servlet 轉發、包含（七）

1.什麼是servlet轉發？

　　servlet將用戶的請求通過一組servlet來處理，每個servlet都有自己特定的職責，當該servlet完成自己的工作後將請求轉發到下一個servlet來處理，這種通過不同servelet相互轉發請求來完成對某個請求的處理過程就叫做servlet轉發。

2.什麼是servlet包含？

　　servlet包含就是源servlet將其他web組件（servlet，html,jsp）,生成的結果包含到自己的結果中。

3.為什麼需要servlet轉發和包含？

　　servlet對象是由Servlet容器創建，Servlet對象的Service（）方法是由servlet容器調用，servlet之間是不允許相互調用（servlet API 2.1以前版本可以）。但是我們又需要通過多個servlet組件協調完成任務，同時方便代碼維護管理，提高開發效率。這樣就可以需要servlet轉發和包含的技術支持。

4、轉發和包含的相同點

　　（1）都是對同一個請求對象處理，源組件和目標組件共用同一個ServletRequest對象和ServletResponse對象。

　　（2）目標組件都可以是servlet，jsp，html。

　　（3）都依賴javax,servlet.RequestDispatcher 介面。

5.RequestDIspatcher 對象

　　該介面表示請求分發器

　　（1）獲取RequestDispatcher對象方法

　　　　調用ServletContext的getRequestDispather(String path)方法獲得。

　　　　調用ServletRequest的getRequestDispather(String path)方法獲得。

　　（2）兩種方法的區別

　　　　ServletContext調用方法的參數必須為絕對路徑，而ServletRequest調用方法參數可以是絕對路徑也可以是相對路徑。

6.請求轉發案例

CheckServlet 類用於處理用戶請求參數。

package com.learn;

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

/**
 * Created by Administrator on 2017/09/26.
 */
public class CheckServlet extends HttpServlet{

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

        String userName = req.getParameter("name");
        String message = "";

        if(userName == null){
            message = "please input your name";
        } else {
            message = userName;
        }
        req.setAttribute("mess",message);
        RequestDispatcher requestDispatcher = getServletContext().getRequestDispatcher("/out");

        PrintWriter out = resp.getWriter();
        out.print("before forward ");
        System.out.println("before forward");

        requestDispatcher.forward(req,resp);

        out.print("after forward");
        System.out.println("after forward");

        out.close();
    }


}

OutServlet  用於輸出處理結果

package com.learn;

import org.json.HTTP;

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

/**
 * Created by Administrator on 2017/09/26.
 */
public class OutServlet extends HttpServlet {

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

        String message = (String) req.getAttribute("mess");

        PrintWriter printWriter = resp.getWriter();
        printWriter.print(message);
        printWriter.close();

    }
}

註意：   1.在轉發之前不要提交響應結果。

　　　　2.轉發過程中，源組件的響應結果不會返回，只會返回目標組件的響應結果。

web.xml配置

  <servlet>
        <servlet-name>check</servlet-name>
        <servlet-class>com.learn.CheckServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>check</servlet-name>
        <url-pattern>/check</url-pattern>
    </servlet-mapping>
    
    <servlet>
        <servlet-name>out</servlet-name>
        <servlet-class>com.learn.OutServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>out</servlet-name>
        <url-pattern>/out</url-pattern>
    </servlet-mapping>

7.包含（include）案例　

MainServlet  用於包含其他組件，並返迴響應結果

package com.learn;

import javax.servlet.RequestDispatcher;
import javax.servlet.Servlet;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.io.PrintWriter;

/**
 * Created by Administrator on 2017/09/26.
 */
public class MainServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        resp.setContentType("text/html");

        PrintWriter out = resp.getWriter();
        out.println("<html><head><title>mainServlet</title></head>");
        out.println("<body>");

        ServletContext servletContext = getServletContext();

        RequestDispatcher headDisPatcher = servletContext.getRequestDispatcher("/header.html");
        RequestDispatcher greetDisPatcher = servletContext.getRequestDispatcher("/greet");
        RequestDispatcher footeDisPatcher = servletContext.getRequestDispatcher("/footer.html");

        headDisPatcher.include(req,resp);
        greetDisPatcher.include(req,resp);
        footeDisPatcher.include(req,resp);

        out.print("</body></html>");
        out.close();


    }
}

header.html

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>header</title>
</head>
<body>
<font size="6">Welcome to ab</font>
</body>
</html>

GreetServlet 

package com.learn;

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

/**
 * Created by Administrator on 2017/09/26.
 */
public class GreetServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

        PrintWriter out = resp.getWriter();

        out.print("hi! "+req.getParameter("name")+", <p>");
        out.close();
    }
}

footer.html

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>footer</title>
</head>
<body>
<span>Thanks for stopping</span>
</body>
</html>

web.xm配置

 <servlet>
        <servlet-name>main</servlet-name>
        <servlet-class>com.learn.MainServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>main</servlet-name>
        <url-pattern>/main</url-pattern>
    </servlet-mapping>

    <servlet>
        <servlet-name>greet</servlet-name>
        <servlet-class>com.learn.GreetServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>greet</servlet-name>
        <url-pattern>/greet</url-pattern>
    </servlet-mapping>