prevayler

Prevayler是一个把Java对象都保持在内存中的持久层框架,不需要数据库.并提供类似于数据库的快照存储，事务，查询等，同时还支持xml存储，远程存储等。
几个重要的接口。

Prevayler ： 核心对象，使用PrevaylerFactory把pojo对象创建
如：Prevayler prevayler = PrevaylerFactory.createPrevayler(new NumberKeeper(), "demo1");
“demo1”代表存储路径

Transation, Query, TransationWithQuery
//以上3个接口定义了事务处理的接口，并通过Prevayler 执行。

通过官方的DEMO我们可以看到Prevayler框架 提供的仅仅是持久化的功能，且其设计上对数据结构设计有很大的影响，比如要求一个系统最好存在一个核心对象用来封装其他对象，从而实现存储和事务等功能，参考Demo2就知道。

[color=red]所以Prevayler仅仅适用于小型系统，且要求数据结构关联性强，针对于复杂的查询，对象存储比起数据库存储显然也不够灵活[/color]

Helloword Demo

package org.prevayler.demo.helloword;


import java.io.IOException;

import org.prevayler.Prevayler;
import org.prevayler.PrevaylerFactory;
import org.prevayler.demos.demo2.business.Account;
import org.prevayler.demos.demo2.business.AccountEntry;
import org.prevayler.demos.demo2.business.Bank;
import org.prevayler.foundation.serialization.XStreamSerializer;

import com.thoughtworks.xstream.XStream;

public class Helloworld {

	Prevayler prevayler;

	Prevayler xmlPrevayler;

	public Helloworld() throws Exception {
		prevayler = PrevaylerFactory.createPrevayler(new HelloworldBean(), "helloworld");
		PrevaylerFactory factory = new PrevaylerFactory();
//		配置存储目录
		factory.configurePrevalenceDirectory("helloworldXml");

//		配置存储序列化工具
		factory.configureSnapshotSerializer(new XStreamSerializer() {
			protected XStream createXStream() {
//			使用XStream生成Xml
				XStream x =  new XStream();
				x.alias("helloworld", HelloworldBean.class);
				return x;
			}
		});
//		配置持久化对象
		factory.configurePrevalentSystem(new HelloworldBean());
		xmlPrevayler = factory.create();
	}

	public void sayHello(int time) throws Exception {
		prevayler.execute(new SaveHelloworld(time));
	}

	public void sayHelloXml(int time) throws Exception {
		xmlPrevayler.execute(new SaveHelloworld(time));
	}

	public HelloworldBean helloBean() {
		return (HelloworldBean) prevayler.prevalentSystem();
	}

	public HelloworldBean helloXmlBean() {
		return (HelloworldBean) xmlPrevayler.prevalentSystem();
	}

	public void save() throws Exception {
//		保存
		prevayler.takeSnapshot();
		xmlPrevayler.takeSnapshot();
	}

	public static void main(String[] args) throws Exception {
		Helloworld helloworld = new Helloworld();
		System.out.println("before say , there has say for " + helloworld.helloBean().getHelloTime() + " times");
		for (int i =0; i < 10; i++) {
			helloworld.sayHello((int)(Math.random()*10));
		}
		System.out.println("after say , there has say for " + helloworld.helloBean().getHelloTime() + " times");


		System.out.println("==========current save as xml===========");

		System.out.println("before say xml , there has say for " + helloworld.helloXmlBean().getHelloTime() + " times");
		for (int i =0; i < 10; i++) {
			helloworld.sayHelloXml((int)(Math.random()*10));
		}
		System.out.println("after say xml , there has say for " + helloworld.helloXmlBean().getHelloTime() + " times");
		helloworld.save();
	}
}

package org.prevayler.demo.helloword;

import java.io.Serializable;
import java.util.Date;

public class HelloworldBean implements Serializable {

	/**
	 * 
	 */
	private static final long serialVersionUID = -2624803617449327171L;

	private int helloTime;

	private Date lastHelloTime;

	public Date getLastHelloTime() {
		return lastHelloTime;
	}

	public void setLastHelloTime(Date lastHelloTime) {
		this.lastHelloTime = lastHelloTime;
	}

	public int getHelloTime() {
		return helloTime;
	}

	public void setHelloTime(int helloTime) {
		this.helloTime = helloTime;
	}

}

package org.prevayler.demo.helloword;

import java.util.Date;

import org.prevayler.Transaction;

public class SaveHelloworld implements Transaction {

	/**
	 * 
	 */
	private static final long serialVersionUID = 3241456994096604069L;

	private int addHelloTime;

	public SaveHelloworld(int addHelloTime) {
		this.addHelloTime = addHelloTime;
	}

	public void executeOn(Object prevalentSystem, Date executionTime) {
		HelloworldBean bean = (HelloworldBean) prevalentSystem;
		bean.setHelloTime(bean.getHelloTime() + addHelloTime);
		bean.setLastHelloTime(executionTime);
	}

}

运行helloworld

第一次输出：
before say , there has say for 0 times
after say , there has say for 69 times
==========current save as xml===========
before say xml , there has say for 0 times
after say xml , there has say for 38 times

第二次输出：
before say , there has say for 69 times
after say , there has say for 127 times
==========current save as xml===========
before say xml , there has say for 38 times
after say xml , there has say for 87 times

再看一下2种方式保存的文件
1.JavaSerializer
很多乱码。。。无法粘贴，大致就是序列化文件
2.XStreamSerializer
<helloworld>
<helloTime>87</helloTime>
<lastHelloTime>2009-10-14 10:49:36.796 CST</lastHelloTime>
</helloworld>

注： 假设不执行快照方法“helloworld.save()”,则程序运行完毕时，会自动以JavaSerializer的方式保存。。下次可以自动恢复。