从URL检索上次修改的内容

/*

 * Copyright 2014 astamuse company,Ltd.

 * 

 * Licensed under the Apache License, Version 2.0 (the "License");

 * you may not use this file except in compliance with the License.

 * You may obtain a copy of the License at

 * 

 *     http://www.apache.org/licenses/LICENSE-2.0

 * 

 * Unless required by applicable law or agreed to in writing, software

 * distributed under the License is distributed on an "AS IS" BASIS,

 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.

 * See the License for the specific language governing permissions and

 * limitations under the License.

 * 

 *//*from nowjava - 时  代  Java*/

import java.io.FileNotFoundException;

import java.io.IOException;

import java.io.InputStream;

import java.net.MalformedURLException;

import java.net.URL;

import java.net.URLConnection;

import javax.servlet.ServletContext;


public class Main{

    private final static long retrieveLastModifiedFromURL(URL url) {

        try {

            URLConnection con = url.openConnection();

            try {

                return con.getLastModified();

            } catch (Exception ex) {