Servlet Source Analysis (2)
2.Servlet
Servlet API的核心就是Servlet接口，所有的Servlet 类都必须实现这个接口
,我们使用Servlet的时候，通常都是这样写的:

public class MyServlet extends HttpServlet {}

然后覆写里面的doGet()和doPost()方法。
那么我们来看一下HttpServlet这个类。HttpServlet其实是一个抽象的类，它继承自GenericServlet，而GenericServlet实现了,ServletConfig和Servlet接口。
看一下Servlet接口的层次图。


GenericServlet实现了Servlet里一些通用的方法。但是与任何的协议无关。而HttpServlet实现了Http协议上的实现。
其实呢，当我们调用Servlet的doGet()和doPost()方式，首先调用的是service()方法。

 public void service(ServletRequest req, ServletResponse res)        throws ServletException, IOException {        HttpServletRequest  request;        HttpServletResponse response;                try {            request = (HttpServletRequest) req;            response = (HttpServletResponse) res;        } catch (ClassCastException e) {            throw new ServletException("non-HTTP request or response");        }        service(request, response);    }

HttpServletRequest和HttpServletResponse分别继承了ServletRequest和ServletResponse，这里做了一次转型。然后调用的service(HttpServletRequest req, HttpServletResponse resp);

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) {                // servlet doesn't support if-modified-since, no reason                // to go through further expensive logic                doGet(req, resp);            } else {                long ifModifiedSince = req.getDateHeader(HEADER_IFMODSINCE);                if (ifModifiedSince < (lastModified / 1000 * 1000)) {                    // If the servlet mod time is later, call doGet()                    // Round down to the nearest second for a proper compare                    // A ifModifiedSince of -1 will always be less                    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 {            //            // Note that this means NO servlet supports whatever            // method was requested, anywhere on this server.            //            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);        }    }

那咱们看一下doGet()方法里，做了些什么

protected void doGet(HttpServletRequest req, HttpServletResponse resp)        throws ServletException, IOException    {        String protocol = req.getProtocol();        String msg = lStrings.getString("http.method_get_not_supported");        if (protocol.endsWith("1.1")) {            resp.sendError(HttpServletResponse.SC_METHOD_NOT_ALLOWED, msg);        } else {            resp.sendError(HttpServletResponse.SC_BAD_REQUEST, msg);        }    }

这里只是判断了请求的URL是否被HTTP1.1协议支持。doPost()方法和这里一样。
我们很少能出现这里的405,400的请求错误，所以这个方法没有什么用。