关于DB4O的学习总结

一下是一个DB4O的操作DAO。

作为DB4O在WEB项目中的操作，如果每次调用接口NEW一个对象去取得查询文件的时候，系统会报错，表示所读的DB文件已经被锁住了。查看了网上的一些解决方式。最后自己实现，解决了问题。

 

解决方法。在WEB容器启动的时候，新建一个监听器

WebListener.java

package common.listener;

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

import common.util.db4o.LogMessageDao;

public class WebListener implements ServletContextListener  {

 
 LogMessageDao logdao;
 private static final ThreadLocal local = new ThreadLocal();
 public void contextDestroyed(ServletContextEvent sce) {
  // TODO Auto-generated method stub
  
 }

 public void contextInitialized(ServletContextEvent event) {
  
  String path =event.getServletContext().getRealPath("/log/log.db");
  logdao = logdao.getInstance(path);
  //local.set(logdao);
  event.getServletContext().setAttribute("LogDao", logdao);

//在监听器中将要用到查DB文件的DAO类放入到ServletContext中
  // TODO Auto-generated method stub
  
  
 }

 
}

然后在我们用用的时候，在一个JSP页面的用法，从ServletContext中拿出来。

LogMessageDao logdao = (LogMessageDao)config.getServletContext().getAttribute("LogDao");
  
   System.out.println("logdao:"+logdao);
   List<LogMessage> list= logdao.getAllMessage();
   for(int i=0;i<list.size();i++)
   {
    LogMessage lg = (LogMessage)list.get(i);
    %>
     <log>
     <perid><%=lg.getPerID() %></perid>
     <remoteip><%=lg.getRemoteIP() %></remoteip>
     <date><%=lg.getDate() %></date>
     </log>
    <%
   }

 

//这样就不会造成文件被锁住了。

 

下面是DAO的JAVA文件

 

package common.util.db4o;

import java.io.File;
import java.util.ArrayList;
import java.util.List;

import com.db4o.Db4o;
import com.db4o.ObjectContainer;
import com.db4o.ObjectSet;
import com.db4o.config.Configuration;
import com.db4o.io.CachedIoAdapter;
import com.db4o.io.RandomAccessFileAdapter;
import com.db4o.query.Constraint;
import com.db4o.query.Query;

public class LogMessageDao {

  private static LogMessageDao messageDao = null;
     private Configuration configuration = null;
     private ObjectContainer db = null;
     private String path;

     private LogMessageDao(String path) {
         configuration = Db4o.newConfiguration();
//         configuration.io(new RandomAccessFileAdapter());
         configuration.io(new CachedIoAdapter(new RandomAccessFileAdapter()));
         configuration.optimizeNativeQueries(true);
        
         configuration.objectClass(LogMessage.class).objectField("perID").indexed(true);
   configuration.objectClass(LogMessage.class).objectField("remoteIP").indexed(true);
   configuration.objectClass(LogMessage.class).objectField("date").indexed(true);

         this.path = path;
         openDB4O(path);
     }

     /**
      * 获取实例
      * @param path
      * @return
      */
     public static LogMessageDao getInstance(String path) {
         if (messageDao == null) {
             messageDao = new LogMessageDao(path);
         }
         return messageDao;
     }
    
         public List<LogMessage> getHistory(String to, String beginDate, String endDate,int msgType) {
         List<LogMessage> msgList = null;
         Query query = db.query();
         query.constrain(LogMessage.class);
        
         Constraint constr = query.descend("from").constrain(to);
        
         query.descend("date").constrain(beginDate).greater().equal();
         query.descend("date").constrain(endDate).smaller().equal();
        
         if(msgType == 0){  //msgType=0 表示查询普通聊天记录
             query.descend("to").constrain(to).or(constr);
         } else if(msgType == 1){  //msgType=1 表示查询群聊天记录
             query.descend("from").constrain(to);
         }
        
         query.descend("date_time").orderAscending();
        
         ObjectSet result = query.execute();
         msgList = (result == null) ? new ArrayList<LogMessage>() : result;
         return msgList;
     }

     /**
      * 查询外部ＩＭ的历史记录
      * @param to  查询对象
      * @param me  查询人的jid
      * @param con  当前xmpp连接实例
      * @param beginDate  查询的开始日期
      * @param endDate　查询的结束日期
      * @return
      */
     public List<LogMessage> getImHistory(String to, String me, String con, String beginDate, String endDate) {
         List<LogMessage> msgList;
         Query query = db.query();
         query.constrain(LogMessage.class);
        
         Constraint constr1 = query.descend("from").constrain(me);
         Constraint constr2 = query.descend("from").constrain(to);

         Constraint constr3 = query.descend("to").constrain(con).and(constr2);
        
         query.descend("date").constrain(beginDate).greater().equal();
         query.descend("date").constrain(endDate).smaller().equal();
        
         query.descend("to").constrain(to).and(constr1).or(constr3);
         query.descend("date_time").orderAscending();
        
         ObjectSet result = query.execute();       
         msgList = (result == null) ? new ArrayList() : result;
         return msgList;
     }
    
     /**
      * 获取一个群最后一个消息的收发时间
      * @param room
      * @return
      */
     public long getLastMsgDate(String room){
         Query query = db.query();
         query.constrain(LogMessage.class);
         query.descend("from").constrain(room);
         query.descend("date_time").orderDescending();
         ObjectSet<LogMessage> result = query.execute();
        
         long date_time = (result != null && result.size() > 0) ? result.get(0).getDate_time() : 0;
         return date_time;
     }

     /**
      * 保存一条消息到数据库
      * @param msg
      */
     public void saveMsg(LogMessage msg) {
         if (db.ext().isClosed()) {
             openDB4O(path);
         }
       
         db.store(msg);
         db.commit();
     }

     /**
      * 打开Ｄb４o数据库
      * @param path
      */
     public void openDB4O(String path) {
         File tempFile = new File(path);
         String tempStr = tempFile.getAbsolutePath();
         int index = tempStr.lastIndexOf(File.separatorChar);
         if (index > 0) {
             String path2 = tempStr.substring(0, tempStr.lastIndexOf(File.separatorChar));
             File file = new File(path2);
             if (!file.exists()) {
                 file.mkdirs();
             }
         }
         db = Db4o.openFile(configuration, path);
     }

     public void closeDB4O() {
         db.close();
     }
     public List<LogMessage> getAllMessage()
  {
   List<LogMessage> mymessage=null;
   
   Query query = this.db.query();
   query.constrain(LogMessage.class);
   
   ObjectSet objset = query.execute();
   mymessage = objset;
  
   return mymessage;
  }
    
     public void save(String path,String perID,String remoteIP)
  {
      if (db.ext().isClosed()) {
             openDB4O(path);
         }
   java.util.Date date = new java.util.Date();
   java.text.SimpleDateFormat df = new java.text.SimpleDateFormat("yyyy-MM-dd hh:mm:ss");
   String strDate = df.format(date); //当天日期
   
   
   LogMessage lm = new LogMessage();
   lm.setPerID(perID);
   lm.setRemoteIP(remoteIP);
   lm.setDate(strDate);
   db.store(lm);
   db.commit();
   
   
   
  }
     public static void main(String[] args) {
//          }
}