ServletContext

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对象通讯。ServletContext对象通常也被称之为Context域对象。

查看ServletContext API文档，了解ServletContext对象的功能。

 

ServletContext应用

多个Servlet通过ServletContext对象实现数据共享

 1 package cn.itcast.context;
 2 
 3 import java.io.IOException;
 4 
 5 import javax.servlet.ServletContext;
 6 import javax.servlet.ServletException;
 7 import javax.servlet.annotation.WebServlet;
 8 import javax.servlet.http.HttpServlet;
 9 import javax.servlet.http.HttpServletRequest;
10 import javax.servlet.http.HttpServletResponse;
11 
12 /**
13  * Servlet implementation class ContextDemo1
14  * 多个ServletContext实现数据共享
15  */
16 
17 public class ContextDemo1 extends HttpServlet {
18     private static final long serialVersionUID = 1L;
19        
20     
21     /**
22      * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
23      */
24     protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
25         String data = "abcdefd";
26         ServletContext context = this.getServletConfig().getServletContext();
27         context.setAttribute("data", data);
28     }
29 
30     /**
31      * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
32      */
33     protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
34         // TODO Auto-generated method stub
35         doGet(request, response);
36     }
37 
38 }
 1 package cn.itcast.context;
 2 
 3 import java.io.IOException;
 4 
 5 import javax.servlet.ServletContext;
 6 import javax.servlet.ServletException;
 7 import javax.servlet.annotation.WebServlet;
 8 import javax.servlet.http.HttpServlet;
 9 import javax.servlet.http.HttpServletRequest;
10 import javax.servlet.http.HttpServletResponse;
11 
12 /**
13  * Servlet implementation class ContextDemo2
14  */
15 
16 public class ContextDemo2 extends HttpServlet {
17     private static final long serialVersionUID = 1L;
18        
19     
20 
21     /**
22      * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
23      */
24     protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
25         ServletContext context = this.getServletContext();
26         String data = (String) context.getAttribute("data");
27         System.out.println(data);
28     }
29 
30     /**
31      * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
32      */
33     protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
34         // TODO Auto-generated method stub
35         doGet(request, response);
36     }
37 
38 }

 可以在web.xml文件中插入<context-param>

1 <context-param>
2       <param-name>url</param-name>
3       <param-value>yyy</param-value>
4   </context-param>

获取WEB应用的初始化参数

实现Servlet的转发

利用ServletContext对象读取资源文件

得到文件路径

读取资源文件的三种方式

.properties文件（属性文件）

 

通过类装载器读取资源文件的注意事项：不适合装在大文件，否则会导师java文件内存溢出

如果需要装载大文件怎么做？

 1 //读取pranavMistry.mp4文件，并拷贝到e:\根目录下
 2     //文件比较大，只能用servletContext
 3     public void test4() throws IOException{
 4         //获取文件名
 5         String path = this.getServletContext().getRealPath("/WEB-INF/classes/pranavMistry.mp4");
 6         String filename = path.substring(path.lastIndexOf("\\"+1));
 7         
 8         InputStream in = this.getServletContext().getResourceAsStream("/WEB-INF/classes//pranavMistry.mp4");
 9         byte buffer[] = new byte[1024];
10         int len = 0;
11         
12         FileOutputStream out = new FileOutputStream("e:\\" + filename);
13         while((len=in.read(buffer))>0){
14             out.write(buffer, 0, len);
15         }
16         out.close();
17         in.close();
18     }

 

Servlet缓存技术

String data = "abcdefg";
response.setDateHeader("expires", System.currentTimeMillis()+ 24*3600*1000);
response.getOutpotStream().write(data.getBytes());

这样可以减轻服务器的压力，提高服务器性能。

 1 package cn.itcast.context;
 2 
 3 import java.io.IOException;
 4 
 5 import javax.servlet.ServletContext;
 6 import javax.servlet.ServletException;
 7 import javax.servlet.annotation.WebServlet;
 8 import javax.servlet.http.HttpServlet;
 9 import javax.servlet.http.HttpServletRequest;
10 import javax.servlet.http.HttpServletResponse;
11 
12 /**
13  *获取整个web站点的初始化参数
14  * Servlet implementation class ContextDemo3
15  */
16 
17 public class ContextDemo3 extends HttpServlet {
18     private static final long serialVersionUID = 1L;
19        
20     
21     /**
22      * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
23      */
24     protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
25         ServletContext context = this.getServletContext();
26         String url = context.getInitParameter("url");
27         System.out.println(url);
28     }
29 
30     /**
31      * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
32      */
33     protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
34         // TODO Auto-generated method stub
35         doGet(request, response);
36     }
37 
38 }

 

