關於Servlet

Servlet的生命週期方法

通過Log輸出的方式，查看Servlet的生命週期方法,下面模擬發起的是get請求

public class ServletTest01 extends HttpServlet {

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp)throws ServletException, IOException {
        resp.getWriter().write("this is a pig...........");
        System.out.println("---doGet------wyk--->");
    }

    @Override
    public void init() throws ServletException {
        System.out.println("----init--wyk>");
        super.init();
    }

    @Override
    protected void service(HttpServletRequest req, HttpServletResponse resp)throws ServletException, IOException {
        resp.getWriter().write("this is a test demo");
        System.out.println("----service--wyk>");
        super.service(req, resp);
    }

    @Override
    public void destroy() {
        System.out.println("----destroy--wyk>");
        super.destroy();
    }
}

輸出結果

----init--wyk>
----service--wyk>
---doGet------wyk--->
----destroy--wyk>

可以得知執行順序： init—–>service—–>doGet——>destroy

生命週期方法的職責：

方法	作用
init	Servlet被創建的時候調用
service	每次請求過來會被調用,將請求進行分發，將請求傳遞給doGet()或doPost()或者其他請求方法
destroy	Servlet被移除出服務器的時候調用

HttpRequest

1.獲取請求信息

public class MyHttpServlet01 extends HttpServlet {

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

            String method = req.getMethod();            //請求方式
            String uri = req.getRequestURI();           //請求資源
            String protocol = req.getProtocol();        //協議版本

            System.out.println("請求行：--請求方式---->" + method + "--請求資源---->" +uri + "--協議版本--->" +protocol);

            //獲取請求頭信息
            Enumeration<String> requestHeaders = req.getHeaderNames();
            for(;requestHeaders.hasMoreElements();){
                String name = requestHeaders.nextElement();
                String header = req.getHeader(name); 
                System.out.println("------請求頭信息------>" + name +"-----value--"+header); 
            }

    //      打印的信息：
    //      
    //      --請求方式---->GET--請求資源---->/HelloWorld/MyHttpServlet--協議版本--->HTTP/1.1
    //      ------請求頭信息------>host-----value--localhost:8080
    //      ------請求頭信息------>user-agent-----value--Mozilla/5.0 (Windows NT 6.1; WOW64; rv:50.0) Gecko/20100101 Firefox/50.0
    //      ------請求頭信息------>accept-----value--text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
    //      ------請求行信息------>accept-language-----value--zh-CN,zh;q=0.8,en-US;q=0.5,en;q=0.3
    //      ------請求頭信息------>accept-encoding-----value--gzip, deflate
    //      ------請求頭信息------>connection-----value--keep-alive
    //      ------請求頭信息------>upgrade-insecure-requests-----value--1

    }
}

2.通過請求頭獲取用戶目前使用的瀏覽器類型

public class MyHttpServlet02 extends HttpServlet{
        /**
         * 注意：contains("")  區分字母大小寫的
         */
        @Override
        protected void doGet(HttpServletRequest req, HttpServletResponse resp)
                throws ServletException, IOException {
            String user_agent = req.getHeader("user-agent");
            if(user_agent.contains("chrome")){
                 System.out.println("谷歌");
            }
            if(user_agent.contains("ie")){
                System.out.println("IE瀏覽器");
            }
            if(user_agent.contains("Firefox")){
                System.out.println("火狐");
            }   
        }

        @Override
        protected void doPost(HttpServletRequest req, HttpServletResponse resp)
                throws ServletException, IOException {  
            doGet(req,resp);
        }
}

3.防盜鏈功能

public class MyHttpServlet03 extends HttpServlet{

        @Override
        protected void doGet(HttpServletRequest req, HttpServletResponse resp)
                throws ServletException, IOException {

            //setContentType():修改response(默認碼錶爲iso8859-1)默認的碼錶，
            //但如果輸出中文，瀏覽器可能還是亂碼，
            //因爲並沒告訴瀏覽器用哪種碼錶解碼，所以瀏覽器還是使用了默認的碼錶，
            //下面的api可以解決這一問題
            //response.setCharacterEncoding("utf-8");   

            resp.setContentType("text/html;charset=UTF-8");         
            PrintWriter writer = resp.getWriter();
            String header = req.getHeader("referer");
            if(header=="" || header.contains("ad.html")){
                writer.write("下載中...");
            }else{
                writer.write("非官方鏈接下載，無效!!!");
            }
        }
        @Override
        protected void doPost(HttpServletRequest req, HttpServletResponse resp)
                throws ServletException, IOException {
            doGet(req, resp);
        }
}

4.使用緩存

    /**
     * 如果Servlet的代碼需要使用緩存，那麼一定要重寫getLastModified方法
     * 進到HttpServlet的源碼裏看Service方法，可以看到doGet和doPost就是在service方法進行判斷調用的，
     * 在裏面可以看到當請求方式爲get的時候，會去判別是否使用瀏覽器的緩存，
     * 可以看到是否使用緩存的關鍵點是getLastModified這個方法，而這個getLastModified方法的默認返回值爲-1，
     * 所以如果我們需要用到緩存，那麼就需要重寫getLastModified這個方法來獲取服務端文件的最近修改時間去返回，
     * 這樣當get請求的時候，纔可能用到瀏覽器的緩存
     * 
     * 如果請求的是一個靜態頁面，一般情況下瀏覽器會使用緩存的；
     * 動態web資源除了get請求以外，其他都不使用緩存的，而Servlet的get請求默認也是不使用緩存的
     */
public class MyHttpServlet04 extends HttpServlet {

        public void doGet(HttpServletRequest request, HttpServletResponse response)
                throws ServletException, IOException {

            try{
            FileInputStream inputStream = new FileInputStream("D:\\ddd.txt");
            OutputStream outputStream = response.getOutputStream();

            byte[] buf = new byte[1024];
            int len = 0;
            while((len=inputStream.read(buf))!=-1){
                outputStream.write(buf,0,len);
            }
            inputStream.close();
            }catch(Exception ex){
                //
            }
            System.out.println("========請求服務端資源成功========="); 
        }
        @Override
        protected long getLastModified(HttpServletRequest req) {
            File file = new File("D:\\ddd.txt");
            return file.lastModified();
        }

        public void doPost(HttpServletRequest request, HttpServletResponse response)
                throws ServletException, IOException {
            doGet(request,response);
        }
}

