ServletContext对象

---

• WEB容器在启动时，它会为每个WEB应用程序都创建一个对应的ServletContext对象，它代表当前web应用。

• ServletContext对象被包含在ServletConfig对象中，开发人员在编写servlet时，可以通过ServletConfig.getServletContext方法获得对ServletContext对象的引用。

• 由于一个WEB应用中的所有Servlet共享同一个ServletContext对象，因此Servlet对象之间可以通过ServletContext对象来实现通讯。ServletContext对象通常也被称之为context域对象。

---

得到servletContext对象的方式：

package servlet;import java.io.IOException;import java.io.PrintWriter;import javax.servlet.ServletContext;import javax.servlet.ServletException;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;/* * servletContext示例 */public class ServletDemo6 extends HttpServlet {    public void doGet(HttpServletRequest request, HttpServletResponse response)            throws ServletException, IOException {        //得到servletContext的方式1        ServletContext context = this.getServletConfig().getServletContext();        //得到servletContext的方式2        context = this.getServletContext();    }    public void doPost(HttpServletRequest request, HttpServletResponse response)            throws ServletException, IOException {    }}

---

ServletContext应用

• 多个Servlet通过ServletContext对象实现数据共享。
  示例代码：

package servlet;import java.io.IOException;import java.io.PrintWriter;import javax.servlet.ServletException;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;/* * ServletContext的应用 * 所有Servlet共享同一个ServletContext * ServletContex对象通常被称为context域对象 *  */public class ServletDemo7 extends HttpServlet {    public void doGet(HttpServletRequest request, HttpServletResponse response)            throws ServletException, IOException {        String data = "aaaa";        this.getServletContext().setAttribute("data",data);    }    public void doPost(HttpServletRequest request, HttpServletResponse response)            throws ServletException, IOException {    }}

package servlet;import java.io.IOException;import java.io.PrintWriter;import javax.servlet.ServletException;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;/* * servletContext域： * 1.这是一个容器 * 2.servletContext域这句话说明了这个容器作用范围，也就应用程序范围 *  *///通过servletContext实现ServletDemo7和servletDemo8的数据交换public class ServletDemo8 extends HttpServlet {    public void doGet(HttpServletRequest request, HttpServletResponse response)            throws ServletException, IOException {        String value = (String) this.getServletContext().getAttribute("data");        System.out.println(value);    }    public void doPost(HttpServletRequest request, HttpServletResponse response)            throws ServletException, IOException {    }}

先访问ServletDemo7然后访问ServletDemo8，发现在命令行输出了：aaaa。

• 获取WEB应用的初始化参数。
  示例代码：

package servlet;import java.io.IOException;import java.io.PrintWriter;import javax.servlet.ServletException;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;//获取web应用的初始化参数/* *  <!-- 获取web应用的初始化参数 -->    <context-param>        <param-name>data</param-name>        <param-value>xxx</param-value>    </context-param> */public class ServletDemo9 extends HttpServlet {    public void doGet(HttpServletRequest request, HttpServletResponse response)            throws ServletException, IOException {        String value = this.getServletContext().getInitParameter("data");        System.out.println(value);    }    public void doPost(HttpServletRequest request, HttpServletResponse response)            throws ServletException, IOException {    }}

在应用程序的web.xml中增加以下内容：

    <!-- 获取web应用的初始化参数 -->    <context-param>        <param-name>data</param-name>        <param-value>xxx</param-value>    </context-param>

访问ServletDemo9，发现命令行输出了：xxx。

• 实现Servlet的转发。
• 示例代码：

package servlet;import java.io.IOException;import java.io.PrintWriter;import javax.servlet.RequestDispatcher;import javax.servlet.ServletException;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;//通过servletContext实现请求转发、public class ServletDemo10 extends HttpServlet {    public void doGet(HttpServletRequest request, HttpServletResponse response)            throws ServletException, IOException {        String data = "aaaaaa";        //把数据带给1.jsp        this.getServletContext().setAttribute("data", data);        RequestDispatcher rd = this.getServletContext().getRequestDispatcher("/1.jsp"); //得到转发到的对象（得到转发到1.jsp的对象）        rd.forward(request, response);    }    public void doPost(HttpServletRequest request, HttpServletResponse response)            throws ServletException, IOException {    }}

