javaweb学习总结(六)——Servlet开发(二)

转载自http://www.cnblogs.com/xdp-gacl/p/3763559.html感谢孤傲苍狼的分享。

 

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：

 1 <servlet>  2 <servlet-name>ServletConfigDemo1</servlet-name>  3 <servlet-class>gacl.servlet.study.ServletConfigDemo1</servlet-class>  4 <!--配置ServletConfigDemo1的初始化参数 -->  5 <init-param>  6 <param-name>name</param-name>  7 <param-value>gacl</param-value>  8 </init-param>  9 <init-param> 10 <param-name>password</param-name> 11 <param-value>123</param-value> 12 </init-param> 13 <init-param> 14 <param-name>charset</param-name> 15 <param-value>UTF-8</param-value> 16 </init-param> 17 </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：

 1 package gacl.servlet.study;  2  3 import java.io.IOException;  4 import java.util.Enumeration;  5 import javax.servlet.ServletConfig;  6 import javax.servlet.ServletException;  7 import javax.servlet.http.HttpServlet;  8 import javax.servlet.http.HttpServletRequest;  9 import javax.servlet.http.HttpServletResponse; 10 11 public class ServletConfigDemo1 extends HttpServlet { 12 13 /** 14  * 定义ServletConfig对象来接收配置的初始化参数 15 */ 16 private ServletConfig config; 17 18 /** 19  * 当servlet配置了初始化参数后，web容器在创建servlet实例对象时， 20  * 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， 21  * 将ServletConfig对象传递给servlet。进而，程序员通过ServletConfig对象就可以 22  * 得到当前servlet的初始化参数信息。 23 */ 24  @Override 25 public void init(ServletConfig config) throws ServletException { 26 this.config = config; 27  } 28 29 public void doGet(HttpServletRequest request, HttpServletResponse response) 30 throws ServletException, IOException { 31 //获取在web.xml中配置的初始化参数 32 String paramVal = this.config.getInitParameter("name");//获取指定的初始化参数 33  response.getWriter().print(paramVal); 34 35 response.getWriter().print("<hr/>"); 36 //获取所有的初始化参数 37 Enumeration<String> e = config.getInitParameterNames(); 38 while(e.hasMoreElements()){ 39 String name = e.nextElement(); 40 String value = config.getInitParameter(name); 41 response.getWriter().print(name + "=" + value + "<br/>"); 42  } 43  } 44 45 public void doPost(HttpServletRequest request, HttpServletResponse response) 46 throws ServletException, IOException { 47 this.doGet(request, response); 48  } 49 50 }

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

 1 package gacl.servlet.study;  2  3 import java.io.IOException;  4 import javax.servlet.ServletContext;  5 import javax.servlet.ServletException;  6 import javax.servlet.http.HttpServlet;  7 import javax.servlet.http.HttpServletRequest;  8 import javax.servlet.http.HttpServletResponse;  9 10 public class ServletContextDemo1 extends HttpServlet { 11 12 public void doGet(HttpServletRequest request, HttpServletResponse response) 13 throws ServletException, IOException { 14 String data = "xdp_gacl"; 15 /** 16  * ServletConfig对象中维护了ServletContext对象的引用，开发人员在编写servlet时， 17  * 可以通过ServletConfig.getServletContext方法获得ServletContext对象。 18 */ 19 ServletContext context = this.getServletConfig().getServletContext();//获得ServletContext对象 20 context.setAttribute("data", data); //将data存储到ServletContext对象中 21  } 22 23 public void doPost(HttpServletRequest request, HttpServletResponse response) 24 throws ServletException, IOException { 25  doGet(request, response); 26  } 27 }

 1 package gacl.servlet.study;  2  3 import java.io.IOException;  4 import javax.servlet.ServletContext;  5 import javax.servlet.ServletException;  6 import javax.servlet.http.HttpServlet;  7 import javax.servlet.http.HttpServletRequest;  8 import javax.servlet.http.HttpServletResponse;  9 10 public class ServletContextDemo2 extends HttpServlet { 11 12 public void doGet(HttpServletRequest request, HttpServletResponse response) 13 throws ServletException, IOException { 14 ServletContext context = this.getServletContext(); 15 String data = (String) context.getAttribute("data");//从ServletContext对象中取出数据 16 response.getWriter().print("data="+data); 17  } 18 19 public void doPost(HttpServletRequest request, HttpServletResponse response) 20 throws ServletException, IOException { 21  doGet(request, response); 22  } 23 }

