JavaWeb-ServletContext舉例解析

ServletContext

Web容器在啓動的時候，它會爲每一個Web程序都創建一個對應的ServletContext對象，它代表了當前的Web應用。

作用：

1.共享數據。

我在此Servlet中保存的數據，在其他Servlet中也可以使用。

存放數據文件：

package com.edwin.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;
/**
 * @author Edwin D
 * @date 2020.6.2 上午 7:59
 */
public class HelloServlet_2 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

//        this.getInitParameter();初始化參數
//        this.getServletContext();Servlet配置
//        this.getServletContext();Servlet上下文
        ServletContext i = this.getServletContext();

        String username = "Edwin";//數據
        i.setAttribute("Username",username);
//        將一個值存在了ServletContext中，名字爲：Username，值爲username(Edwin)。
    }

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

讀取數據文件：

package com.edwin.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;
/**
 * @author Edwin D
 * @date 2020.6.2 上午 8:56
 */
public class GetServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext i = this.getServletContext();
        String username = (String) i.getAttribute("Username");

        resp.setContentType("text/html");
        resp.setCharacterEncoding("UTF-8");
        resp.getWriter().println("姓名："+username);
    }

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

XML文件：

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee
                             http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd"
         version="4.0"
         metadata-complete="true">

  <servlet>
    <servlet-name>Hello</servlet-name>
    <servlet-class>com.edwin.servlet.HelloServlet_2</servlet-class>
  </servlet>
  <servlet-mapping>
    <servlet-name>Hello</servlet-name>
    <url-pattern>/Hello</url-pattern>
  </servlet-mapping>

  <servlet>
    <servlet-name>Get</servlet-name>
    <servlet-class>com.edwin.servlet.GetServlet</servlet-class>
  </servlet>
  <servlet-mapping>
    <servlet-name>Get</servlet-name>
    <url-pattern>/Get</url-pattern>
  </servlet-mapping>

</web-app>

測試結果：

注意訪問頁面的順序：點擊運行，先訪問Hello（此頁面爲空），但是必須要先訪問，確保：

String username = "Edwin";//數據

數據傳入ServletContext，纔可以被後面的GetServlet訪問到。

結果：
主界面：

直接訪問GetServlet：Null值

轉到Hello：空值

再訪問GetServlet：

2.獲取初始化參數

編寫Servlet：

package com.edwin.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;
/**
 * @author Edwin D
 * @date 2020.6.2 下午 10:21
 */
public class ParameterServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext i = this.getServletContext();
        String url = i.getInitParameter("url");
        resp.getWriter().println(url);
    }
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}

設置參數：

<!--配置一些Web應用的初始化參數-->
<context-param>
    <param-name>url</param-name>
    <param-value>jdbc:mysql://localhost:3306/Database</param-value>
</context-param>
<servlet>
    <servlet-name>Url</servlet-name>
    <servlet-class>com.edwin.servlet.ParameterServlet</servlet-class>
</servlet>
<servlet-mapping>
    <servlet-name>Url</servlet-name>
    <url-pattern>/Url</url-pattern>
</servlet-mapping>

輸出效果：

更改路徑：

3.請求轉發

原理圖：

Servlet代碼：

package com.edwin.servlet;

import javax.servlet.RequestDispatcher;
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;
/**
 * @author Edwin D
 * @date 2020.6.2 下午 10:33
 */
public class ForwardServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext i = this.getServletContext();
//        方法一：
//        RequestDispatcher requestDispatcher = i.getRequestDispatcher("/Url");//請求轉發的路徑
//        requestDispatcher.forward(req,resp);//調用forward請求轉發。

//        方法二：簡化版方法一
        i.getRequestDispatcher("/Url").forward(req,resp);
    }
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}

配置代碼：

<!--Forward-->
<servlet>
    <servlet-name>For</servlet-name>
    <servlet-class>com.edwin.servlet.ForwardServlet</servlet-class>
</servlet>
<servlet-mapping>
    <servlet-name>For</servlet-name>
    <url-pattern>/Forward</url-pattern>
</servlet-mapping>

效果：

區別於另外一種模式：

4.讀取資源文件

Properties：

• 在java目錄下新建一個Properties。

• 在resource目錄下新建一個Properties。

運行程序後，你會發現，兩個文件都被打包到同一個路徑下：classes。我們俗稱爲這個路徑爲：classpath

思路：創建一個文件流：

Properties:

username = root
password = 1234

Servlet:

package com.edwin.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;
/**
 * @author Edwin D
 * @date 2020.6.3 下午 12:46
 */
public class CheckPropertiesServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        InputStream i = this.getServletContext().getResourceAsStream("/WEB-INF/classes/com/edwin/servlet/ac.properties");

        Properties prop = new Properties();
        prop.load(i);
        String Un = prop.getProperty("username");
        String Pwd = prop.getProperty("password");
        resp.getWriter().println(Un+":"+Pwd);
    }

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

Xml:

<!--Properties-->
<servlet>
    <servlet-name>Prop</servlet-name>
    <servlet-class>com.edwin.servlet.CheckPropertiesServlet</servlet-class>
</servlet>
<servlet-mapping>
    <servlet-name>Prop</servlet-name>
    <url-pattern>/Prop</url-pattern>
</servlet-mapping>

輸出：

參考文獻

《【狂神說Java】JavaWeb入門到實戰》

視頻連接

2020.06.03