单例模式应用_ _ _同时涉及到监听器的应用

当第一次启动项目的时候，就需要访问一些数据，仔细思考一下，

　　1）就会想到该数据的应该是全局的变量，而且不会被改动，这时候我们想到了单例

　　2）又 “第一次启动项目的时候” 这句话，我们想到了监听器，具体就想到了 ServletContextListener

eg:

单例：FileUploadAppProperties

 1 import java.util.HashMap;
 2 import java.util.Map;
 3 
 4 public class FileUploadAppProperties {
 5     
 6     private Map<String, String> properties = new HashMap<>();
 7     
 8     private FileUploadAppProperties(){}
 9     
10     private static FileUploadAppProperties instance = new FileUploadAppProperties();
11     
12     public static FileUploadAppProperties getInstance() {
13         return instance;
14     }
15     
16     public void addProperty(String propertyName, String propertyValue){
17         properties.put(propertyName, propertyValue);
18     }
19     
20     public String getProperty(String propertyName){
21         return properties.get(propertyName);
22     }
23     
24 }

监听器：FileUploadAppListener

 1 import java.io.File;
 2 import java.io.IOException;
 3 import java.io.InputStream;
 4 import java.util.Map;
 5 import java.util.Properties;
 6 
 7 import javax.servlet.ServletContextEvent;
 8 import javax.servlet.ServletContextListener;
 9 
10 import com.atguigu.fileupload.app.utils.FileUploadAppProperties;
11 
12 /**
13  * Application Lifecycle Listener implementation class FileUploadAppListener
14  *
15  */
16 public class FileUploadAppListener implements ServletContextListener {
17 
18     /**
19      * Default constructor. 
20      */
21     public FileUploadAppListener() {
22         // TODO Auto-generated constructor stub
23     }
24 
25     /**
26      * @see ServletContextListener#contextInitialized(ServletContextEvent)
27      */
28     public void contextInitialized(ServletContextEvent arg0) {
29         InputStream in = getClass().getClassLoader().getResourceAsStream("/upload.properties");
30         
31         Properties properties = new Properties();
32         try {
33             properties.load(in);
34             
35             for(Map.Entry<Object, Object> prop: properties.entrySet()){
36                 String propertyName = (String) prop.getKey();
37                 String propertyValue = (String) prop.getValue();
38                 
39                 FileUploadAppProperties.getInstance().addProperty(propertyName, propertyValue);
40             }
41             
42         } catch (IOException e) {
43             e.printStackTrace();
44         }
45     }
46 
47     /**
48      * @see ServletContextListener#contextDestroyed(ServletContextEvent)
49      */
50     public void contextDestroyed(ServletContextEvent arg0) {
51         // TODO Auto-generated method stub
52     }
53     
54 }

src / upload.properties

1 exts=pptx,docx,doc
2 file.max.size=1048576
3 total.file.max.size=5242880

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