Servlet-使用RequestDispathcher调派请求

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RequestDispathcher

使用HttpServletRequest的getRequestDispatcher(“Servlet-URL”) 获得对象

RequestDispatcher dispatch = req.getRequestDispatcher(“/Test1”);

include()

将另一个servlet包括进进目前的操作流程中。

以下代码会输出

Dispatch

Test1

Done

/Dispatch
    protected void doGet(HttpServletRequest req, HttpServletResponse resp)
            throws ServletException, IOException {

        PrintWriter output = resp.getWriter();
        output.println("Dispatch<br/>");
        RequestDispatcher dispatch = req.getRequestDispatcher("/Test1");
        dispatch.include(req, resp);;

        output.println("Done<br/>");
    }           

/Test1
    protected void doGet(HttpServletRequest req, HttpServletResponse resp)
            throws ServletException, IOException {
            PrintWriter output = resp.getWriter();
            output.println("Test1<br/>");
    }           

forward()

将请求转发给另一个servlet，只是服务器端的转发，对于客户端并无任何影响

PS:调用forWard()之前不能有任何相应确认，如果设置了相应但还未确认(例如println()后还没有flush())，则所有相应会被忽略.如果已经有相应确认，会报出java.lang.IllegalStateException异常

以下代码会转向Test1，而忽略之forward之前的println输出

PrintWriter output = resp.getWriter();
        output.println("Dispatch<br/>");
        RequestDispatcher dispatch = req.getRequestDispatcher("/Test1");
        dispatch.forward(req, resp);;

        output.println("Done<br/>");           

以下代码会直接报错，因为已经有相应确认

PrintWriter output = resp.getWriter();
        output.println("Dispatch<br/>");
        output.flush(); 
        RequestDispatcher dispatch = req.getRequestDispatcher("/Test1");
        dispatch.forward(req, resp);;

        output.println("Done<br/>");           

请求范围属性

当调派请求的过程中有需要共享的属性，可以使用HttpServletRequest的方法

**setAttribute() 指定名称与对象**

**getAttribute() 指定名称获取对象**

**getAttributeNames() 获取所有属性名称**

**removeAttribute() 移除属性**

以下代码的Test1通过getAttribute(“name”)获取到name的集合，需要通过强制转换后才能使用。

/Dispatch
    protected void doGet(HttpServletRequest req, HttpServletResponse resp)
            throws ServletException, IOException {
        PrintWriter output = resp.getWriter();
        List<String> list = new  ArrayList<>();
        list.add("Tom");
        list.add("jerry");

        req.setAttribute("name", list);     // 传入字符串和对象

        RequestDispatcher dispatch = req.getRequestDispatcher("/Test1");
        dispatch.forward(req, resp);

        output.println("Done<br/>");
    }           

/Test1
    protected void doGet(HttpServletRequest req, HttpServletResponse resp)
            throws ServletException, IOException {
            PrintWriter output = resp.getWriter();
            output.println("Test1<br/>");
            List name = (List)req.getAttribute("name");
            output.println(name.get(1)+"</br/>");

    }           

sendRedirect()

要求浏览器重新请求一个URL，客户端的地址栏会改变

protected void doGet(HttpServletRequest req, HttpServletResponse resp)
            throws ServletException, IOException {
        String url = req.getParameter("url");
        resp.sendRedirect(url);
    }           

在跳转之前不可以有相应被确认,否则会产生java.lang.IllegalStateException 异常

protected void doGet(HttpServletRequest req, HttpServletResponse resp)
            throws ServletException, IOException {
        PrintWriter output = resp.getWriter();
        output.println("2333");
        output.flush();
        String url = req.getParameter("url");
        resp.sendRedirect(url);
    }           

跳转时可以附带参数,方式类比于GET方式的URL

String name = req.getParameter("name");
        PrintWriter output = resp.getWriter();
        resp.sendRedirect("http://localhost:8080/Test/Redirect_Para?name="+name);           

PS:

只能用转向方式来访问WEB-INF目录下的JSP,不用采用重定向的方式请求该目录里面的任何资源

同一个请求周期中，HttpServletResponse的getWriter()与getOutputStream()只能择一使用，否则会丢出IllegalStateException