servlet中service() 和doGet() 、doPost() 学习笔记

Sevlet接口定义如下：



与Sevlet接口相关的结构图：



service() 方法是 Servlet 的核心。每当一个客户请求一个HttpServlet 对象，该对象的service() 方法就要被调用(由web容器调用)，在GenericServlet中定义了service抽象方法：

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

在HttpSevlet中覆写了GenericServlet中的service方法，源码如下：

/**
* This method is an HTTP-specific version of the method. There's no need to override this method.
* @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);//调用doGet(rq,rp)
} else {
long ifModifiedSince = req.getDateHeader(HEADER_IFMODSINCE);
if (ifModifiedSince < lastModified) {
// 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);
}
}

从上可知service是用来转向的，所以我们一般写一个servlet，只需要重写doGet或者doPost就可以了，例如，当一个客户通过HTML 表单发出一个HTTP POST请求时，doPost()方法被调用，发出一个HTTP GET请求时doGet()方法被调用。当然还有doTrace()、doDelete()、doHead()类似。