　　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：

 1 <?xml version="1.0" encoding="UTF-8"?>  2 <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  3  http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd">  4 <display-name></display-name>  5 <!-- 配置WEB应用的初始化参数 -->  6 <context-param>  7 <param-name>url</param-name>  8 <param-value>jdbc:mysql://localhost:3306/test</param-value>  9 </context-param> 10 11 <welcome-file-list> 12 <welcome-file>index.jsp</welcome-file> 13 </welcome-file-list> 14 </web-app>

　　获取Web应用的初始化参数，代码如下：

 1 package gacl.servlet.study;  2  3 import java.io.IOException;  4 import javax.servlet.ServletContext;  5 import javax.servlet.ServletException;  6 import javax.servlet.http.HttpServlet;  7 import javax.servlet.http.HttpServletRequest;  8 import javax.servlet.http.HttpServletResponse;  9 10 11 public class ServletContextDemo3 extends HttpServlet { 12 13 public void doGet(HttpServletRequest request, HttpServletResponse response) 14 throws ServletException, IOException { 15 16 ServletContext context = this.getServletContext(); 17 //获取整个web站点的初始化参数 18 String contextInitParam = context.getInitParameter("url"); 19  response.getWriter().print(contextInitParam); 20  } 21 22 public void doPost(HttpServletRequest request, HttpServletResponse response) 23 throws ServletException, IOException { 24  doGet(request, response); 25  } 26 27 }

运行结果：

　　

　　3.3、用servletContext实现请求转发

ServletContextDemo4

 1 package gacl.servlet.study;  2  3 import java.io.IOException;  4 import java.io.PrintWriter;  5 import javax.servlet.RequestDispatcher;  6 import javax.servlet.ServletContext;  7 import javax.servlet.ServletException;  8 import javax.servlet.http.HttpServlet;  9 import javax.servlet.http.HttpServletRequest; 10 import javax.servlet.http.HttpServletResponse; 11 12 public class ServletContextDemo4 extends HttpServlet { 13 14 public void doGet(HttpServletRequest request, HttpServletResponse response) 15 throws ServletException, IOException { 16 String data = "<h1><font color='red'>abcdefghjkl</font></h1>"; 17  response.getOutputStream().write(data.getBytes()); 18 ServletContext context = this.getServletContext();//获取ServletContext对象 19 RequestDispatcher rd = context.getRequestDispatcher("/servlet/ServletContextDemo5");//获取请求转发对象(RequestDispatcher) 20 rd.forward(request, response);//调用forward方法实现请求转发 21  } 22 23 public void doPost(HttpServletRequest request, HttpServletResponse response) 24 throws ServletException, IOException { 25  } 26 }

ServletContextDemo5

 1 package gacl.servlet.study;  2  3 import java.io.IOException;  4 import javax.servlet.ServletException;  5 import javax.servlet.http.HttpServlet;  6 import javax.servlet.http.HttpServletRequest;  7 import javax.servlet.http.HttpServletResponse;  8  9 public class ServletContextDemo5 extends HttpServlet { 10 11 public void doGet(HttpServletRequest request, HttpServletResponse response) 12 throws ServletException, IOException { 13 response.getOutputStream().write("servletDemo5".getBytes()); 14  } 15 16 public void doPost(HttpServletRequest request, HttpServletResponse response) 17 throws ServletException, IOException { 18 this.doGet(request, response); 19  } 20 21 }

