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JAVA使用XML作为持久存储介质实现方法

来源：长理培训发布时间：2017-12-21 16:17:28

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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.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:

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.

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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 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; } }

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　　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; } }

责编：罗莉

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