這一期我們繼續來聊聊jdo，其實我個人覺得在撰寫
jdo的程式之前，有一個觀念很重要，就是jdo的著眼點是
在物件資料上，是把物件的狀態和內容儲存起來，而不是在
討論儲存的媒介。換言之，把jdo和其他有關資料庫的規格
相比其實是不太合適的。也因此，我們使用jdo技術的時候，
我們要想的是怎麼把我們要儲存的資料以類別正確的表示出來，
至於資料要存到哪裡？這是PersistenceManager(概念上來說，稱為
jdo的driver似乎可以理解)的工作。
        ok,我們來設計一個簡單的類別(之後的操作都以lido為例)：
===================================================
package school;

public class Course
{
	private String id="";
	private String teacher="";
	private String name="";
	
	public String getId(){return id;}
	public void setId(String id){this.id=id;}
	
	public String getTeacher(){return teacher;}
	public void setTeacher(String teacher){this.teacher=teacher;}
	
	public String getName(){return name;}
	public void setName(String name){this.name=name;}
}
===================================================
        編譯好之後，我們要"enhance"編譯出來的bytecode，在enhance之前，
我們要先寫一個metadata：
===================================================
<?xml version="1.0"?>
<!DOCTYPE jdo SYSTEM "jdo.dtd">
<jdo>
    <package name="school">
        <class name="Course"/>
    </package>
</jdo>
===================================================
        然後我們在這個metadata檔所在的路徑進行enhance的動作：
===================================================
java com.libelis.lido.Enhance
===================================================
        經過enhance的動作，bytecode才可以被lido所實作的PersistenceManager
使用。接下來，我們假設儲存媒體是lido內附的McKoi SQL Database，
我們要撰寫一個設定檔，讓lido根據這個設定檔對McKoi SQL Database做操作
(舉例來說，lido會根據設定檔和metadata建立database裡面的table)，設定檔
如下：
===================================================
# lido.properties file

# jdo standard properties

javax.jdo.option.connectionURL=jdbc:mckoi://localhost/
javax.jdo.option.connectionDriverName=com.mckoi.JDBCDriver
javax.jdo.option.connectionUserName=sa
javax.jdo.option.connectionPassword=

javax.jdo.option.msWait=5
javax.jdo.option.multithreaded=false
javax.jdo.option.optimistic=false
javax.jdo.option.retainValues=false
javax.jdo.option.restoreValues=true
javax.jdo.option.nontransactionalRead=true
javax.jdo.option.nontransactionalWrite=false
javax.jdo.option.ignoreCache=false


# set to PM, CACHE, or SQL to have some traces
# ex:
#lido.trace=SQL,DUMP,CACHE

#  set the Statement pool size
lido.sql.poolsize=10
lido.cache.entry-type=weak

# set the max batched statement
# 0: no batch
# default is 20

lido.sql.maxbatch=30
lido.objectpool=90

# set for PersistenceManagerFactory pool limits
lido.minPool=1
lido.maxPool=10

jdo.metadata=metadata.jdo

===================================================
        接著，我們只要撰寫應用程式的主體就可以了，撰寫的時候抓住
幾個大原則：
1.透過JDOHelper.getPersistenceManagerFactory(Properties)取得
PersistenceManagerFactory  
2.透過PersistenceManagerFactory取得PersistenceManager
3.透過PersistenceManager取得Transaction   
4.對persistence物件的操作，要在transaction.begin()和transaction.commit()之間操作
        我們來看看範例程式：
===================================================
package school;

import javax.jdo.Transaction;
import javax.jdo.PersistenceManager;
import javax.jdo.Query;
import javax.jdo.PersistenceManagerFactory;
import javax.jdo.*;
import java.util.*;

public class test {

    public static void main(String[] args) {
    	Properties prop=new Properties();
    	prop.put("javax.jdo.PersistenceManagerFactoryClass","com.libelis.lido.PersistenceManagerFactory");
    	prop.put("javax.jdo.option.ConnectionURL","jdbc:mckoi://localhost/");
    	prop.put("javax.jdo.option.ConnectionDriverName","com.mckoi.JDBCDriver");
    	prop.put("javax.jdo.option.ConnectionUserName","sa");
    	prop.put("javax.jdo.option.ConnectionPassword","");
        PersistenceManagerFactory pmf = JDOHelper.getPersistenceManagerFactory(prop);
        PersistenceManager pm = pmf.getPersistenceManager();
        Transaction tx = pm.currentTransaction();
  
        tx.begin(); //儲存Course
        {
            System.out.println("Creating a new Course");
            Course course=new Course();
            course.setId("id1");
            course.setTeacher("teacher1");
            course.setName("course1");
            pm.makePersistent(course);
        }
        tx.commit();

        tx.begin(); // Query
        {
            System.out.println("Retrieve all Courses");
            Query query = pm.newQuery(Course.class);
           String filter = "name==\"course1\""; 
           query.setFilter(filter);
            Collection courses = (Collection) query.execute();
            Iterator it = courses.iterator();
            while (it.hasNext()) { 
                Course course = (Course) it.next();
                String name = course.getName();
                System.out.println(name);
            }
            pm.deletePersistentAll(courses);
        }
        tx.commit();
        
        System.exit(0);
    }
}

===================================================