深入分析JavaWeb Item6 — servletConfig 与servletContext详解，

来源： javaer 分享于 2019-04-09 点击 46591 次点评：157

深入分析JavaWeb Item6 — servletConfig 与servletContext详解，

一、ServletConfig讲解

首先看ServletConfig API文档

1.1、配置Servlet初始化参数

在Servlet的配置文件web.xml中，可以使用一个或多个<init-param>标签为servlet配置一些初始化参数。

例如：

<servlet>
    <servlet-name>ServletConfigDemo1</servlet-name>
    <servlet-class>gacl.servlet.study.ServletConfigDemo1</servlet-class>
    <!--配置ServletConfigDemo1的初始化参数 -->
    <init-param>
        <param-name>name</param-name>
        <param-value>gacl</param-value>
    </init-param>
     <init-param>
        <param-name>password</param-name>
        <param-value>123</param-value>
    </init-param>
    <init-param>
        <param-name>charset</param-name>
        <param-value>UTF-8</param-value>
    </init-param>
</servlet>

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。

例如：

package gacl.servlet.study;

import java.io.IOException;
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;

public class ServletConfigDemo1 extends HttpServlet {

    /**
     * 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;
    }

    public void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        //获取在web.xml中配置的初始化参数
        String paramVal = this.config.getInitParameter("name");//获取指定的初始化参数
        response.getWriter().print(paramVal);

        response.getWriter().print("<hr/>");
        //获取所有的初始化参数
        Enumeration<String> e = config.getInitParameterNames();
        while(e.hasMoreElements()){
            String name = e.nextElement();
            String value = config.getInitParameter(name);
            response.getWriter().print(name + "=" + value + "<br/>");
        }
    }

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

}

运行结果如下：

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，但是还有更简洁的this.getServletContext()方法；

<font color=”red”>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：1，是一个容器 2。作用范围是应用程序范围。

三、ServletContext的应用

3.1、多个Servlet通过ServletContext对象实现数据共享

范例：ServletContextDemo1和ServletContextDemo2通过ServletContext对象实现数据共享

package gacl.servlet.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 ServletContextDemo1 extends HttpServlet {

    public void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        String data = "xdp_gacl";
        /**
         * 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.getServletConfig().getServletContext();//获得ServletContext对象
        context.setAttribute("data", data);  //将data存储到ServletContext对象中
    }

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

package gacl.servlet.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 ServletContextDemo2 extends HttpServlet {

    public void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        ServletContext context = this.getServletContext();
        String data = (String) context.getAttribute("data");//从ServletContext对象中取出数据
        response.getWriter().print("data="+data);
    }

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

先运行ServletContextDemo1，将数据data存储到ServletContext对象中，然后运行ServletContextDemo2就可以从ServletContext对象中取出数据了，这样就实现了数据共享，如下图所示：

3.2、获取WEB应用的初始化参数

如果想在所有的Servlet应用中都要配置并读取初始化参数，则可以在web.xml文件的<web-app>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 version="3.0" xmlns="http://java.sun.com/xml/ns/javaee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee 

http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd">

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

    <welcome-file-list>
        <welcome-file>index.jsp</welcome-file>
    </welcome-file-list>
</web-app>

获取Web应用的初始化参数，代码如下：

package gacl.servlet.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 ServletContextDemo3 extends HttpServlet {

    public 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 contextInitParam = context.getInitParameter("url");
        response.getWriter().print(contextInitParam);
    }

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

}

运行结果：

3.3、用servletContext实现请求转发

实现Servlet的转发。

ServletContextDemo4

package gacl.servlet.study;

import java.io.IOException;
import java.io.PrintWriter;
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 ServletContextDemo4 extends HttpServlet {

    public void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        String data = "<h1><font color='red'>abcdefghjkl</font></h1>";
        response.getOutputStream().write(data.getBytes());
        ServletContext context = this.getServletContext();//获取ServletContext对象
        RequestDispatcher rd = context.getRequestDispatcher("/servlet/ServletContextDemo5");//获取请求转发对象(RequestDispatcher)
        rd.forward(request, response);//调用forward方法实现请求转发
    }

    public void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
    }
}

ServletContextDemo5

package gacl.servlet.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 ServletContextDemo5 extends HttpServlet {

    public void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        response.getOutputStream().write("servletDemo5".getBytes());
    }

