//*******************The Log class
import java.io.BufferedWriter;
import java.io.File;
import java.io.FileWriter;
import java.io.IOException;
import java.uitl.Date;
import java.text.DateFormat;

public class Log{
   private static final String filePath = PropertyReader.getResource("Log_File_Path");//Supposing we have define in the last ProperyReader class and the file
  
   public static final String EXCEPTION                  =   "Exception";
   public static final String CREATE_STAFF           =   "Create Staff";
   public static final String EDIT_STAFF                 =   "Edit Staff";
   public static final String DELETE_STAFF            =   "Delete Staff";
   public static final String RECORD_HAS_EXIST  =   "Record Has Exist";

   public static void log(String msg_type, Exception e){
      StringBuffer errMsg = new StringBuffer(e.toString);
     
      for(int i=0;i<e.getStackTrace().length;i++){
         errMsg.append("\n\t at");
         errMsg.append(e.getStackTrace()[i].toString);
      }
      log(msg_type,errMsg.toString());
      OptionPanel.showErrMsg("Sorry,System may have an error \n System will exit");
      System.exit(-1);
   }

  public static void log(String msg.type,Staff staff){
     String msg = null;
     if(msg_type == CREATE_STAFF){
         msg = staff.toString() + "has benn created";
     }else if(msg_type == EDIT_STAFF){
         msg = staff.toString() + "has been Changed";
     }else if(msg_type == DELETE_STAFF){
         msg = staff.toString() + "has been Deleted";
     }else if(msg_type == RECORD_HAS_EXIST){
         msg = staff.toString() + "has exist in the database";
     }
     log(msg_type,msg);
  }

  private static void log(String msg_type,String msg){
      BufferedWriter out = null;
      DateFormat df = DateFormat.getDateInstance(DateFormat.MEDIUM);
     
      try{
        out = new BufferedWriter(new FileWriter(getLogFilePath(),true));//如果為 true，則將字節(jié)寫入文件末尾處，而不是寫入文件開始處
        out.write("["+df.format(new Date()) + "] <" + msg_type + "> :" + msg);
        out.newline();
        out.newline();
      }catch(IOException e){
          e.printStackTrace();
      }finally{
         try{
           if(out!=null){
              out.close();
           }
         }catch(IOException e){
            e.printStackTrace();
         }
      }
  }

  private static String getLogFilePath(){
     File logDir = new File(filePath);
     if(!logDir.exists()){
       logDir.mkdir();
     }
    
     int i = 1;
     String fileName = filePath + "log_";
     File file = new File(fileName + i + ".txt");
   
     while(file.exists() && file.length() > 30000L) {
         i++;
         file = new File(fileName + i + ".txt");
     }
   
      return fileName + i + ".txt"
  }
}

//*****************************The OptionPanel Dialog Class for the Log Class
import javax.swing.JOptionPane;

public class OptionPanel {
   private static final String appTitle = PropertyReader.getResource("App_Title");//suposing the file has been established and the property app-title stands for the name of application
   private static final MainFrame frame = MainFrame.getMainFrame();

   public static void showWarningMsg(String msg){
      JOptionPane.showMessageDialog(frame,msg,appTitle,JOptionPane.WARNING_MESSAGE);
   }
   public static void showErrMsg(String msg){
       JOptionPane.showMessageDialog(frame,msg,appTitle,JOptionPane.Error_MESSAGE);
   }
   public static int showConfirmMsg(String msg){
        return JOptionPane.showConfirmDialog(frame,msg,appTitle,JOptionPane.YES_NO_OPTON,JOptionPane.QUESTION_MESSAGE);
   }
}

In a project, we can write a class to read the properties.As following,

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

public class PropertyReader{
   private static Properties property = null;
  
   static{
      InputSteam stream = null;
      try{
        stream=PropertyReader.class.getResourceAsStream("/resource/properties.properties");
        property = new Properties();
        property.load(stream);
      }catch(IOException e){
          e.printStackTrace();
      }finally{
          if(stream != null){
             try{
                stream.close();
             }catch(IOException e){
                e.printStackTrace();
             }           
          }
      }
   }
   public static String getResource(String key){
     if(property == null){
       return null;// init error;
     }
     
     return property.getProperty(key);
   }
}