请求包含（Include）和请求转发（Forward）

定义

　　请求包含是指将另一个Servlet的请求处理流程包含进来。
　　请求转发是指将请求转发给别的Servlet处理。

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实现

　　实现请求包含和请求转发，可以使用HttpServletRequest的getRequestDispatcher()方法取得RequestDispatcher接口的实现对象实例，调用时传入请求、响应对象即可。
　　取得RequestDispatcher还有两个方式，通过ServletContext的getRequestDispatcher()或getNamedDispatcher()。

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demo

package com.test;

import javax.servlet.RequestDispatcher;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.io.PrintWriter;

@WebServlet({"/some.controller", "/world"})
public class Some extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        PrintWriter out = response.getWriter();
        out.println("Some do one...");
        RequestDispatcher dispatcher = request.getRequestDispatcher("other.view");
        // 请求包含
        dispatcher.include(request, response);
        // 请求转发
        // dispatcher.forward(request, response);
        out.println("Some do two...");
    }
}

package com.test;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.util.Arrays;

@WebServlet({"/other.view", "/view"})
public class Other extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {

        response.getWriter().println("Other do one...");
    }
}

请求转发 forward 的响应结果是：

　　Other do one...

请求包含 include 的响应结果是：

　　Some do one...
　　Other do one...
　　Some do two...

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区别

　　forward：目前的Servlet不能有任何响应确认，如果在目前的Servlet中通过响应对象设置了一些响应但未确认（响应缓冲区未满或未调用任何清除方法），则所有响应设置会被忽略，如果已经有响应确认且调用了forward()方法，则会抛出IllegalStateException。

　　include：被包含的Servlet中任何对请求标头的设置都会被忽略。但被包含的Servlet中可以使用getSession()方法取得HttpSession对象。

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相同点

1、在取得RequestDispatcher时，也可以包括查询字符串。
　　例如：req.getRequestDispatcher("other.view?data=123456").include(req, resp);
　　这样在被包含或转发的Servlet中就可以使用getParameter（"data"）取得请求参数值。
2．查询字符串的方式，仅适用于传递字符串值给另一个Servlet；如果有必须共享的“对象”，可以设置给请求对象成为属性。
　　HttpServletRequest上与请求范围属性有关的几个方法如下。
　　setAttribute()：指定名称与对象设置属性。
　　getAttribute()：指定名称取得属性。
　　getAttributeNames()：取得所有属性名称。
　　removeAttribute()：指定名称移除属性。

example:

package com.test;

import javax.servlet.RequestDispatcher;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.io.PrintWriter;

@WebServlet({"/some.controller", "/world"})
public class Some extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        request.setAttribute("setAttr", new int[]{1, 2, 3});

        PrintWriter out = response.getWriter();
        out.println("Some do one...");
        RequestDispatcher dispatcher = request.getRequestDispatcher("other.view?data=123456");
        // 请求包含
        // dispatcher.include(request, response);
        // 请求转发
        dispatcher.forward(request, response);
        out.println("Some do two...");
    }
}

package com.test;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.util.Arrays;

@WebServlet({"/other.view", "/view"})
public class Other extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {

        String data = request.getParameter("data");
        int[] getAttr = (int[])request.getAttribute("setAttr");

        response.getWriter().println("Other do one..." + " | "
                + "include or forward parameter: " + data + " | "    // 请求或转发时携带的查询参数
                + "setAttrbute object: " + Arrays.toString(getAttr));  // 共享对象
    }
}

请求转发 forward 的响应结果是：

　　Other do one... | include or forward parameter: 123456 | setAttrbute object: [1, 2, 3]

请求包含 include 的响应结果是：

　　Some do one...
　　Other do one... | include or forward parameter: 123456 | setAttrbute object: [1, 2, 3]
　　Some do two...

---

相似点

请 求 包 含	javax.servlet. include.request_uri	javax.servlet. include.context_path	javax.servlet. include.servlet_path	javax.servlet. include.path_info	javax.servlet. include.query_string
请 求 转 发	javax.servlet. forward.request_uri	javax.servlet. forward.context_path	javax.servlet. forward.servlet_path	javax.servlet. forward.path_info	javax.servlet. forward.query_string
表 示 上 一 个 Servlet 的	Request URI	Context path	Servlet path	Path info	取得RequestDispatcher时 给定的查询参数

如果被包含或转发的Servlet还包含或转发其他的Servlet，则这些属性名称的对应值也会被代换。

example:

package com.test;

import javax.servlet.RequestDispatcher;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.io.PrintWriter;

@WebServlet({"/some.controller", "/world"})
public class Some extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        PrintWriter out = response.getWriter();
        out.println("Some do one...");
        RequestDispatcher dispatcher = request.getRequestDispatcher("other.view?data=123456");
        // 请求包含
        dispatcher.include(request, response);
        // 请求转发
        // dispatcher.forward(request, response);
        out.println("Some do two...");
    }
}

package com.test;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

@WebServlet({"/other.view", "/view"})
public class Other extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {

        String request_uri, context_path, servlet_path, path_info, query_string;

        // 默认的请求包含时的参数
        /*request_uri = (String)request.getAttribute("javax.servlet.include.request_uri");
        context_path = (String)request.getAttribute("javax.servlet.include.context_path");
        servlet_path = (String)request.getAttribute("javax.servlet.include.servlet_path");
        path_info = (String)request.getAttribute("javax.servlet.include.path_info");
        query_string = (String)request.getAttribute("javax.servlet.include.query_string");*/
        // 默认的请求转发时的参数
        request_uri = (String)request.getAttribute("javax.servlet.forward.request_uri");
        context_path = (String)request.getAttribute("javax.servlet.forward.context_path");
        servlet_path = (String)request.getAttribute("javax.servlet.forward.servlet_path");
        path_info = (String)request.getAttribute("javax.servlet.forward.path_info");
        query_string = (String)request.getAttribute("javax.servlet.forward.query_string");


        response.getWriter().println("Other do one..." + " | "
                + "request_uri: " + request_uri + " | "
                + "context_path: " + context_path + " | "
                + "servlet_path: " + servlet_path + " | "
                + "path_info: " + path_info + " | "
                + "query_string: " + query_string
        );
    }
}

请求转发 forward 的响应结果是：

　　Other do one... | request_uri: /hello/some.controller | context_path: /hello | servlet_path: /some.controller | path_info: null | query_string: null

请求包含 include 的响应结果是：

　　Some do one...
　　Other do one... | request_uri: /hello/other.view | context_path: /hello | servlet_path: /other.view | path_info: null | query_string: data=123456
　　Some do two...