Servlet的生存时间是由init,service,destory方法构成，这里分析一下service这个方法
Servlet接口中定义了一个service()方法，而我们一般是使用HttpServlet，HttpServlet中对它进行了实现，将ServletRequest和ServletResponse转变成为HttpServletRequest和HttpServletResponse具体实现如下:
@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);
}
}
HttpServlet
而对于service方法，一般来说这个方法是不需要重写的，因为在HttpServlet中已经有了很好的实现，它会根据请求的方法名（GET，POST），调用doGet，doPos以及其他的doXXX方法，也就是说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。如果重写了service方法，那么servlet容器就会把请求交给这个方法来处理，倘若你重写的service方法没有调用doXXX，即使你在Servlet中又重写了其他doGet doPost等也是不会被调用的 因为Servlet的service被自动调用（就像init destory一样）。