先码后看 severlet开发基础侵立删

转自：www.cnblogs.com/xdp-gacl/p/3763559.html

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     @Override25     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 gacl18  * 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 response42      * @throws IOException43      */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 response63      * @throws FileNotFoundException64      * @throws IOException65      */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 response87      * @throws IOException88      */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 response112      * @throws IOException113      */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 gacl19  *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 response42      * @throws IOException43      */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 response65      * @throws IOException66      */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 space92          */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 IOException101      */102     public void test4() throws IOException {103         // path=G:\Java学习视频\JavaWeb学习视频\JavaWeb\day05视频\01.avi104         // path=01.avi105         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 }

原文：www.cnblogs.com/xdp-gacl/p/3763559.html