SERVLET

ServletContext

　　web容器在啟動的時候，它會為每個web程式都建立一個對應的ServletContext物件，它代表了當前的web應用。

接下來，讓我們來看看ServletContext物件有什麼用處吧。

1. 資料傳輸

　　我們可以在儲存一個類中的servlet的同時，在另一個類獲取該servlet資料。

程式碼展示：

類1：

package com.charels.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;

public class HelloServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
　　　　　//this.getServletContext() Servlet上下文
        ServletContext context 
 
= this.getServletContext();

        String name = "小明";
        context.setAttribute("name",name);

    }
}

類2.

package com.charels.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;

public class GetServlet extends HttpServlet {
        @Override
        protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
            ServletContext context = this.getServletContext();

            String name = (String)context.getAttribute("name");
            resp.setContentType("text/html");
            resp.setCharacterEncoding("GBK");
            resp.getWriter().print("名字"+name);
        }

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

在web.xml註冊，兩個servlet。

  <!--  註冊servlet-->
  <servlet>
    <servlet-name>hello</servlet-name>
    <servlet-class>com.charels.servlet.HelloServlet</servlet-class>
  </servlet>

  <!--  servlet的請求路徑-->
  <servlet-mapping>
    <servlet-name>hello</servlet-name>
    <url-pattern>/hello</url-pattern>
  </servlet-mapping>

  <servlet>
    <servlet-name>getc</servlet-name>
    <servlet-class>com.charels.servlet.GetServlet</servlet-class>
  </servlet>
  <servlet-mapping>
    <servlet-name>getc</servlet-name>
    <url-pattern>/getc</url-pattern>
  </servlet-mapping>

接下來，讓我們康康執行Tomcat後的展示結果：

在這裡要注意，根據程式碼的意思，要獲取上下文資料，則需要先訪問第一個類的servlet，再訪問第二個才能獲取資料。

第一個類的網頁：

第二個類的網頁：

2. 獲取初始化引數

程式碼展示：

首先在web.xml中配置初始化資料：

    <!--配置一些web應用初始化引數-->
    <context-param>
        <param-name>url</param-name>
        <param-value>jdbc:mysql://localhost:3306/mybatis</param-value>
    </context-param>

接下來編寫獲取初始化資料的程式碼：

package com.charels.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;

public class GetUrl extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext context = this.getServletContext();
        String url = context.getInitParameter("url");

        resp.getWriter().print(url);
    }

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

編寫完畢後，註冊servlet，然後執行Tomcat：

3. 請求與轉發

　　我們可以通過一個servlet的類來實現頁面的轉發。

程式碼展示：

package com.charels.servlet;

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

public class Dri extends GetUrl{
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        System.out.println("進入Dri");

        ServletContext context = this.getServletContext();
        context.getRequestDispatcher("/g1").forward(req,resp);
    }

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

結果展示：

　　控制檯：

　　頁面展示：

4. 讀取資原始檔

　　首先，在properties的包中建立一個db.properties並編寫資料

　　在servlet類中編寫讀取資源的程式碼：

package com.charels.servlet;

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.InputStream;
import java.util.Properties;

public class GetPro extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        resp.setCharacterEncoding("gbk");

        InputStream is = this.getServletContext().getResourceAsStream("/WEB-INF/classes/db.properties");

        Properties pro = new Properties();
        pro.load(is);
        String username = pro.getProperty("username");
        String passsword= pro.getProperty("password");

        resp.getWriter().print(username + '：' + passsword);
    }

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

編寫完畢後，執行Tomcat，展示結果：