使用XML作为持久层解决方案的,它的基本功能要有:

1.对象的CRUD功能(本例中基本对象是Member类).

2.保证线程安全,对桌面程序和Web程序都一样适用.

3.有缓存,在存储介质即XML文件突然丢失的情况下还能有效工作.

本例采用了MemberService和MemberPersistence两个类来协作完成这些功能.

MemberService是业务层和持久层之间的桥梁,用于对Member对象的CRUD操作,内置一个hashtable来缓存Member对象,即使文件突然丢失,缓存中的数据也不会被影响.它内置一个MemberPersistence成员来完成与持久介质的交互.

实现添加,删除,更新的三个函数add(),delete(),update()都用lockObj实现了同步,这样就无需担心线程安全问题.其它函数对members成员没有修改,故不需要实现同步.

try{ 

memberPersistence.add(member); 
members.put(member.getId(), member); 
return true; 
}
catch(XmlFileWriteException ex){ 
   System.out.println("Member:" + member.getId() + " add error!"); 
   return false; 
}

当MemberPersistence添加对象成功后,这个对象才会被添加到members中,这样保证了缓存和实际数据的同步;如果颠倒一下顺序,那末MemberPersistence添加对象不成功时,出现XmlFileWriteException异常,这是还需要在catch中删除刚才添加的对象,这样做人为加大了程序的复杂度,不如上面的做法简单高效.

关于查询函数的做法不明白的请见 http://www.blogjava.net/sitinspring/archive/2007/06/05/122119.html 中形式三.

下面是MemberService类的全部代码:

package com.

import java.util.ArrayList; 
import java.util.Collections; 
import java.util.Hashtable; 
import java.util.Iterator; 
import java.util.List; 
import com.sitinpsring.domain.Member; 
import com.sitinpsring.domain.MemberFilter; 
import com.sitinpsring.exception.XmlFileReadException; 
import com.sitinpsring.exception.XmlFileWriteException; 
import com.sitinpsring.persistence.MemberPersistence; 
public class MemberService { 
private static Hashtable members; 
private static MemberPersistence memberPersistence; 
private static final Object lockObj = new Object(); 
static { 
try { 
memberPersistence = new MemberPersistence("member.xml"); 
members = memberPersistence.loadMemberFromFile(); 
} catch (XmlFileReadException ex) { 
System.out.println("Can’t read the file:member.xml"); 
}catch (XmlFileWriteException ex) { 
System.out.println("Can’t write to the file:member.xml"); 
} 
} 
public MemberService() { 
} 
public boolean hasMember(String id) { 
return members.containsKey(id); 
} 
public boolean hasMember(Member member) { 
return hasMember(member.getId()); 
} 
public boolean add(Member member) { 
if (hasMember(member)) { 
System.out.println("Member:" + member.getId() + " has been exist!"); 
return false; 
} else { 
synchronized (lockObj) { 
try{ 
memberPersistence.add(member); 
members.put(member.getId(), member); 
return true; 
} 
catch(XmlFileWriteException ex){ 
System.out.println("Member:" + member.getId() + " add error!"); 
return false; 
} 
} 
} 
} 
public boolean update(Member member) { 
if (hasMember(member)) { 
synchronized (lockObj) { 
try{ 
memberPersistence.update(member); 
Member oldMember = members.get(member.getId()); 
oldMember.setName(member.getName()); 
oldMember.setAge(member.getAge()); 
return true; 
} 
catch(XmlFileWriteException ex){ 
System.out.println("Member:" + member.getId() + " upate error!"); 
return false; 
} 
} 
} else { 
System.out.println("Member:" + member.getId() 
+ " can’t been found!"); 
return false; 
} 
} 
public boolean saveOrUpdate(Member member) { 
if (hasMember(member)) { 
return update(member); 
} else { 
return add(member); 
} 
} 
public boolean delete(Member member) { 
if (hasMember(member)) { 
synchronized (lockObj) { 
try{ 
memberPersistence.delete(member.getId()); 
members.remove(member.getId()); 
return true; 
}catch(XmlFileWriteException ex){ 
System.out.println("Member:" + member.getId() + " delete error!"); 
return false; 
} 
} 
} else { 
System.out.println("Member:" + member.getId() 
+ " can’t been found!"); 
return false; 
} 
} 
@SuppressWarnings("unchecked") 
public List search(MemberFilter memberFilter) { 
ArrayList retval = new ArrayList(); 
for (Iterator it = members.keySet().iterator(); it.hasNext();) { 
String key = (String) it.next(); 
Member member = members.get(key); 
if (memberFilter.accept(member)) { 
retval.add(member); 
} 
} 
Collections.sort(retval); 
return retval; 
} 
public List getAll() { 
MemberFilter filter = new MemberFilter() { 
public boolean accept(Member member) { 
return true; 
} 
}; 
return search(filter); 
} 
public Member getMemberById(String id) { 
for (Iterator it = members.keySet().iterator(); it.hasNext();) { 
String key = (String) it.next(); 
Member member = members.get(key); 
if (member.getId().equals(id)) { 
return member; 
} 
} 
return null; 
} 
}