　　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

  1 package gacl.servlet.study;  2  3 import java.io.FileInputStream;  4 import java.io.FileNotFoundException;  5 import java.io.IOException;  6 import java.io.InputStream;  7 import java.text.MessageFormat;  8 import java.util.Properties;  9 import javax.servlet.ServletException;  10 import javax.servlet.http.HttpServlet;  11 import javax.servlet.http.HttpServletRequest;  12 import javax.servlet.http.HttpServletResponse;  13  14 /**  15  * 使用servletContext读取资源文件  16  *  17  * @author gacl  18  *  19 */  20 public class ServletContextDemo6 extends HttpServlet {  21  22 public void doGet(HttpServletRequest request, HttpServletResponse response)  23 throws ServletException, IOException {  24 /**  25  * response.setContentType("text/html;charset=UTF-8");目的是控制浏览器用UTF-8进行解码；  26  * 这样就不会出现中文乱码了  27 */  28 response.setHeader("content-type","text/html;charset=UTF-8");  29 readSrcDirPropCfgFile(response);//读取src目录下的properties配置文件  30 response.getWriter().println("<hr/>");  31 readWebRootDirPropCfgFile(response);//读取WebRoot目录下的properties配置文件  32 response.getWriter().println("<hr/>");  33 readPropCfgFile(response);//读取src目录下的db.config包中的db3.properties配置文件  34 response.getWriter().println("<hr/>");  35 readPropCfgFile2(response);//读取src目录下的gacl.servlet.study包中的db4.properties配置文件  36  37  }  38  39 /**  40  * 读取src目录下的gacl.servlet.study包中的db4.properties配置文件  41  * @param response  42  * @throws IOException  43 */  44 private void readPropCfgFile2(HttpServletResponse response)  45 throws IOException {  46 InputStream in = this.getServletContext().getResourceAsStream("/WEB-INF/classes/gacl/servlet/study/db4.properties");  47 Properties prop = new Properties();  48  prop.load(in);  49 String driver = prop.getProperty("driver");  50 String url = prop.getProperty("url");  51 String username = prop.getProperty("username");  52 String password = prop.getProperty("password");  53 response.getWriter().println("读取src目录下的gacl.servlet.study包中的db4.properties配置文件：");  54  response.getWriter().println(  55  MessageFormat.format(  56 "driver={0},url={1},username={2},password={3}",  57  driver,url, username, password));  58  }  59  60 /**  61  * 读取src目录下的db.config包中的db3.properties配置文件  62  * @param response  63  * @throws FileNotFoundException  64  * @throws IOException  65 */  66 private void readPropCfgFile(HttpServletResponse response)  67 throws FileNotFoundException, IOException {  68 //通过ServletContext获取web资源的绝对路径  69 String path = this.getServletContext().getRealPath("/WEB-INF/classes/db/config/db3.properties");  70 InputStream in = new FileInputStream(path);  71 Properties prop = new Properties();  72  prop.load(in);  73 String driver = prop.getProperty("driver");  74 String url = prop.getProperty("url");  75 String username = prop.getProperty("username");  76 String password = prop.getProperty("password");  77 response.getWriter().println("读取src目录下的db.config包中的db3.properties配置文件：");  78  response.getWriter().println(  79  MessageFormat.format( 80 "driver={0},url={1},username={2},password={3}", 81 driver,url, username, password)); 82 } 83 84 /** 85 * 通过ServletContext对象读取WebRoot目录下的properties配置文件 86 * @param response 87 * @throws IOException 88 */ 89 private void readWebRootDirPropCfgFile(HttpServletResponse response) 90 throws IOException { 91 /** 92 * 通过ServletContext对象读取WebRoot目录下的properties配置文件 93 * “/”代表的是项目根目录 94 */ 95 InputStream in = this.getServletContext().getResourceAsStream("/db2.properties"); 96 Properties prop = new Properties(); 97 prop.load(in); 98 String driver = prop.getProperty("driver"); 99 String url = prop.getProperty("url"); 100 String username = prop.getProperty("username"); 101 String password = prop.getProperty("password"); 102 response.getWriter().println("读取WebRoot目录下的db2.properties配置文件："); 103 response.getWriter().print( 104 MessageFormat.format( 105 "driver={0},url={1},username={2},password={3}", 106 driver,url, username, password)); 107 } 108 109 /** 110 * 通过ServletContext对象读取src目录下的properties配置文件 111 * @param response 112 * @throws IOException 113 */ 114 private void readSrcDirPropCfgFile(HttpServletResponse response) throws IOException { 115 /** 116 * 通过ServletContext对象读取src目录下的db1.properties配置文件 117 */ 118 InputStream in = this.getServletContext().getResourceAsStream("/WEB-INF/classes/db1.properties"); 119 Properties prop = new Properties(); 120 prop.load(in); 121 String driver = prop.getProperty("driver"); 122 String url = prop.getProperty("url"); 123 String username = prop.getProperty("username"); 124 String password = prop.getProperty("password"); 125 response.getWriter().println("读取src目录下的db1.properties配置文件："); 126 response.getWriter().println( 127 MessageFormat.format( 128 "driver={0},url={1},username={2},password={3}", 129 driver,url, username, password)); 130 } 131 132 public void doPost(HttpServletRequest request, HttpServletResponse response) 133 throws ServletException, IOException { 134 this.doGet(request, response); 135 } 136 137 }

运行结果如下：

　　