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

}

运行结果：

访问的是ServletContextDemo4，浏览器显示的却是ServletContextDemo5的内容，这就是使用ServletContext实现了请求转发

3.4、利用ServletContext对象读取资源文件

利用ServletContext对象读取资源文件,因为文件的位置不同，所有读取的方式也不同，一般来说分为两种情况：

在Servlet的context域中读取文件，工程目录下的src目录发布到服务器中，会映射到“/WEB-INF/classes”文件夹下。所以要一一对应。而且这个是相对目录，相对于web服务器的目录。如果要用传统的文件读取文件，则要使用绝对路劲

PrintWriter out = response.getWriter();
        ServletContext context = this.getServletContext();
        String path = context.getRealPath("/WEB-INF/classes/itcast.properties");
        InputStream in = new FileInputStream(path);
        Properties pro = new Properties();
        pro.load(in);

如果是非servlet中读取配置文件，则要使用类加载器去读取。稍后讲到

项目目录结构如下：

代码范例：使用servletContext读取资源文件

package gacl.servlet.study;

import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.InputStream;
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;

/**
 * 使用servletContext读取资源文件
 * 
 * @author gacl
 * 
 */
public class ServletContextDemo6 extends HttpServlet {

    public 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；
         * 这样就不会出现中文乱码了
         */
        response.setHeader("content-type","text/html;charset=UTF-8");
        readSrcDirPropCfgFile(response);//读取src目录下的properties配置文件
        response.getWriter().println("<hr/>");
        readWebRootDirPropCfgFile(response);//读取WebRoot目录下的properties配置文件
        response.getWriter().println("<hr/>");
        readPropCfgFile(response);//读取src目录下的db.config包中的db3.properties配置文件
        response.getWriter().println("<hr/>");
        readPropCfgFile2(response);//读取src目录下的gacl.servlet.study包中的db4.properties配置文件

    }

    /**
     * 读取src目录下的gacl.servlet.study包中的db4.properties配置文件
     * @param response
     * @throws IOException
     */
    private void readPropCfgFile2(HttpServletResponse response)
            throws IOException {
        InputStream in = this.getServletContext().getResourceAsStream("/WEB-INF/classes/gacl/servlet/study/db4.properties");
        Properties prop = new Properties();
        prop.load(in);
        String driver = prop.getProperty("driver");
        String url = prop.getProperty("url");
        String username = prop.getProperty("username");
        String password = prop.getProperty("password");
        response.getWriter().println("读取src目录下的gacl.servlet.study包中的db4.properties配置文件：");
        response.getWriter().println(
                MessageFormat.format(
                        "driver={0},url={1},username={2},password={3}", 
                        driver,url, username, password));
    }

    /**
     * 读取src目录下的db.config包中的db3.properties配置文件
     * @param response
     * @throws FileNotFoundException
     * @throws IOException
     */
    private void readPropCfgFile(HttpServletResponse response)
            throws FileNotFoundException, IOException {
        //通过ServletContext获取web资源的绝对路径
        String path = this.getServletContext().getRealPath("/WEB-INF/classes/db/config/db3.properties");
        InputStream in = new FileInputStream(path);
        Properties prop = new Properties();
        prop.load(in);
        String driver = prop.getProperty("driver");
        String url = prop.getProperty("url");
        String username = prop.getProperty("username");
        String password = prop.getProperty("password");
        response.getWriter().println("读取src目录下的db.config包中的db3.properties配置文件：");
        response.getWriter().println(
                MessageFormat.format(
                        "driver={0},url={1},username={2},password={3}", 
                        driver,url, username, password));
    }

