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Java使用XML作为持久存储介质实现方法

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对一些需要将数据持久化的小型程序中,传统的关系型数据库显得庞大而不实用,OO数据库有一个学习曲线的问题,而使用XML是一种较好的选择.本文将就设计一个合理的XML持久化的解决方案进行探讨.

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.sitinpsring.service;
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<String, Member> 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<Member> search(MemberFilter memberFilter) {
ArrayList<Member> retval = new ArrayList<Member>();
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<Member> 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 = "<members></members>";
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<String, Member> loadMemberFromFile() {
Hashtable<String, Member> retval=new Hashtable<String, Member>();
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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