ServletRequest.getRequestDispatcher和ServletContext.getRequestDispatcher的区别。

本节介绍以下几种跳转方法。

1.用RequestDispatcher跳转：

马克-to-win: javax.servlet.RequestDispatcher是Sun公司编写的一个Interface，顾名思义，它可以把用户请求"a资源"的request派遣（dispatch）到"b资源"上，从而实现跳转。（本来人家要访问'a资源"，你的程序却让人家自动的跳转到"b资源"上。） 注意：RequestDispatcher有两种形式：这两种形式都能达到跳转的目的。一种是ServletRequest.getRequestDispatcher(String url)。一种是ServletContext.getRequestDispatcher(String url)。但正常来讲，二者都只能访问同一web应用的资源。见下面理论论述和实验。

马克-to-win：注意以下两者的而区别：ServletContext.getRequestDispatcher(String
url)中的url只能使用绝对路径； 而ServletRequest.getRequestDispatcher(String
url)中的url可以使用相对路径（两个资源在同一个目录下，所以省去路径），也可以绝对路径。假如你写成相对路径时，比如
request.getRequestDispatcher("add").forward(request, response);
系统会在同一目录下找add，见后面的例子。

但是假如你写成request.getRequestDispatcher(/WebModule1/jsp1.jsp).forward
(request, response); 系统也会傻傻的变成：request.getRequestDispatcher(http://localhost:
8080/WebModule1/WebModule1/jsp1.jsp).forward(request, response);
当然你更不要写成：request.getRequestDispatcher(http://localhost:
8083/WebModule1/jsp1.jsp).forward(request, response).道理是一样的。<

下面这段话来自于Sun公司的官方文档，大家可以参考一下。对于初学者来讲，没必要完全搞懂。

from documentation: request.getRequestDispatcher: The pathname specified may be relative, although it cannot extend outside the current servlet context. If the path begins with a "/" it is interpreted as relative to the current context root.（马克-to-win：上面的翻译就是相对时不能访问到当前servlet的外面，用/就代表当前应用的根目录，见我下面的例子）


底下我们就通过一个实验来说明这二者的区别在。本个实验中，web.xml主要内容是这样的：

    <servlet>
        <servlet-name>ServletHello1</servlet-name>
        <servlet-class>com.ServletHello1</servlet-class>
    </servlet>
    <servlet>
        <servlet-name>AddCookie</servlet-name>
        <servlet-class>com.AddCookie</servlet-class>
    </servlet>
    <servlet>
        <description>
        </description>
        <servlet-name>DeleteCookie</servlet-name>
        <servlet-class>com.DeleteCookie</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>ServletHello1</servlet-name>
        <url-pattern>/Hello/MarkToWinServletHello1</url-pattern>
    </servlet-mapping>
    <servlet-mapping>
        <servlet-name>AddCookie</servlet-name>
        <url-pattern>/Cookie/AddCookie</url-pattern>
    </servlet-mapping>
    <servlet-mapping>
        <servlet-name>DeleteCookie</servlet-name>
        <url-pattern>/Cookie/DeleteCookie</url-pattern>
    </servlet-mapping>


AddCookie.java:


package com;
import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.http.Cookie;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
public class AddCookie extends HttpServlet {
    protected void doGet(HttpServletRequest request,
            HttpServletResponse response) throws ServletException, IOException {
            String rav=(String)request.getAttribute("requestkey");
            String scs=(String)this.getServletContext().getAttribute("ServletContextKey");
            System.out.println(rav+" "+scs);
    }
}




package com;
import java.io.IOException;
import java.io.InputStream;
import javax.servlet.ServletException;
import javax.servlet.ServletOutputStream;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
public class ServletHello1 extends HttpServlet {
    public void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        String rav=(String)request.getAttribute("requestkey");
        String scs=(String)this.getServletContext().getAttribute("ServletContextKey");
        System.out.println(rav+" "+scs);
    }
}


1.html是这样的:

test


例：3.1.1

package com;
import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.http.Cookie;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class DeleteCookie extends HttpServlet {
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        request.setAttribute("requestkey","requstvalue1");
        getServletContext().setAttribute("ServletContextKey","ServletContextValue");
/*下面任意一种方法都能正确执行。*/        
        request.getRequestDispatcher("/Cookie/AddCookie").forward(request, response);
 //       request.getRequestDispatcher("AddCookie").forward(request, response);
 //       request.getRequestDispatcher("/WEB-INF/1.html").forward(request, response);
 //       request.getRequestDispatcher("/Hello/MarkToWinServletHello1").forward(request, response);
 //       getServletContext().getRequestDispatcher("/Hello/MarkToWinServletHello1").forward(request, response);
 //       getServletContext().getRequestDispatcher("/WEB-INF/1.html").forward(request, response);

/*下面的写法错误，找不到AddCookie*/
//      request.getRequestDispatcher("/AddCookie").forward(request, response);
// getServletContext().getRequestDispatcher("http://localhost:8080/ServletHello/Hello/MarkToWinServletHello1").forward(request, response);
    }
}

更多内容请见原文，原文转载自：https://blog.csdn.net/qq_44594371/article/details/103163543