ServletConfig和ServletContext对象

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mb638413918930b 2022-11-28 18:27:42 博主文章分类：JavaEE ©著作权

文章标签 servlet config context 读取资源文件 java 文章分类 OpenStack 云计算

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ServletConfig对象

　　ServletConfig对象，用于封装Servlet的配置信息。
　　当Servlet在Web.xml中配置初始化参数后，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。
　　Servlet配置初始化参数，如：

<servlet>
    <servlet-name>ServletDemo4</servlet-name>
    <servlet-class>com.wm103.ServletDemo4</servlet-class>

    <init-param>
        <param-name>username</param-name>
        <param-value>DreamBoy</param-value>
    </init-param>
    <init-param>
        <param-name>password</param-name>
        <param-value>123456</param-value>
    </init-param>
</servlet>

　　ServletDemo4.java 获取Servlet的配置参数：

package com.wm103;

import javax.servlet.ServletConfig;
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.util.Enumeration;

/**
 * Created by DreamBoy on 2017/4/27.
 */

/**
 * ServletConfig对象，用于封装Servlet的配置信息
 */
public class ServletDemo4 extends HttpServlet {
    /*private ServletConfig config;

    @Override
    public void init(ServletConfig config) {
        this.config = config;
    }*/

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

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        ServletConfig config = this.getServletConfig();
        String value = config.getInitParameter("username");
        System.out.println(value);

        Enumeration<String> e = config.getInitParameterNames();
        while (e.hasMoreElements()) {
            String name = e.nextElement();
            String val = config.getInitParameter(name);
            System.out.println(val);
        }

    }
}

　　注解方式如下：

package com.wm103;

import javax.servlet.ServletConfig;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebInitParam;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.util.Enumeration;

/**
 * Created by DreamBoy on 2018/3/3.
 */
@WebServlet(name = "ServletDemo2", urlPatterns = {"/ServletDemo2"}, initParams = {
        @WebInitParam(name = "data1", value = "data1value"),
        @WebInitParam(name = "data2", value = "data2value"),
        @WebInitParam(name = "data3", value = "data3value")
})
public class ServletDemo2 extends HttpServlet {

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

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        ServletConfig config = this.getServletConfig();
        Enumeration<String> e = config.getInitParameterNames();
        while (e.hasMoreElements()) {
            String name = e.nextElement();
            String value = config.getInitParameter(name);
            System.out.println(name + "=" + value);
        }
    }
}

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。Web容器关闭或Web应用被删除时，对应的Web应用的ServletContext对象将被销毁。
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方法获取ServletContext对象。

package com.wm103;

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

/**
 * Created by DreamBoy on 2017/4/27.
 */
@WebServlet(name = "ServletDemo5", urlPatterns={"/ServletDemo5"})
public class ServletDemo5 extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request, response);
    }

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

        // 获取ServletContext对象 1：
        // ServletContext context = this.getServletConfig().getServletContext();

        // 获取ServletContext对象 2：
        ServletContext context = this.getServletContext();


        response.getWriter().println("This is ServletDemo5.");
    }
}

ServletContext对象实现多个Servlet数据共享

　　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。
　　通过ServletContext对象的setAttribute方法设置共享的数据；通过ServletContext对象的getAttribute方法获取共享的数据；通过ServletContext对象的removeAttribute方法删除共享的数据。

获取Web应用的初始化参数

　　在web.xml文件中配置Web应用的初始化参数：

<!-- 为整个Web应用配置初始化参数 -->
<context-param>
    <param-name>data1</param-name>
    <param-value>data1value</param-value>
</context-param>
<context-param>
    <param-name>data2</param-name>
    <param-value>data2value</param-value>
</context-param>

　　在Servlet中获取Web应用的初始化参数：

package com.wm103;

import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.util.Enumeration;

/**
 * Created by DreamBoy on 2017/4/27.
 */

/**
 * 获取Web应用的初始化参数
 */
@WebServlet(name = "ServletDemo6", urlPatterns={"/ServletDemo6"})
public class ServletDemo6 extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request, response);
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        ServletContext context = this.getServletContext();

