Servlet

摘要: Servlet内部方法解析

Servlet的方法剖析:

1.service()方法里面做了什么?

2.doGet()与doPost()做了什么?应该怎么写?

回答

1.service()方法里面做了什么?





如果你的service方法中没有写super.service(req, resp); 那么后果是doget()和dopost()方法永远不会调用.这是为什么呢?

我们可以查看一下HttpServlet的源代码:(Ps:已经有删除 , 补录中有完整的代码....)

------------------------Service()--------------------------------------------------

protected void service(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException {

String method = req.getMethod();

if (method.equals(METHOD_GET)) {//如果" 请求 "是get方法 , 那就调用doget , 否则看看是不是post
long lastModified = getLastModified(req);
if (lastModified == -1) {
doGet(req, resp);//调用
}else if (method.equals(METHOD_POST)) {
doPost(req, resp);

.......后面是诺干个if-else语句

从这个源代码中就可以看出Service()主要是干什么了吧-----他就是用来区分是get方法还是post方法的

2.doGet里面做了什么?

下卖个关子 , 请看这句代码对不对,会不会出现异常.....





答案是:一定会出现异常,原因就是你写了一个super.doGet();

这个会调用父类的doGet()方法 , 这样做的结果就是: 会出现405(请求错误)或者是illegalXXXException

Why? 为什么doget()不可以调用父类的方法?

原因就是:





这段源代码就是在告诉我们:无论如何,他都会调用requset.sendError方法的 ,

如果你还想在super.doGet()下面获取参数的话那就是错误的 ,因为sendError之后上下文已经改变了 , 你就没法获取参数了;

例如:

这个:








然后调用之后:





-------补录------------------------------------------------
/**
* Receives standard HTTP requests from the public
* <code>service</code> method and dispatches
* them to the <code>do</code><i>Method</i> methods defined in
* this class. This method is an HTTP-specific version of the
* {@link javax.servlet.Servlet#service} method. There's no
* need to override this method.
*
* @param req the {@link HttpServletRequest} object that
* contains the request the client made of
* the servlet
*
* @param resp the {@link HttpServletResponse} object that
* contains the response the servlet returns
* to the client
*
* @exception IOException if an input or output error occurs
* while the servlet is handling the
* HTTP request
*
* @exception ServletException if the HTTP request
* cannot be handled
*
* @see javax.servlet.Servlet#service
*/
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);
}
}