Java使用XML作為持久存儲介質(zhì)實(shí)現(xiàn)方法

作者：佚名 2009-04-09 13:58:58

對一些需要將數(shù)據(jù)持久化的小型程序中,傳統(tǒng)的關(guān)系型數(shù)據(jù)庫顯得龐大而不實(shí)用,OO數(shù)據(jù)庫有一個學(xué)習(xí)曲線的問題,而使用XML是一種較好的選擇.本文將就設(shè)計(jì)一個合理的XML持久化的解決方案進(jìn)行探討.

使用XML作為持久層解決方案的,它的基本功能要有:

1.對象的CRUD功能(本例中基本對象是Member類).

2.保證線程安全,對桌面程序和Web程序都一樣適用.

3.有緩存,在存儲介質(zhì)即XML文件突然丟失的情況下還能有效工作.

本例采用了MemberService和MemberPersistence兩個類來協(xié)作完成這些功能.

MemberService是業(yè)務(wù)層和持久層之間的橋梁,用于對Member對象的CRUD操作,內(nèi)置一個hashtable來緩存Member對象,即使文件突然丟失,緩存中的數(shù)據(jù)也不會被影響.它內(nèi)置一個MemberPersistence成員來完成與持久介質(zhì)的交互.

實(shí)現(xiàn)添加,刪除,更新的三個函數(shù)add(),delete(),update()都用lockObj實(shí)現(xiàn)了同步,這樣就無需擔(dān)心線程安全問題.其它函數(shù)對members成員沒有修改,故不需要實(shí)現(xiàn)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; 
}

當(dāng)MemberPersistence添加對象成功后,這個對象才會被添加到members中,這樣保證了緩存和實(shí)際數(shù)據(jù)的同步;如果顛倒一下順序,那末MemberPersistence添加對象不成功時,出現(xiàn)XmlFileWriteException異常,這是還需要在catch中刪除剛才添加的對象,這樣做人為加大了程序的復(fù)雜度,不如上面的做法簡單高效.

關(guān)于查詢函數(shù)的做法不明白的請見 http://www.blogjava.net/sitinspring/archive/2007/06/05/122119.html 中形式三.

下面是MemberService類的全部代碼:

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類是與XML文件打交道的類,通過它,數(shù)據(jù)才能真正存入持久介質(zhì)-XML文件.它的函數(shù)都很好理解.這些函數(shù)工作時實(shí)際只會引發(fā)兩種異常--讀XML文件異常(一般由多個根節(jié)點(diǎn)導(dǎo)致)和寫XML文件異常,會觸發(fā)這些異常的函數(shù)都應(yīng)該對他們進(jìn)行捕獲和拋出處理.

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; 
} 
}