JavaWeb学习心得之Servlet（二）

一、ServletConfig说明

1.配置Servlet初始化参数

在Servlet配置文件web.xml中，可以使用一个或多个<init-param>标签配置初始化参数。

<?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_3_0.xsd" id="WebApp_ID" version="3.0">
  <display-name>ServletDemo</display-name>
  <welcome-file-list>
    <welcome-file>index.html</welcome-file>
    <welcome-file>index.htm</welcome-file>
    <welcome-file>index.jsp</welcome-file>
    <welcome-file>default.html</welcome-file>
    <welcome-file>default.htm</welcome-file>
    <welcome-file>default.jsp</welcome-file>
  </welcome-file-list>
  <servlet>
      <servlet-name>ServletDemo02</servlet-name>
      <servlet-class>com.hanxin.ServletDemo02</servlet-class>
      <!-- 初始化参数 -->
      <init-param>
          <param-name>username</param-name>
          <param-value>hanxin</param-value>
      </init-param>
      <init-param>
          <param-name>password</param-name>
          <param-value>123456</param-value>
      </init-param>
  </servlet>
  <servlet-mapping>
      <servlet-name>ServletDemo02</servlet-name>
      <url-pattern>/servletDemo02.do</url-pattern>
  </servlet-mapping>
</web-app>

2.获取Servlet初始化参数

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;
public class ServletDemo02 extends HttpServlet{
	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，通过ServletConfig对象就可以
	 * 得到当前servlet的初始化参数信息。
	 * @see javax.servlet.GenericServlet#init(javax.servlet.ServletConfig)
	 */
	@Override
	public void init(ServletConfig config) throws ServletException {
		this.config = config;
	}

	@Override
	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		//获取所有初始化参数
		Enumeration<String> e =  config.getInitParameterNames();
		//获取输出流
		PrintWriter pw = response.getWriter();
		//输出所有参数到浏览器
		while(e.hasMoreElements()){
			String name = e.nextElement();
			//获取ServletConfig对象中的初始化参数
			String value = config.getInitParameter(name);
			pw.print(name+"="+value+"\r");
		}
		pw.flush();
		pw.close();
	}

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

访问结果：

JavaWeb学习心得之Servlet（二）

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。由于web应用的所有servlet共享一个ServletContext对象，因此可以通过ServletContext对象来实现通讯，ServletContext也叫做context域对象。

1.多个Servlet通过ServletContext对象实现数据共享

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 ServletDemo01 extends HttpServlet{

	@Override
	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		//获取ServletContext对象
		ServletContext context = this.getServletContext();
		//保存变量到ServletContext中
		context.setAttribute("data", "hanxin");
	}

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

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 ServletDemo02 extends HttpServlet{
	@Override
	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		//获取ServletContext对象
		ServletContext context = this.getServletContext();
		//获取ServletContext中的变量
		String data = String.valueOf(context.getAttribute("data"));
		response.getWriter().println(data);
	}

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

配置web.xml（可以同时配置多个Servlet），参考文章。

先后访问ServletDemo01和ServletDemo02，访问结果。

JavaWeb学习心得之Servlet（二）

2.获取web应用的初始化数据

web.xml中<web-app>根标签，使用标签<context-param>来配置初始化参数。

<!-- 配置web应用初始化参数 -->
  <context-param>
      <param-name>webAppName</param-name>
      <param-value>ServletDemo</param-value>
  </context-param>

获取web应用初始化参数

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 ServletDemo03 extends HttpServlet{
	@Override
	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		//获取ServletContext对象
		ServletContext context = this.getServletContext();
		//获取ServletContext初始参数
		String data = context.getInitParameter("webAppName");
		response.getWriter().println(data);
	}

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

访问结果

JavaWeb学习心得之Servlet（二）

3.实现请求转发

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 ServletDemo04 extends HttpServlet{
	@Override
	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		ServletContext context = getServletContext();
		//获取转发对象
		RequestDispatcher rd = context.getRequestDispatcher("/servletDemo05.do");
		rd.forward(request, response);//调用forward方法实现请求转发
	}

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

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 ServletDemo05 extends HttpServlet{
	@Override
	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		ServletContext context = getServletContext();
		response.getWriter().print("this is demo05");
	}

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

访问结果：

JavaWeb学习心得之Servlet（二）

4.利用ServletContext对象读取资源文件

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 ServletDemo06 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com.hanxin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/com/hanxin/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com.hanxin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);
    }

}

JavaWeb学习心得之Servlet（二）

5.利用类加载器读取资源文件

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;

/**
 * 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 ServletDemo07 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);
        
    }
    
    /**
     * 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 = ServletDemo07.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 = ServletDemo07.class.getClassLoader();
        //用类装载器读取src目录下的com.hanxin包中的db4.properties配置文件
        InputStream in = loader.getResourceAsStream("com/hanxin/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com.hanxin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));
    }
    

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

        this.doGet(request, response);
    }

}

JavaWeb学习心得之Servlet（二）

三、在客户端缓存Servlet的输出

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 ServletDemo08 extends HttpServlet {

    public void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        String data = "hello world";
        /**
         * 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);
    }

}

JavaWeb学习心得之Servlet（二）

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