1.jsp的文件内容如下：

<%@ page language="java" import="java.util.*" pageEncoding="ISO-8859-1"%><!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"><html>  <head>    <title>My JSP '1.jsp' starting page</title>  </head>  <body>        <h1>      <font color="red">         <%            String data = (String)application.getAttribute("data");            out.write(data);         %>       </font>     </h1>  </body></html>

访问ServletDemo10，可以在浏览器看到如下结果：

• 利用ServletContext对象读取资源文件。
  示例代码：

package servlet;import java.io.FileInputStream;import java.io.IOException;import java.io.InputStream;import java.util.Properties;import javax.servlet.ServletException;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;public class ServletDemo11 extends HttpServlet {//通过ServletContext去读取资源文件（db.properties）    //读取资源文件(模板代码)    public void doGet(HttpServletRequest request, HttpServletResponse response)            throws ServletException, IOException {            test1();            System.out.println("---------------");            test2();            System.out.println("---------------");            test3();            System.out.println("---------------");            test4();            System.out.println("---------------");            test5();    }    public void doPost(HttpServletRequest request, HttpServletResponse response)            throws ServletException, IOException {        doGet(request, response);    }    //读取/src下面的db.properties文件    private void test1() throws IOException {        InputStream in = this.getServletContext().getResourceAsStream("/WEB-INF/classes/db.properties");//把资源文件作为流返回        Properties props = new Properties();    //map        props.load(in);        String url = props.getProperty("url");        String username = props.getProperty("username");        String password = props.getProperty("password");        System.out.println(url);        System.out.println(username);        System.out.println(password);    }    //读取/src/servlet下面的db.properties文件    private void test2() throws IOException {        InputStream in = this.getServletContext().getResourceAsStream("/WEB-INF/classes/servlet/db.properties");//把资源文件作为流返回        Properties props = new Properties();    //map        props.load(in);        String url = props.getProperty("url");        String username = props.getProperty("username");        String password = props.getProperty("password");        System.out.println(url);        System.out.println(username);        System.out.println(password);    }    //读取/WebRoot下面的db.properties文件    private void test3() throws IOException {        InputStream in = this.getServletContext().getResourceAsStream("/db.properties");//把资源文件作为流返回        Properties props = new Properties();    //map        props.load(in);        String url = props.getProperty("url");        String username = props.getProperty("username");        String password = props.getProperty("password");        System.out.println(url);        System.out.println(username);        System.out.println(password);    }    //读取资源文件需要注意的问题(不要采用传统方式，要采用ServletContext)    //传统方式采用相对路径去读取资源文件    private void test4() throws IOException {        FileInputStream in = new FileInputStream("classes/db.properties");//把资源文件作为流返回,相对路径，相对于java虚拟机，就是Tomcat的启动目录D:\Tomcat\apache-tomcat-6.0.20\bin        //在D:\Tomcat\apache-tomcat-6.0.20\bin目录下创建一个classes文件，里面放入db.properties文件。        Properties props = new Properties();    //map        props.load(in);        String url = props.getProperty("url");        String username = props.getProperty("username");        String password = props.getProperty("password");        System.out.println(url);        System.out.println(username);        System.out.println(password);    }    //传统方式用绝对路径去读取资源文件    private void test5() throws IOException{        String path = this.getServletContext().getRealPath("/WEB-INF/classes/db.properties");//返回资源文件的绝对路径        //好处：可以截取到资源文件的名称        String filename = path.substring(path.lastIndexOf("\\")+1);        System.out.println("当前读取到的资源名称是："+filename);        //之后用传统方式        FileInputStream in = new FileInputStream(path);//把资源文件作为流返回        Properties props = new Properties();    //map        props.load(in);        String url = props.getProperty("url");        String username = props.getProperty("username");        String password = props.getProperty("password");        System.out.println("当前读取到的资源数据是：");        System.out.println(url);        System.out.println(username);        System.out.println(password);    }}