，一般我们写的Servlet就是继承于它。

重写父类的service方法，把请求参数ServletRequest 、ServletResponse转化成HttpServletRequest、HttpServletResponse，然后调用内部方法
根据HTTP方法不同，路由到不同的doXXX方法中，doXXX方法由子类实现，子类没实现默认抛出异常。

    public void service(ServletRequest req, ServletResponse res)
        throws ServletException, IOException
    {
        HttpServletRequest  request;
        HttpServletResponse response;
        
        if (!(req instanceof HttpServletRequest &&
                res instanceof HttpServletResponse)) {
            throw new ServletException("non-HTTP request or response");
        }

        request = (HttpServletRequest) req;
        response = (HttpServletResponse) res;

        service(request, response);
    }
}

    protected void service(HttpServletRequest req, HttpServletResponse resp)
        throws ServletException, IOException
    {
        String method = req.getMethod();

        if (method.equals(METHOD_GET)) {
            long lastModified = getLastModified(req);
            if (lastModified == -1) {
                doGet(req, resp);
            } else {
                long ifModifiedSince = req.getDateHeader(HEADER_IFMODSINCE);
                if (ifModifiedSince < lastModified) {
                    maybeSetLastModified(resp, lastModified);
                    doGet(req, resp);
                } else {
                    resp.setStatus(HttpServletResponse.SC_NOT_MODIFIED);
                }
            }

        } else if (method.equals(METHOD_HEAD)) {
            long lastModified = getLastModified(req);
            maybeSetLastModified(resp, lastModified);
            doHead(req, resp);

        } else if (method.equals(METHOD_POST)) {
            doPost(req, resp);
            
        } else if (method.equals(METHOD_PUT)) {
            doPut(req, resp);
            
        } else if (method.equals(METHOD_DELETE)) {
            doDelete(req, resp);
            
        } else if (method.equals(METHOD_OPTIONS)) {
            doOptions(req,resp);
            
        } else if (method.equals(METHOD_TRACE)) {
            doTrace(req,resp);
            
        } else {

            String errMsg = lStrings.getString("http.method_not_implemented");
            Object[] errArgs = new Object[1];
            errArgs[0] = method;
            errMsg = MessageFormat.format(errMsg, errArgs);
            
            resp.sendError(HttpServletResponse.SC_NOT_IMPLEMENTED, errMsg);
        }
    }

HttpServletBean

HttpServletBean主要参与创建工作，并没有涉及请求的处理。

FrameworkServlet

FrameworkServlet实现了doXXX的方法，内部交给processRequest(request, response)处理，以doGet为例，其代码如下

    protected final void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {

        processRequest(request, response);
    }

这里我们可以看出HttpServletBean把各种方法的请求重新聚集到processRequest方法集中处理，下面我们看下processRequest的处理过程，其简化代码如下

    protected final void processRequest(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {

        long startTime = System.currentTimeMillis();
        Throwable failureCause = null;
        // 获取LocaleContextHolder中保存的原始LocaleContext
        LocaleContext previousLocaleContext = LocaleContextHolder.getLocaleContext();
        // 创建当前请求的LocaleContext
        LocaleContext localeContext = buildLocaleContext(request);

        // 获取RequestContextHolder中保存的原始RequestAttributes
        RequestAttributes previousAttributes = RequestContextHolder.getRequestAttributes();
        // 创建当前请求的RequestAttributes
        ServletRequestAttributes requestAttributes = buildRequestAttributes(request, response, previousAttributes);

        WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
        asyncManager.registerCallableInterceptor(FrameworkServlet.class.getName(), new FrameworkServlet.RequestBindingInterceptor());

        //将localeContext、requestAttributes分别设置到LocaleContextHolder和RequestContextHolder
        initContextHolders(request, localeContext, requestAttributes);

        try {
            // 模板方法，DispatcherServlet中实现
            doService(request, response);
        }
        catch (ServletException ex) {
            failureCause = ex;
            throw ex;
        }
        catch (IOException ex) {
            failureCause = ex;
            throw ex;
        }
        catch (Throwable ex) {
            failureCause = ex;
            throw new NestedServletException("Request processing failed", ex);
        }

        finally {
            // 恢复原来的LocaleContext和RequestAttributes
            resetContextHolders(request, previousLocaleContext, previousAttributes);
            if (requestAttributes != null) {
                requestAttributes.requestCompleted();
            }

            // 发布ServletRequestHandledEvent消息
            publishRequestHandledEvent(request, response, startTime, failureCause);
        }
    }

我们可以看出其中最重要的语句是doService(request, response)，它是一个模板方法，具体实现在DispatcherServlet中。