    /**
     * 通过ServletContext对象读取WebRoot目录下的properties配置文件
     * @param response
     * @throws IOException
     */
    private void readWebRootDirPropCfgFile(HttpServletResponse response)
            throws IOException {
        /**
         * 通过ServletContext对象读取WebRoot目录下的properties配置文件
         * “/”代表的是项目根目录
         */
        InputStream in = this.getServletContext().getResourceAsStream("/db2.properties");
        Properties prop = new Properties();
        prop.load(in);
        String driver = prop.getProperty("driver");
        String url = prop.getProperty("url");
        String username = prop.getProperty("username");
        String password = prop.getProperty("password");
        response.getWriter().println("读取WebRoot目录下的db2.properties配置文件：");
        response.getWriter().print(
                MessageFormat.format(
                        "driver={0},url={1},username={2},password={3}", 
                        driver,url, username, password));
    }

    /**
     * 通过ServletContext对象读取src目录下的properties配置文件
     * @param response
     * @throws IOException
     */
    private void readSrcDirPropCfgFile(HttpServletResponse response) throws IOException {
        /**
         * 通过ServletContext对象读取src目录下的db1.properties配置文件
         */
        InputStream in = this.getServletContext().getResourceAsStream("/WEB-INF/classes/db1.properties");
        Properties prop = new Properties();
        prop.load(in);
        String driver = prop.getProperty("driver");
        String url = prop.getProperty("url");
        String username = prop.getProperty("username");
        String password = prop.getProperty("password");
        response.getWriter().println("读取src目录下的db1.properties配置文件：");
        response.getWriter().println(
                MessageFormat.format(
                        "driver={0},url={1},username={2},password={3}", 
                        driver,url, username, password));
    }

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

}

运行结果如下：

使用类装载器读取资源文件

我们在非servlet中读取资源文件时（比如在数据库的dao层读取配置文件），采用类装载器 classLoader,你可以先采用servlet服务先读取，然后在把servlet传递给dao，这样虽然可以实现，但是，这样损坏了我们编代码的设计原则，就是层之间不能有交织在一起的东西。

package gacl.servlet.study;

import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
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;

/**
 * 用类装载器读取资源文件
 * 通过类装载器读取资源文件的注意事项:不适合装载大文件，否则会导致jvm内存溢出
 * @author gacl
 *
 */
public class ServletContextDemo7 extends HttpServlet {

    public 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；
         * 这样就不会出现中文乱码了
         */
        response.setHeader("content-type","text/html;charset=UTF-8");
        test1(response);
        response.getWriter().println("<hr/>");
        test2(response);
        response.getWriter().println("<hr/>");
        //test3();
        test4();

    }

    /**
     * 读取类路径下的资源文件
     * @param response
     * @throws IOException
     */
    private void test1(HttpServletResponse response) throws IOException {
        //获取到装载当前类的类装载器
        ClassLoader loader = ServletContextDemo7.class.getClassLoader();
        //用类装载器读取src目录下的db1.properties配置文件
        InputStream in = loader.getResourceAsStream("db1.properties");
        Properties prop = new Properties();
        prop.load(in);
        String driver = prop.getProperty("driver");
        String url = prop.getProperty("url");
        String username = prop.getProperty("username");
        String password = prop.getProperty("password");
        response.getWriter().println("用类装载器读取src目录下的db1.properties配置文件：");
        response.getWriter().println(
                MessageFormat.format(
                        "driver={0},url={1},username={2},password={3}", 
                        driver,url, username, password));
    }

    /**
     * 读取类路径下面、包下面的资源文件
     * @param response
     * @throws IOException
     */
    private void test2(HttpServletResponse response) throws IOException {
        //获取到装载当前类的类装载器
        ClassLoader loader = ServletContextDemo7.class.getClassLoader();
        //用类装载器读取src目录下的gacl.servlet.study包中的db4.properties配置文件
        InputStream in = loader.getResourceAsStream("gacl/servlet/study/db4.properties");
        Properties prop = new Properties();
        prop.load(in);
        String driver = prop.getProperty("driver");
        String url = prop.getProperty("url");
        String username = prop.getProperty("username");
        String password = prop.getProperty("password");
        response.getWriter().println("用类装载器读取src目录下的gacl.servlet.study包中的db4.properties配置文件：");
        response.getWriter().println(
                MessageFormat.format(
                        "driver={0},url={1},username={2},password={3}", 
                        driver,url, username, password));
    }