MemberPersistence类是与XML文件打交道的类,通过它,数据才能真正存入持久介质-XML文件.它的函数都很好理解.这些函数工作时实际只会引发两种异常--读XML文件异常(一般由多个根节点导致)和写XML文件异常,会触发这些异常的函数都应该对他们进行捕获和抛出处理.

package com.sitinpsring.persistence; 

import java.io.File; 
import java.io.FileWriter; 
import java.util.Hashtable; 
import java.util.Iterator; 
import java.util.List; 
import org.dom4j.Document; 
import org.dom4j.DocumentHelper; 
import org.dom4j.Element; 
import org.dom4j.io.SAXReader; 
import org.dom4j.io.XMLWriter; 
import com.sitinpsring.domain.Member; 
import com.sitinpsring.exception.XmlFileReadException; 
import com.sitinpsring.exception.XmlFileWriteException; 
public class MemberPersistence { 
private String xmlFilePos; 
private Document document; 
public MemberPersistence(String xmlFilePos) 
throws XmlFileReadException,XmlFileWriteException{ 
this.xmlFilePos = xmlFilePos; 
if (isFileExist(this.xmlFilePos) == false) { 
// Create document when file not exist 
            createDocument(); 
return; 
} else { 
// Get Docunent when file exist 
            SAXReader reader = new SAXReader(); 
try { 
document = reader.read(new File(this.xmlFilePos)); 
} catch (Exception ex) { 
throw new XmlFileReadException(ex.getMessage()); 
} 
} 
} 
private void createDocument() throws XmlFileWriteException{ 
String text = ""; 
try { 
document = DocumentHelper.parseText(text); 
writeDocumentToFile(); 
} catch (XmlFileWriteException ex) { 
throw ex; 
}catch (Exception ex) { 
ex.printStackTrace(); 
} 
} 
private void writeDocumentToFile() throws XmlFileWriteException{ 
try { 
XMLWriter writer = new XMLWriter(new FileWriter(this.xmlFilePos)); 
writer.write(document); 
writer.close(); 
} catch (Exception ex) { 
throw new XmlFileWriteException(ex.getMessage()); 
} 
} 
public Hashtable loadMemberFromFile() { 
Hashtable retval=new Hashtable(); 
List nodes = document.getRootElement().elements("member"); 
for (Iterator it = nodes.iterator(); it.hasNext();) { 
Element elm = (Element) it.next(); 
Member member = new Member(elm.attributeValue("id"),elm.elementText("name"),
Integer.parseInt(elm.elementText("age"))); 
retval.put(member.getId(), member); 
} 
return retval; 
} 
public boolean add(Member member) throws XmlFileWriteException{ 
try { 
Element rootElm = document.getRootElement(); 
Element newMemberElm = rootElm.addElement("member"); 
newMemberElm.addAttribute("id", member.getId()); 
Element nameElm=newMemberElm.addElement("name"); 
nameElm.setText(member.getName()); 
Element ageElm=newMemberElm.addElement("age"); 
ageElm.setText(String.valueOf(member.getAge())); 
writeDocumentToFile(); 
return true; 
} catch (XmlFileWriteException ex) { 
throw ex; 
} 
} 
public boolean update(Member member) throws XmlFileWriteException{ 
try { 
Element rootElm = document.getRootElement(); 
List nodes = rootElm.elements("member"); 
for (Iterator it = nodes.iterator(); it.hasNext();) { 
Element elm = (Element) it.next(); 
if(elm.attributeValue("id").equals(member.getId())){ 
elm.element("name").setText(member.getName()); 
elm.element("age").setText(String.valueOf(member.getAge())); 
break; 
} 
} 
writeDocumentToFile(); 
return false; 
} catch (XmlFileWriteException ex) { 
throw ex; 
} 
} 
public boolean delete(String id) throws XmlFileWriteException{ 
try { 
Element rootElm = document.getRootElement(); 
List nodes = rootElm.elements("member"); 
for (Iterator it = nodes.iterator(); it.hasNext();) { 
Element elm = (Element) it.next(); 
Member member = new Member(elm.attributeValue("id"),elm.elementText("name"),
Integer.parseInt(elm.elementText("age"))); 
if(member.getId().equals(id)){ 
rootElm.remove(elm); 
break; 
} 
} 
writeDocumentToFile(); 
return false; 
} catch (XmlFileWriteException ex) { 
throw ex; 
} 
} 
public static boolean isFileExist(String filePath) { 
boolean exists = (new File(filePath)).exists(); 
return exists; 
} 
}

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