06月06, 2019
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Java Web--Servlet--ServletConfig与ServletContext

ServletConfig与ServletContext

一、ServletConfig讲解

1.1、配置Servlet初始化参数

在通过eclipse创建servlet的时候就可以创建init初始化参数,如图: 初始化参数

创建好之后web.xml文件中就会加入新的内容:

<servlet>
    <description></description>
    <display-name>ServletDemo02</display-name>
    <servlet-name>ServletDemo02</servlet-name>
    <servlet-class>com.lovo.study.ServletDemo02</servlet-class>
    <init-param>
      <description></description>
      <param-name>name</param-name>
      <param-value>lovo</param-value>
    </init-param>
    <init-param>
      <description></description>
      <param-name>pwd</param-name>
      <param-value>123456</param-value>
    </init-param>
  </servlet>
  <servlet-mapping>
    <servlet-name>ServletDemo02</servlet-name>
    <url-pattern>/ServletDemo02</url-pattern>
  </servlet-mapping>

其中<init-param>包含的部分就是初始化参数

1.2、通过ServletConfig获取Servlet的初始化参数

　　当servlet配置了初始化参数后，web容器在创建servlet实例对象时，会自动将这些初始化参数封装到ServletConfig对象中，并在调用servlet的init方法时，将ServletConfig对象传递给servlet。进而，我们通过ServletConfig对象就可以得到当前servlet的初始化参数信息。

package com.lovo.study;

import java.io.IOException;
import java.io.PrintWriter;
import java.util.Enumeration;

import javax.servlet.ServletConfig;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

/**
 * Servlet implementation class ServletDemo02
 */
public class ServletDemo02 extends HttpServlet {
    private static final long serialVersionUID = 1L;

    public ServletDemo02() {
        super();
    }

    /**
    * 定义ServletConfig对象来接收配置的初始化参数
    */
    private ServletConfig config;

    /**
    * 当servlet配置了初始化参数后，web容器在创建servlet实例对象时，
    * 会自动将这些初始化参数封装到ServletConfig对象中，并在调用servlet的init方法时，
    * 将ServletConfig对象传递给servlet。进而，程序员通过ServletConfig对象就可以
    * 得到当前servlet的初始化参数信息。
    */

    @Override
    public void init(ServletConfig config) throws ServletException {
        this.config = config;
    }


    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //获取在web.xml中配置的初始化参数
        String paramValue = this.config.getInitParameter("name");//指定键名
        PrintWriter out = response.getWriter();
        out.append(paramValue);
        out.append("<br/>");

        //获取所有初始化参数
        Enumeration<String> e = this.config.getInitParameterNames();
        while(e.hasMoreElements()){
            String name = e.nextElement();
            String value = this.config.getInitParameter(name);

            out.append(name + "=" + value + "<br/>");
        }
    }


    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        doGet(request, response);
    }

}

二、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。 　　ServletConfig对象中维护了ServletContext对象的引用，开发人员在编写servlet时，可以通过ServletConfig.getServletContext方法获得ServletContext对象。 　　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。

三、ServletContext的应用

3.1、多个Servlet通过ServletContext对象实现数据共享

新建ServletContext01和ServletContext02实现数据共享示例:

SerlvetContext01:

package com.lovo.study;

import java.io.IOException;

import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class ServletContext01 extends HttpServlet {
    private static final long serialVersionUID = 1L;

    public ServletContext01() {
        super();
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String data = "lovo";
        /**
        * ServletConfig对象中维护了ServletContext对象的引用，开发人员在编写servlet时，
        * 可以通过ServletConfig.getServletContext方法获得ServletContext对象。
        */
        ServletContext context = this.getServletContext();//获得ServletContext对象
        context.setAttribute("name", data);//将data存储到ServletContext对象中
    }

    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        doGet(request, response);
    }

}

ServletContext02:

package com.lovo.study;

import java.io.IOException;

import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class ServletContext02 extends HttpServlet {
    private static final long serialVersionUID = 1L;

    public ServletContext02() {
        super();
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        ServletContext context = this.getServletContext();
        String value = context.getAttribute("name").toString();//从ServletContext对象中取出数据
        response.getWriter().append(value);
    }

    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request, response);
    }

}

思考: 1.关闭浏览器之后,再次直接访问ServletContext02会不会有问题?

2.先关闭tomcat服务器再重新启动,然后再直接访问ServletContext02会不会有问题?