    /**
     * 通过类装载器读取资源文件的注意事项:不适合装载大文件，否则会导致jvm内存溢出
     */
    public void test3() {
        /**
         * 01.avi是一个150多M的文件，使用类加载器去读取这个大文件时会导致内存溢出：
         * java.lang.OutOfMemoryError: Java heap space
         */
        InputStream in = ServletContextDemo7.class.getClassLoader().getResourceAsStream("01.avi");
        System.out.println(in);
    }

    /**
     * 读取01.avi,并拷贝到e:\根目录下
     * 01.avi文件太大，只能用servletContext去读取
     * @throws IOException
     */
    public void test4() throws IOException {
        // path=G:\Java学习视频\JavaWeb学习视频\JavaWeb\day05视频\01.avi
        // path=01.avi
        String path = this.getServletContext().getRealPath("/WEB-INF/classes/01.avi");
        /**
         * path.lastIndexOf("\\") + 1是一个非常绝妙的写法
         */
        String filename = path.substring(path.lastIndexOf("\\") + 1);//获取文件名
        InputStream in = this.getServletContext().getResourceAsStream("/WEB-INF/classes/01.avi");
        byte buffer[] = new byte[1024];
        int len = 0;
        OutputStream out = new FileOutputStream("e:\\" + filename);
        while ((len = in.read(buffer)) > 0) {
            out.write(buffer, 0, len);
        }
        out.close();
        in.close();
    }

    public void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {

        this.doGet(request, response);
    }

}

　　运行结果如下：

使用类装载器读取资源文件，存在的问题是；类装载器，每次只会装载一次。

//如果读取资源文件的程序不是servlet的话，
//就只能通过类转载器去读了，文件不能太大
//用传递参数方法不好，耦合性高
public class UserDao {

    private static Properties dbconfig=new Properties();
    static {
        InputStream in=UserDao.class.getClassLoader().getResourceAsStream("db.properties");
        try {
            dbconfig.load(in);
        } catch (IOException e) {
            throw new ExceptionInInitializerError(e);
        }    
        //上面代码类装载器只装载一次,下面代码用类装载方式得到文件位置
        URL url=UserDao.class.getClassLoader().getResource("db.properties");
        String str=url.getPath();
        //file:/C:/apache-tomcat-7.0.22/webapps/day05/WEB-INF/classes/db.properties
        try {
            InputStream in2=new FileInputStream(str);
            try {
                dbconfig.load(in2);
            } catch (IOException e) {
                throw new ExceptionInInitializerError(e);
            }
        } catch (FileNotFoundException e1) {
            throw new ExceptionInInitializerError(e1);
        }        
    }
    public void update() {
        System.out.println(dbconfig.get("url"));
    }
}

四、在客户端缓存Servlet的输出

对于不经常变化的数据，在servlet中可以为其设置合理的缓存时间值，以避免浏览器频繁向服务器发送请求，提升服务器的性能。例如：

package gacl.servlet.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 ServletDemo5 extends HttpServlet {

    public void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        String data = "abcddfwerwesfasfsadf";
        /**
         * 设置数据合理的缓存时间值，以避免浏览器频繁向服务器发送请求，提升服务器的性能
         * 这里是将数据的缓存时间设置为1天
         */
        response.setDateHeader("expires",System.currentTimeMillis() + 24 * 3600 * 1000);
        response.getOutputStream().write(data.getBytes());
    }

    public void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {

        this.doGet(request, response);
    }

}

深入分析JavaWeb Item6 — servletConfig 与servletContext详解，