Java Web学习总结（3）——Servlet详解-白红宇

Java Web学习总结（3）——Servlet详解

阅读量：6194 次

发布时间：2019-06-21

本文共 20056 字，大约阅读时间需要 66 分钟。

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 
      
        2     
       
        ServletConfigDemo1
        3     
       
        gacl.servlet.study.ServletConfigDemo1
        4     
        5     
       
         6         
        
         name
         7         
        
         gacl
         8     
        9      
       
        10         
        
         password
        11         
        
         123
        12     
       13     
       
        14         
        
         charset
        15         
        
         UTF-8
        16     
       17

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("
      
");36         //获取所有的初始化参数37         Enumeration
      
        e = config.getInitParameterNames();38         while(e.hasMoreElements()){39             String name = e.nextElement();40             String value = config.getInitParameter(name);41             response.getWriter().print(name + "=" + value + "
       
");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 
       2 
      
        4     
        5     
        6     
       
         7         
        
         url
         8         
        
         jdbc:mysql://localhost:3306/test
         9     
       10 11     
       
        12         
        
         index.jsp
        13     
       14

　　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 = "abcdefghjkl
";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("
      
"); 31         readWebRootDirPropCfgFile(response);//读取WebRoot目录下的properties配置文件 32         response.getWriter().println("
      
"); 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("
      
"); 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 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 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("
      
"); 32         test2(response); 33         response.getWriter().println("
      
"); 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 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 }

转载于:https://www.cnblogs.com/zhanghaiyang/p/7213252.html

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