        String value = context.getInitParameter("data1");
        System.out.println(value);

        Enumeration<String> en = context.getInitParameterNames();
        while (en.hasMoreElements()) {
            String name = en.nextElement();
            String val = context.getInitParameter(name);
            System.out.println("name: " + name + ", value: " + val);
        }

    }
}

实现Servlet的转发

RequestDispatcher rd = this.getServletContext().getRequestDispatcher("/1.jsp");
rd.forward(request, response);

利用ServletContext对象读取资源文件

　　如读取 .properties文件。在Web工程的src创建db.properties文件，内容如下：

url=jdbc:mysql://localhost:3306/test
username=root
password=root

　　使用ServletContext对象读取db.properties文件的内容：

package com.wm103;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.util.Properties;

/**
 * Created by DreamBoy on 2017/4/27.
 */

/**
 * 通过ServletContext读取资源文件
 */
@WebServlet(name = "ServletDemo8", urlPatterns={"/ServletDemo8"})
public class ServletDemo8 extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request, response);
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        test2();
    }

    /**
     * 读取资源文件
     * @throws IOException
     */
    private void test1() throws IOException {
        InputStream in = this.getServletContext().getResourceAsStream("/WEB-INF/classes/db.properties");

        Properties props = new Properties();
        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);
    }

    /**
     * 通过ServletContext的getRealPath得到资源的绝对路径后，再通过读取资源文件
     * @throws IOException
     */
    private void test2() throws IOException {
        String path = this.getServletContext().getRealPath("/WEB-INF/classes/db.properties");
        System.out.println(path);
        String filename = path.substring(path.lastIndexOf("\\") + 1);
        System.out.println(filename);
        FileInputStream in = new FileInputStream(path);

        Properties props = new Properties();
        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);
    }
}

在非Servlet程序中读取资源文件

　　通过类装载器的方式，读取资源文件。示例：
　　UserDao.java

package com.wm103.dao;

/**
 * Created by DreamBoy on 2017/4/28.
 */

import java.io.IOException;
import java.io.InputStream;
import java.util.Properties;

/**
 * 如果读取资源文件的程序不是servlet的话，就只能通过类装载器进行读取了
 * 通过类装载器读取资源文件，资源文件不能太大！！！
 */
public class UserDao {
    private static Properties dbconfig = new Properties();

    // 静态代码块
    static {
        try {
            // 获取当前类的类装载器
            // 通过类装载器加载，资源文件只被装载一次。当我们修改/WEB-INF/classes/db.properties文件时，读取结果仍是修改前的结果。
            InputStream in = UserDao.class.getClassLoader().getResourceAsStream("db.properties");
            dbconfig.load(in);
        } catch (IOException e) {
            throw new ExceptionInInitializerError(e);
        }
    }


    public void update() throws IOException {
        // 以下代码虽然可以读取资源文件的数据，但是无法获取更新后的数据。
        // 通过类装载器加载，资源文件只被装载一次。当我们修改/WEB-INF/classes/db.properties文件时，读取结果仍是修改前的结果。
        /*InputStream in = UserDao.class.getClassLoader().getResourceAsStream("db.properties");
        dbconfig.load(in);*/

        // 通过类装载的方式得到资源文件的位置，再通过传统方式读取资源文件的数据，这样可以读取到更新后的数据。
        // 通过这样的方式，Web应用运行过程中，修改/WEB-INF/classes/db.properties文件后，将读取到修改后的内容。
        /*String path = UserDao.class.getClassLoader().getResource("db.properties").getPath();
        FileInputStream in = new FileInputStream(path);
        dbconfig.load(in);*/

        String url = dbconfig.getProperty("url");
        String username = dbconfig.getProperty("username");
        String password = dbconfig.getProperty("password");

        System.out.println(url);
        System.out.println(username);
        System.out.println(password);
    }

    public void find() {

    }

    public void delete() {

    }
}

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