Implementing a Revision History Strategy

Hello.

I've been working on some way to keep a revision history in a way that is transparente to the people using the pesistence layer. I've just implemented a proof of concept using @PreUpdate entity listener that does just that. Here it goes:

--Car.java--

package ejb3.tests.entitylistener.persistence;

import javax.persistence.Column;

import javax.persistence.EmbeddedId;

import javax.persistence.Entity;

import javax.persistence.Table;

@Entity

@Table(name = "CAR")

public class Car {

@EmbeddedId

private CarId id;

@Column(name = "HP")

private int horsePower;

//Due to a stupid hibernate bug, when we call persist() on

//the EntityManager, the method @PrePersist, which should be

//called befoe any validation are made, is called after

//hibernate verifies if the non-generated id field is null,

//which result in an Exception. Therefore we must instantiate

//it inside the constructor.

public Car() {

this.id = new CarId();

}

public Car(long carIdId, int version, int horsePower) {

this.id = new CarId();

this.id.setId(carIdId);

this.id.setVersion(version);

this.horsePower = horsePower;

}

public CarId getId() {

return this.id;

}

public void setId(CarId id) {

this.id = id;

}

public int getHorsePower() {

return this.horsePower;

}

public void setHorsePower(int horsePower) {

this.horsePower = horsePower;

}

}

-CarId.java

package ejb3.tests.entitylistener.persistence;

import javax.persistence.Column;

import javax.persistence.Embeddable;

@Embeddable

public class CarId implements java.io.Serializable {

private static final long serialVersionUID = 1;

private long id;

private int version;

public CarId() {

}

public CarId(long id, int version) {

this.id = id;

this.version = version;

}

@Column(name="ID", nullable=false)

public long getId() {

return this.id;

}

public void setId(long id) {

this.id = id;

}

@Column(name="VERSION", nullable=false)

public int getVersion() {

return this.version;

}

public void setVersion(int version) {

this.version = version;

}

public boolean equals(Object other) {

if ( (this == other ) ) return true;

if ( (other == null ) ) return false;

if ( !(other instanceof CarId) ) return false;

CarId castOther = ( CarId ) other;

return (this.getId()==castOther.getId())

&& (this.getVersion()==castOther.getVersion());

}

public int hashCode() {

int result = 17;

result = 37 * result + (int) this.getId();

result = 37 * result + this.getVersion();

return result;

}

}

-CarListener.java

package ejb3.tests.entitylistener.persistence.listener;

import java.sql.Connection;

import java.sql.PreparedStatement;

import java.sql.ResultSet;

import java.sql.SQLException;

import javax.naming.InitialContext;

import javax.naming.NamingException;

import javax.persistence.PrePersist;

import javax.persistence.PreUpdate;

import javax.sql.DataSource;

import ejb3.tests.entitylistener.persistence.Car;

import ejb3.tests.entitylistener.persistence.CarId;

public class CarListener {

private static long initialId = 0;

private Car getCar(long id, int version) throws NamingException, SQLException {

InitialContext ic = new InitialContext();

Object o = ic.lookup(

"java:/DefaultDS");

DataSource ds = (DataSource)o;

Connection conn = ds.getConnection();

PreparedStatement ps =

conn.prepareStatement("SELECT * FROM CAR WHERE ID = ? AND VERSION = ?");

ps.setObject(1, id);

ps.setObject(2, version);

ResultSet rs = ps.executeQuery();

rs.next();

long otherId = (Long)rs.getObject(1);

int otherVersion = (Integer)rs.getObject(2);

int otherHorsePower = (Integer)rs.getObject(3);

rs.close();

ps.close();

conn.close();

return new Car(otherId, otherVersion, otherHorsePower);

}

private void setCar(long id, int version, int horsePower) throws NamingException, SQLException {

InitialContext ic = new InitialContext();

Object o = ic.lookup(

"java:/DefaultDS");

DataSource ds = (DataSource)o;

Connection conn = ds.getConnection();

PreparedStatement ps =

conn.prepareStatement("INSERT INTO CAR VALUES (?, ?, ?)");

ps.setObject(1, id);

ps.setObject(2, version);

ps.setObject(3, horsePower);

ps.executeUpdate();

ps.close();

conn.close();

}

@PreUpdate

public void postUpdate(Car toUpdate) throws NamingException, SQLException {

synchronized(CarListener.class) {

Car c = this.getCar(toUpdate.getId().getId(), toUpdate.getId().getVersion());

boolean bool1 = c.getId().equals(toUpdate.getId());

boolean bool2 = c.getHorsePower() == toUpdate.getHorsePower();

if(!(bool1 && bool2)) {

CarId toUpdateId = toUpdate.getId();

long carIdId = toUpdateId.getId();

int currentVersion = toUpdateId.getVersion();

int newVersion = ++currentVersion;

int horsePower = toUpdate.getHorsePower();

setCar(carIdId, newVersion, horsePower);

toUpdateId.setVersion(newVersion);

}

}

}

@PrePersist

public void createNewId(Car toPersist) {

if(toPersist.getId().getId() == 0) {

synchronized(CarListener.class) {

toPersist.getId().setId(++initialId);

toPersist.getId().setVersion(1);

}

}

}

}

--

Tie the CarListener to the Car via orm.xml's <entity-listeners> node.

If you find a better way to do this, please advise.

Hugo Oliveira

hugom.oliveira.ext@siemens.com

Hugo_Oliveiraa at 2007-7-10 3:19:29 >