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1.HttpServlet介绍（具体方法可参考：java WEB学习笔记08：HttpServletRequest & ServletRequest） HttpServletResponse
1）public abstract class HttpServlet extends GenericServlet implements java.io.Serializable 是继承GenericServlet 的一个针对HTTP 协议定制的Servlet。
2）在service()方法中直接把ServletRequest 和 ServletResponse 转为 HttpServletRequest 和 HttpServletResponse，并且调用了重载的service(HttpServletRequest req, HttpServletResponse resp)
获取请求方式：request.getMethod().根据请求方式又创建了doXxx（）方法（xxx 为具体的请求方式，比如doGet, doPost）
@Override
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); 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;
try {
ifModifiedSince = req.getDateHeader(HEADER_IFMODSINCE);
} catch (IllegalArgumentException iae) {
// Invalid date header - proceed as if none was set
ifModifiedSince = -1;
}
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);
}
}
3.一般在实际开发中，直接继承HttpServlet，并根据请求方法doXxx() 方法接口
优点：直接针对性的覆盖doXxx() 方法；直接使用 HttpServletRequest 和 HttpServletResponse
4.关于Servlet小结
1）了解GenericServlet 原理和实现
2）掌握 HttpServlet 实现的原理 及 源码
