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

import java.util.HashMap;
import java.util.Map;

public class FileUploadAppProperties {
    
    private Map<String, String> properties = new HashMap<>();
    
    private FileUploadAppProperties(){}
    
    private static FileUploadAppProperties instance = new FileUploadAppProperties();
    
    public static FileUploadAppProperties getInstance() {
        return instance;
    }
    
    public void addProperty(String propertyName, String propertyValue){
        properties.put(propertyName, propertyValue);
    }
    
    public String getProperty(String propertyName){
        return properties.get(propertyName);
    }
    
}

监听器：FileUploadAppListener

import java.io.File;
import java.io.IOException;
import java.io.InputStream;
import java.util.Map;
import java.util.Properties;

import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;

import com.atguigu.fileupload.app.utils.FileUploadAppProperties;

/**
 * Application Lifecycle Listener implementation class FileUploadAppListener
 *
 */
public class FileUploadAppListener implements ServletContextListener {

    /**
     * Default constructor. 
     */
    public FileUploadAppListener() {
        // TODO Auto-generated constructor stub
    }

    /**
     * @see ServletContextListener#contextInitialized(ServletContextEvent)
     */
    public void contextInitialized(ServletContextEvent arg0) {
        InputStream in = getClass().getClassLoader().getResourceAsStream("/upload.properties");
        
        Properties properties = new Properties();
        try {
            properties.load(in);
            
            for(Map.Entry<Object, Object> prop: properties.entrySet()){
                String propertyName = (String) prop.getKey();
                String propertyValue = (String) prop.getValue();
                
                FileUploadAppProperties.getInstance().addProperty(propertyName, propertyValue);
            }
            
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    /**
     * @see ServletContextListener#contextDestroyed(ServletContextEvent)
     */
    public void contextDestroyed(ServletContextEvent arg0) {
        // TODO Auto-generated method stub
    }
    
}

src / upload.properties

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