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;
}
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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;
}
}
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