代码范例：使用类装载器读取资源文件

  1 package gacl.servlet.study;  2  3 import java.io.FileOutputStream;  4 import java.io.IOException;  5 import java.io.InputStream;  6 import java.io.OutputStream;  7 import java.text.MessageFormat;  8 import java.util.Properties;  9  10 import javax.servlet.ServletException;  11 import javax.servlet.http.HttpServlet;  12 import javax.servlet.http.HttpServletRequest;  13 import javax.servlet.http.HttpServletResponse;  14  15 /**  16  * 用类装载器读取资源文件  17  * 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  18  * @author gacl  19  *  20 */  21 public class ServletContextDemo7 extends HttpServlet {  22  23 public void doGet(HttpServletRequest request, HttpServletResponse response)  24 throws ServletException, IOException {  25 /**  26  * response.setContentType("text/html;charset=UTF-8");目的是控制浏览器用UTF-8进行解码；  27  * 这样就不会出现中文乱码了  28 */  29 response.setHeader("content-type","text/html;charset=UTF-8");  30  test1(response);  31 response.getWriter().println("<hr/>");  32  test2(response);  33 response.getWriter().println("<hr/>");  34 //test3();  35  test4();  36  37  }  38  39 /**  40  * 读取类路径下的资源文件  41  * @param response  42  * @throws IOException  43 */  44 private void test1(HttpServletResponse response) throws IOException {  45 //获取到装载当前类的类装载器  46 ClassLoader loader = ServletContextDemo7.class.getClassLoader();  47 //用类装载器读取src目录下的db1.properties配置文件  48 InputStream in = loader.getResourceAsStream("db1.properties");  49 Properties prop = new Properties();  50  prop.load(in);  51 String driver = prop.getProperty("driver");  52 String url = prop.getProperty("url");  53 String username = prop.getProperty("username");  54 String password = prop.getProperty("password");  55 response.getWriter().println("用类装载器读取src目录下的db1.properties配置文件：");  56  response.getWriter().println(  57  MessageFormat.format(  58 "driver={0},url={1},username={2},password={3}",  59  driver,url, username, password));  60  }  61  62 /**  63  * 读取类路径下面、包下面的资源文件  64  * @param response  65  * @throws IOException  66 */  67 private void test2(HttpServletResponse response) throws IOException {  68 //获取到装载当前类的类装载器  69 ClassLoader loader = ServletContextDemo7.class.getClassLoader();  70 //用类装载器读取src目录下的gacl.servlet.study包中的db4.properties配置文件  71 InputStream in = loader.getResourceAsStream("gacl/servlet/study/db4.properties");  72 Properties prop = new Properties();  73  prop.load(in);  74 String driver = prop.getProperty("driver");  75 String url = prop.getProperty("url");  76 String username = prop.getProperty("username");  77 String password = prop.getProperty("password");  78 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：");  79  response.getWriter().println(  80  MessageFormat.format(  81 "driver={0},url={1},username={2},password={3}", 82 driver,url, username, password)); 83 } 84 85 /** 86 * 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 87 */ 88 public void test3() { 89 /** 90 * 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： 91 * java.lang.OutOfMemoryError: Java heap space 92 */ 93 InputStream in = ServletContextDemo7.class.getClassLoader().getResourceAsStream("01.avi"); 94 System.out.println(in); 95 } 96 97 /** 98 * 读取01.avi,并拷贝到e:\根目录下 99 * 01.avi文件太大，只能用servletContext去读取 100 * @throws IOException 101 */ 102 public void test4() throws IOException { 103 // path=G:\Java学习视频\JavaWeb学习视频\JavaWeb\day05视频\01.avi 104 // path=01.avi 105 String path = this.getServletContext().getRealPath("/WEB-INF/classes/01.avi"); 106 /** 107 * path.lastIndexOf("\\") + 1是一个非常绝妙的写法 108 */ 109 String filename = path.substring(path.lastIndexOf("\\") + 1);//获取文件名 110 InputStream in = this.getServletContext().getResourceAsStream("/WEB-INF/classes/01.avi"); 111 byte buffer[] = new byte[1024]; 112 int len = 0; 113 OutputStream out = new FileOutputStream("e:\\" + filename); 114 while ((len = in.read(buffer)) > 0) { 115 out.write(buffer, 0, len); 116 } 117 out.close(); 118 in.close(); 119 } 120 121 public void doPost(HttpServletRequest request, HttpServletResponse response) 122 throws ServletException, IOException { 123 124 this.doGet(request, response); 125 } 126 127 }

　　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：

 1 package gacl.servlet.study;  2  3 import java.io.IOException;  4  5 import javax.servlet.ServletException;  6 import javax.servlet.http.HttpServlet;  7 import javax.servlet.http.HttpServletRequest;  8 import javax.servlet.http.HttpServletResponse;  9 10 public class ServletDemo5 extends HttpServlet { 11 12 public void doGet(HttpServletRequest request, HttpServletResponse response) 13 throws ServletException, IOException { 14 String data = "abcddfwerwesfasfsadf"; 15 /** 16  * 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 17  * 这里是将数据的缓存时间设置为1天 18 */ 19 response.setDateHeader("expires",System.currentTimeMillis() + 24 * 3600 * 1000); 20  response.getOutputStream().write(data.getBytes()); 21  } 22 23 public void doPost(HttpServletRequest request, HttpServletResponse response) 24 throws ServletException, IOException { 25 26 this.doGet(request, response); 27  } 28 29 }