HttpResponse

1.通過響應頭告知瀏覽器要以下載的方式處理文件

public class DownLoadServlet extends HttpServlet {

        public void doGet(HttpServletRequest request, HttpServletResponse response)
                throws ServletException, IOException {

            File file = new File("d:\\yh.png");
            System.out.println("====Name===>" + file.getName());

            //可以從瀏覽器拿到的響應頭 看到 content-disposition: attachment;filename=yh.png(content設置：附件、文件名)
        //  response.setHeader("content-disposition","attachment");通知瀏覽器以文件下載的形式處理
            //文件下載的時候對文件名稱進行URL編碼，使用utf-8，各大廠商定的規則
            response.setHeader("content-disposition","attachment;filename="+URLEncoder.encode(file.getName(),"utf-8"));

            OutputStream outputStream = response.getOutputStream();
            FileInputStream inputStream = new FileInputStream(file);
            byte[] buf = new byte[1024];
            int len = 0;
            while((len=inputStream.read(buf))!=-1){
                outputStream.write(buf,0,len);
            }
            inputStream.close();
        }

        public void doPost(HttpServletRequest request, HttpServletResponse response)
                throws ServletException, IOException {
            doGet(request, response);
        }
}

2.將文件進行壓縮後輸出到瀏覽器

public class GzipServlet extends HttpServlet{

        @Override
        protected void doGet(HttpServletRequest req, HttpServletResponse resp)
                throws ServletException, IOException {
            //告訴瀏覽器我傳輸給你的文件是gzip壓縮的，先解壓再顯示
            resp.setHeader("content-encoding", "gzip");
            OutputStream outputStream = resp.getOutputStream();

            StringBuilder builder = new StringBuilder();
            for(int i=0;i<150;i++){
                builder.append("abcde" + i);
            }
            System.out.println("----No Gzip---->" + builder.toString().length());       //1090
            //壓縮之後，通過谷歌瀏覽器的請求頭信息查看大小是310

            GZIPOutputStream gzipOutputStream = new GZIPOutputStream(outputStream);
            gzipOutputStream.write(builder.toString().getBytes());

            gzipOutputStream.finish();
        }
        @Override
        protected void doPost(HttpServletRequest req, HttpServletResponse resp)
                throws ServletException, IOException {
            doGet(req, resp);
        }
}

3.設置響應頭，讓其重定向，頁面重新跳轉

public class LocationServlet extends HttpServlet {

        public void doGet(HttpServletRequest request, HttpServletResponse response)
                throws ServletException, IOException {

            /*response.setStatus(302);      //302是重定向；
            response.setHeader("location","/HelloWorld/ad.html");*/

            response.sendRedirect("/HelloWorld/ad.html");   //該代碼==上面的兩句
        }
        public void doPost(HttpServletRequest request, HttpServletResponse response)
                throws ServletException, IOException {
            doGet(request,response);
        }
}

4.通知瀏覽器不要產生緩存文件

    /**
     *  Expires: -1                 　　  --不緩存
     *  Cache-Control: no-cache         --不緩存
     *  Pragma: no-cache                --不緩存
     */
public class NoCacheServlet extends HttpServlet {

        public void doGet(HttpServletRequest request, HttpServletResponse response)
                throws ServletException, IOException {
            //不同瀏覽器所需要的頭不一樣, 凡是涉及到密碼的頁面信息都不要產生緩存文件， 
            //容易造成信息外泄。（所以三種都寫）
            response.setHeader("Expires", "-1");
            response.setHeader("Cache-Control","no-cache");
            response.setHeader("Pragma", "no-cache");

            //response.setCharacterEncoding("utf-8");
            response.setContentType("text/html;charset=utf-8");

            response.getWriter().write("當前的系統時間==" + System.currentTimeMillis());

        }
        public void doPost(HttpServletRequest request, HttpServletResponse response)
                throws ServletException, IOException {
            doGet(request, response);
        }
}

5.設置瀏覽器定時刷新或跳轉

public class RefreshServlet extends HttpServlet {

        public void doGet(HttpServletRequest request, HttpServletResponse response)
                throws ServletException, IOException {

            //response.setHeader("refresh", "1");       //告知瀏覽器，每秒刷新一次
            response.setHeader("refresh","3;url=/HelloWorld/index.jsp");//指定秒數，然後定時跳轉
        }
        public void doPost(HttpServletRequest request, HttpServletResponse response)
                throws ServletException, IOException {
            doGet(request, response);
        }
}

補充

get和post請求方式的區別

• 1.get的數據攜帶在Url後面，post的數據則在實體內容；

• 2.get攜帶的數據信息不可大於1kb，post則無限制；

• 3.get不能用於提交敏感數據，post可用於提交敏感數據；

• 4.get提交方式會產生緩存文件，post提交方式不會產生緩存文件