javaweb Servlet开发总结（二）

2016-05-05 09:24:34 作者：孤傲苍狼

这篇文章主要为大家详细介绍了javaweb Servlet开发总结的第二篇，感兴趣的小伙伴们可以参考一下

一、ServletConfig讲解

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。
　　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

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

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 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

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

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：

代码范例：使用类装载器读取资源文件

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：

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);
 }

}

以上就是本文的全部内容，希望对大家掌握javaweb Servlet开发技术有所帮助。

javaweb Servlet开发总结（二）

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