3.2、获取WEB应用的初始化参数

在web.xml文件中使用<context-param>标签配置WEB应用的初始化参数，如下所示：

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://java.sun.com/xml/ns/javaee" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd" id="WebApp_ID" version="2.5">
  <display-name>ServletDemo</display-name>

    <welcome-file>default.jsp</welcome-file>
  </welcome-file-list>

  <!-- 配置WEB应用的初始化参数 -->
  <context-param>
      <param-name>url</param-name>
      <param-value>jdbs:mysql://localhost:3306/test</param-value>
  </context-param>
</web-app>

获取Web应用的初始化参数，代码如下：

package com.lovo.study;

import java.io.IOException;

import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class ServletContext03 extends HttpServlet {
    private static final long serialVersionUID = 1L;

    public ServletContext03() {
        super();
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        ServletContext context = this.getServletContext();
        //获取整个web站点的初始化参数
        String value = context.getInitParameter("url");
        response.getWriter().append(value);
    }

    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request, response);
    }

}

3.3、用servletContext实现请求转发

通过ServletContext还能实现请求转发,具体如下面的代码:

ServletContext04:

package com.lovo.study;

import java.io.IOException;

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;


public class ServletContext04 extends HttpServlet {
    private static final long serialVersionUID = 1L;

    public ServletContext04() {
        super();
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        ServletContext context = this.getServletContext();
        //获取转发对象
        RequestDispatcher rd = context.getRequestDispatcher("/ServletContext05");
        rd.forward(request, response);
    }

    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request, response);
    }

}

ServletContext05: 

package com.lovo.study;

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

public class ServletContext05 extends HttpServlet {
    private static final long serialVersionUID = 1L;

    public ServletContext05() {
        super();
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        response.getWriter().append("ServletContext05");
    }

    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request, response);
    }

}

效果如图: requestDispatche

3.4、利用ServletContext对象读取资源文件

项目目录结构如下: properties

可以看到几个properties文件分别在不同的位置,现在我们通过ServletContext06.java文件来读取这几个在不同位置的属性文件

ServletContext06.java

package com.lovo.study;

import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.text.MessageFormat;
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 ServletContext06 extends HttpServlet {
    private static final long serialVersionUID = 1L;

    public ServletContext06() {
        super();
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        /**
         * response.setContentType("text/html;charset=UTF-8");
         * 目的是控制浏览器用UTF-8进行解码
         * 防止出现中文乱码
         */
        response.setHeader("Content-Type","text/html;charset=UTF-8");

        //读取com.lovo.study包中的db1.properties属性文件
        InputStream in = this.getServletContext()
                .getResourceAsStream("/WEB-INF/classes/com/lovo/study/db1.properties");

        //读取src中的db2.properties属性文件
//        InputStream in = this.getServletContext()
//                .getResourceAsStream("/WEB-INF/classes/db2.properties");

        //读取webContent中的db3.properties属性文件
//        InputStream in = this.getServletContext()
//                .getResourceAsStream("db3.properties");

        //读取config包中的db4.properties属性文件
//        InputStream in = this.getServletContext()
//                .getResourceAsStream("/WEB-INF/classes/config/db4.properties");

        //获取属性对象
        Properties prop = new Properties();
        prop.load(in);
        String name = prop.getProperty("name");
        String pwd = prop.getProperty("pwd");
        String db = prop.getProperty("db");
        PrintWriter out = response.getWriter();
        out.append(MessageFormat.format("name={0},pwd={1},db={2}", 
                name, pwd, db));

    }

    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request, response);
    }

}

当然,除了通过ServletContext来读取文件,通过类加载器ClassLoader同样可以来读取属性文件.

ServletContext07.java

package com.lovo.study;

import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.text.MessageFormat;
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 ServletContext07 extends HttpServlet {
    private static final long serialVersionUID = 1L;

    public ServletContext07() {
        super();
    }
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        response.setHeader("Content-type", "text/html;charset=UTF-8");
        //通过反射获取ClassLoader类加载器
        ClassLoader loader = ServletContext07.class.getClassLoader();

        //获取com.lovo.study包中的db1.properties文件
        //InputStream in = loader.getResourceAsStream("com/lovo/study/db1.properties");

        //获取src中的db2.properties文件
        InputStream in = loader.getResourceAsStream("db2.properties");

        Properties prop = new Properties();
        prop.load(in);
        String name = prop.getProperty("name");
        String pwd = prop.getProperty("pwd");
        String db = prop.getProperty("db");
        PrintWriter out = response.getWriter();
        out.append(MessageFormat.format("name={0},pwd={1},db={2}", 
                name, pwd, db));

    }

    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request, response);
    }

}

本文链接：http://www.yanhongzhi.com/post/context.html

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作者 admin 发表于 2019-06-06 17:35:33 ，添加在分类 Java Web 下 ，并被添加「 java web Java 」标签 ，最后修改于 2019-06-07 18:32:06

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