JAVA WEB学习总结(六)——Servlet开发(二)
JAVA WEB学习总结(六)——Servlet开发(二)
小南超子 发表于6个月前
JAVA WEB学习总结(六)——Servlet开发(二)
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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 }

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