 1 package cn.itcast.context;
 2 
 3 import java.io.IOException;
 4 
 5 import javax.servlet.RequestDispatcher;
 6 import javax.servlet.ServletContext;
 7 import javax.servlet.ServletException;
 8 import javax.servlet.annotation.WebServlet;
 9 import javax.servlet.http.HttpServlet;
10 import javax.servlet.http.HttpServletRequest;
11 import javax.servlet.http.HttpServletResponse;
12 
13 /**用servletContext实现请求转发：mvc
14  * 注意：
15  * 1.转发之前的所有写入无效
16  * 2.转发之前response不能提交，否则转发的时候服务器会抛出异常。Cannot forward after response has been committed
17  * 
18  * 
19  * Servlet implementation class ContextDemo4
20  */
21 
22 public class ContextDemo4 extends HttpServlet {
23     private static final long serialVersionUID = 1L;
24        
25     
26     protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
27         ServletContext context = this.getServletContext();
28         RequestDispatcher rd = context.getRequestDispatcher("/ContextDemo5");
29         rd.forward(request, response);
30     }
31 
32     /**
33      * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
34      */
35     protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
36         // TODO Auto-generated method stub
37         doGet(request, response);
38     }
39 
40 }
 1 package cn.itcast.context;
 2 
 3 import java.io.IOException;
 4 import javax.servlet.ServletException;
 5 import javax.servlet.annotation.WebServlet;
 6 import javax.servlet.http.HttpServlet;
 7 import javax.servlet.http.HttpServletRequest;
 8 import javax.servlet.http.HttpServletResponse;
 9 
10 /**
11  * Servlet implementation class ContextDemo5
12  */
13 
14 public class ContextDemo5 extends HttpServlet {
15     private static final long serialVersionUID = 1L;
16        
17     /**
18      * @see HttpServlet#HttpServlet()
19      */
20     public ContextDemo5() {
21         super();
22         // TODO Auto-generated constructor stub
23     }
24 
25     /**
26      * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
27      */
28     protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
29         response.getOutputStream().write("servletdemo5".getBytes());
30     }
31 
32     /**
33      * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
34      */
35     protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
36         // TODO Auto-generated method stub
37         doGet(request, response);
38     }
39 
40 }
 1 package cn.itcast.context;
 2 
 3 import java.io.FileInputStream;
 4 import java.io.IOException;
 5 import java.io.InputStream;
 6 import java.util.Properties;
 7 
 8 import javax.servlet.ServletException;
 9 import javax.servlet.annotation.WebServlet;
10 import javax.servlet.http.HttpServlet;
11 import javax.servlet.http.HttpServletRequest;
12 import javax.servlet.http.HttpServletResponse;
13 
14 /**
15  * Servlet implementation class ContextDemo6
16  */
17 
18 public class ContextDemo6 extends HttpServlet {
19     private static final long serialVersionUID = 1L;
20        
21     /**
22      * @see HttpServlet#HttpServlet()
23      */
24     public ContextDemo6() {
25         super();
26         // TODO Auto-generated constructor stub
27     }
28 
29     /**
30      * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
31      */
32     protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
33         //在做web工程的时候，不建议采用传统方式读取文件数据。
34         //FileInputStream in = new FileInputStream("src/db.properties");
35         //System.out.println(in);
36         
37         //使用Context读取文件
38         //读取web工程中资源文件的模板代码
39         //InputStream in = this.getServletContext().getResourceAsStream("/WEB-INF/classes/db.properties");
40         //Properties prop = new Properties();
41         //prop.load(in);
42         //String driver = prop.getProperty("driver");
43         //String url = prop.getProperty("url");
44         //String username = prop.getProperty("username");
45         //String password = prop.getProperty("password");
46         //System.out.println(driver);
47         
48         
49         //获得web资源的绝对路径
50         String path = this.getServletContext().getRealPath("/WEB-INF/classes/db.properties");
51         FileInputStream in = new FileInputStream(path);
52         Properties prop = new Properties();
53         prop.load(in);
54         String driver = prop.getProperty("driver");
55         String url = prop.getProperty("url");
56         String username = prop.getProperty("username");
57         String password = prop.getProperty("password");
58         System.out.println(driver);
59     }
60 
61     /**
62      * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
63      */
64     protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
65         // TODO Auto-generated method stub
66         doGet(request, response);
67     }
68 
69 }
 1 package cn.itcast.context;
 2 
 3 import java.io.FileNotFoundException;
 4 import java.io.FileOutputStream;
 5 import java.io.IOException;
 6 import java.io.InputStream;
 7 import java.util.Properties;
 8 
 9 import javax.servlet.ServletException;
10 import javax.servlet.annotation.WebServlet;
11 import javax.servlet.http.HttpServlet;
12 import javax.servlet.http.HttpServletRequest;
13 import javax.servlet.http.HttpServletResponse;
14 
15 /**
16  * Servlet implementation class ContextDemo7
17  */
18 //用类装载器读取资源文件
19 public class ContextDemo7 extends HttpServlet {
20     private static final long serialVersionUID = 1L;
21        
22     /**
23      * @see HttpServlet#HttpServlet()
24      */
25     public ContextDemo7() {
26         super();
27         // TODO Auto-generated constructor stub
28     }
29 
30     /**
31      * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
32      */
33     protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
34         ClassLoader loader = ContextDemo7.class.getClassLoader();
35         InputStream in = loader.getResourceAsStream("db.properties");
36         Properties prop = new Properties();
37         prop.load(in);
38         String driver = prop.getProperty("driver");
39         String url = prop.getProperty("url");
40         String username = prop.getProperty("username");
41         String password = prop.getProperty("password");
42         System.out.println(driver);
43     }
44     
45     //读取pranavMistry.mp4文件，并拷贝到e:\根目录下
46     //文件比较大，只能用servletContext
47     public void test4() throws IOException{
48         //获取文件名
49         String path = this.getServletContext().getRealPath("/WEB-INF/classes/pranavMistry.mp4");
50         String filename = path.substring(path.lastIndexOf("\\"+1));
51         
52         InputStream in = this.getServletContext().getResourceAsStream("/WEB-INF/classes//pranavMistry.mp4");
53         byte buffer[] = new byte[1024];
54         int len = 0;
55         
56         FileOutputStream out = new FileOutputStream("e:\\" + filename);
57         while((len=in.read(buffer))>0){
58             out.write(buffer, 0, len);
59         }
60         out.close();
61         in.close();
62     }
63 
64     /**
65      * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
66      */
67     protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
68         // TODO Auto-generated method stub
69         doGet(request, response);
70     }
71 
72 }

 

posted @ 2017-02-11 20:06  alittlecomputer  阅读(193)  评论(0)  编辑  收藏  举报