简单地分析一下HTTP请求是怎么落到DispatcherServlet的doDispatch方法头上
版本说明
基于Spring Boot 2.4.3，对应Spring MVC 5.3.4
首先给出DispatcherServlet类的继承关系图，我们主要关心HttpServlet➡️HttpServletBean➡️FrameworkServlet➡️DispatcherServlet这一条继承关系线
HttpServlet作为抽象类，声明定义了doGet、doPost、doPut等方法处理HTTP请求(以下统称为doXxx方法)，然而它的反应一律是“不好意思，我是废物，不支持处理xxx这个HTTP方法”。因此，需要其子类来重写doXxx方法来处理请求
protected void doGet(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException
{
String msg = lStrings.getString("http.method_get_not_supported");
sendMethodNotAllowed(req, resp, msg);
}
private void sendMethodNotAllowed(HttpServletRequest req, HttpServletResponse resp, String msg) throws IOException {
String protocol = req.getProtocol();
// Note: Tomcat reports "" for HTTP/0.9 although some implementations
// may report HTTP/0.9
if (protocol.length() == 0 || protocol.endsWith("0.9") || protocol.endsWith("1.0")) {
resp.sendError(HttpServletResponse.SC_BAD_REQUEST, msg;
} else {
resp.sendError(HttpServletResponseSC_METHOD_NOT_ALLOWED, msg);
}
}
挖坑
注意到javax/servlet/http/HttpServlet.java来自tomcat-embed-core-9.0.43-sources.jar包，再往外一层思考，web容器是怎么调用这些doXxx方法的？
然而作为HttpServlet的子类，HttpServletBean并没有重写doXxx方法，它是HttpServlet的简单扩展，用于处理web.xml中的serlvet配置参数
接着来到HttpServletBean的子类FrameworkServlet，它终于重写了来自爷爷HttpServlet的doXxx方法，并且实现很简单，都调用processRequest方法代为处理
/**
* Delegate GET requests to processRequest/doService.
* <p>Will also be invoked by HttpServlet's default implementation of {@code doHead},
* with a {@code NoBodyResponse} that just captures the content length.
* @see #doService
* @see #doHead
*/
@Override
protected final void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
processRequest(request, response);
}
在processRequest方法中，它又通过doService方法处理请求并捕获处理时抛出的异常，然后记录结果、发布一个请求已处理事件
protected final void processRequest(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
... // 一些初始化代码
try {
doService(request, response);
}
catch (ServletException | IOException ex) {
failureCause = ex;
throw ex;
}
catch (Throwable ex) {
failureCause = ex;
throw new NestedServletException("Request processing failed", ex);
}
finally {
resetContextHolders(request, previousLocaleContext, previousAttributes);
if (requestAttributes != null) {
requestAttributes.requestCompleted();
}
logResult(request, response, failureCause, asyncManager);
publishRequestHandledEvent(request, response, startTime, failureCause);
}
}
可doService还是不中用，它是一个抽象方法，因此还是需要FrameworkServlet的子类来实现doService方法。终于，轮到DispatcherServlet出场了
protected abstract void doService(HttpServletRequest request, HttpServletResponse response)
throws Exception;
作为FrameworkServlet的子类，DispatcherServlet类实现了doSerive方法，并且它又将处理请求又交给自家的doDispatch方法去做。至此，请求处理到达了最核心的一站
总结
请求处理主要涉及4个类HttpServlet、HttpServletBean、FrameworkServlet和DispatcherServlet。看起来一层套一层，又是子类重写父类方法，又是子类实现父类抽象方法，但是每一层都干了一点事：比如HttpServletBean类处理web.xml中的servlet配置参数，FrameworkServlet是发布请求处理的结果事件，最后DispatcherServlet干最重要的活。一个庞大复杂的系统框架不能通过一个类把所有事都干完了，那样代码臃肿并且紧耦合。通过类继承，每一层干一点事，最后形成一个完整的系统