转发的原理及代码演示

一、转发的概念

一个Web组件（Servlet/JSP）将未完成的处理通过容器转交给另外一个Web组件继续处理，转发的各个组件会共享Request和Response对象。

二、代码演示

1、编写转发界面

创建空工程，在工程中创建javaEE模块
配置中设置tomcat的部署

编写register界面

<!DOCTYPE html>
<html lang="en">
<head><meta charset="UTF-8"><title>转发测试</title>
</head>
<body><form action="forwardServlet" method="post"><input type="submit" value="转发"></form>
</body>
</html>

2、编写Servlet 转发

编写servlet

ForwardServlet

package com.example.forward_demo03;import javax.servlet.*;
import javax.servlet.http.*;
import javax.servlet.annotation.*;
import java.io.IOException;@WebServlet(name = "ForwardServlet", urlPatterns = "/forwardServlet")
public class ForwardServlet extends HttpServlet {@Overrideprotected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {this.doPost(request, response);}@Overrideprotected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {System.out.println("接收到了转发请求!");//获取另一个web组件，让它进行转发RequestDispatcher requestDispatcher = request.getRequestDispatcher("/targetServlet");requestDispatcher.forward(request, response);}
}

这里看到，需要再编写一个 targetServlet，处理转发请求。它会执行真正的转发请求。

targetServlet

package com.example.forward_demo03;import javax.servlet.*;
import javax.servlet.http.*;
import javax.servlet.annotation.*;
import java.io.IOException;
import java.io.PrintWriter;@WebServlet(name = "TargetServlet", value = "/targetServlet")
public class TargetServlet extends HttpServlet {@Overrideprotected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {this.doPost(request, response);}@Overrideprotected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {System.out.println("接收到了传递过来的转发请求！");response.setContentType("text/html;charset=utf-8");PrintWriter writer = response.getWriter();writer.write("<h1>" + "转发成功！" + "</h1>");}
}

部署测试：
- 运行tomcat
- 访问转发界面，发送转发请求
- 转发请求发送，浏览器界面显示
- 控制台输出显示
- F12查看调试信息
  
  可以看到只进行一次请求。

3、转发的其他功能

绑定数据到Request对象

修改ForwardServlet

package com.example.forward_demo03;import javax.servlet.*;
import javax.servlet.http.*;
import javax.servlet.annotation.*;
import java.io.IOException;@WebServlet(name = "ForwardServlet", urlPatterns = "/forwardServlet")
public class ForwardServlet extends HttpServlet {@Overrideprotected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {this.doPost(request, response);}@Overrideprotected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {System.out.println("接收到了转发请求!");request.setAttribute("key1", "value1");//获取另一个web组件，让它进行转发RequestDispatcher requestDispatcher = request.getRequestDispatcher("/targetServlet");requestDispatcher.forward(request, response);}
}

修改TargetServlet

package com.example.forward_demo03;import javax.servlet.*;
import javax.servlet.http.*;
import javax.servlet.annotation.*;
import java.io.IOException;
import java.io.PrintWriter;@WebServlet(name = "TargetServlet", value = "/targetServlet")
public class TargetServlet extends HttpServlet {@Overrideprotected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {this.doPost(request, response);}@Overrideprotected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {System.out.println("接收到了传递过来的转发请求！");Object value = request.getAttribute("key1");System.out.println("获取到的转发来的key1对应的值：" + value);response.setContentType("text/html;charset=utf-8");PrintWriter writer = response.getWriter();writer.write("<h1>" + "转发成功！" + "</h1>");}
}

测试结果

从这也可以看出，转发的request用的是同一个。

三、转发和重定向的比较

	转发	重定向
转发/重定向之后浏览器地址栏的URL会不会发生改变	不会	会
转发/重定向过程中是否共享Request对象	是	否
转发/重定向的URL可不可以是其它项目工程	不可以	可以

在这里插入图片描述