EJB 3.0 Outside the Container and Inside the JVM - Part 1: The POJO Model

NOTE: This is a more truncated version of an entry I started previously. The aim is to keep it simpler and to actually finish this it...

ANOTHER NOTE: I use Netbeans. It's not popular but I like it. If you don't have Netbeans you'll still be able to follow this series but won't be able to drag and drop a Swing UI in part 4 using Matisse. My heart bleeds...

EJB 3.0 and JPA 1.0 have made it easy for people like me (RDBMS-phobics) to persist our O-O domain models. What's more, Derby (aka JavaDB) has made it easy to have within-JVM RDBMS persistence that is transparent to the user of your thick client application. What follows is a simple example application I wrote to learn some of the basic concepts (and also track my expenses claims)

Step 0: Setting up my project.

I like Maven; Maven 1.0.x to be precise. Before anything else I set up a basic maven project called "expenses-tracker-model" in the standard fashion:


You'll also need to manually get hold of the JPA and Derby Jars (toplink-essentials.jar, derby-10.1.jar and javaee-9.jar) and place them in your repository wherever you see fit.

Step 1: Coding the Domain Model

I like to model in OO land. My model is very simple - a single, serializable POJO class called "Claim" - which will eventually map to a single Derby table called "CLAIM". I create my java class and add attributes as follows:

private Long id;
private String refNumber;
private String projectCode;
private String description;
private Date dateFrom;
private Date dateTo;
private String status;
private Date submitDate;
private Date paidDate;
private Long totalClaimed;
private Long totalFromCurrentAccount;

I now need to add the annotations which will allow JPA to work its magic. First I tell the class that it is an entity (i.e. will be represented in the RDBMS) with the following annotation:

import javax.persistence.Entity;
...
@Entity
public class Claim implements Serializable {
...

The compiler is now clever enough to know that, unless told otherwise, the attributes on this class will be represented as columns in our table. That's how EJB 3.0 works. It assumes the default unless you tell it different. Very handy.

The observant amongst you will realise that we most likely need a primary key. You're right. We need to declare which will be our primary key field and how we will work with it. This is done with the @Id annotation. The @GeneratedValue annotation tells JPA that we'dlike the RDBMS to auto generate our PKs for us (which makes things even easier at our O-O end):

import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
...
/** Unique, datastore generated id */
@Id
@GeneratedValue(strategy = GenerationType.AUTO)
private Long id;
...

Finally, because we have java.util.Date fields, we need to provide some extra information about these for JPA also:

import javax.persistence.Temporal;
import javax.persistence.TemporalType;
...
@Temporal(TemporalType.TIMESTAMP)
private Date dateFrom;

We're nearly there. Another quick IDE aided step to encapsulate our attributes with some getters and setters and we're ready to go. You should now have something like this:

package com.andrewharmellaw.exptracker.model;

import java.io.Serializable;
import java.util.Date;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.Temporal;
import javax.persistence.TemporalType;

@Entity
public class Claim implements Serializable {

/** Unique, datastore generated id */
@Id
@GeneratedValue(strategy = GenerationType.AUTO)
private Long id;

private String refNumber;

private String projectCode;

private String description;

@Temporal(TemporalType.TIMESTAMP)
private Date dateFrom;

@Temporal(TemporalType.TIMESTAMP)
private Date dateTo;

private String status;

@Temporal(TemporalType.TIMESTAMP)
private Date submitDate;

@Temporal(TemporalType.TIMESTAMP)
private Date paidDate;

private Long totalClaimed;

private Long totalFromCurrentAccount;

/** Creates a new instance of Claim */
public Claim() {

}

public Long getId() {
return id;
}

public void setId(Long id) {
this.id = id;
}

public String getRefNumber() {
return refNumber;
}

public void setRefNumber(String refNumber) {
this.refNumber = refNumber;
}

public String getProjectCode() {
return projectCode;
}

public void setProjectCode(String projectCode) {
this.projectCode = projectCode;
}

public String getDescription() {
return description;
}

public void setDescription(String description) {
this.description = description;
}

public Date getDateFrom() {
return dateFrom;
}

public void setDateFrom(Date dateFrom) {
this.dateFrom = dateFrom;
}

public Date getDateTo() {
return dateTo;
}

public void setDateTo(Date dateTo) {
this.dateTo = dateTo;
}

public String getStatus() {
return status;
}

public void setStatus(String status) {
this.status = status;
}

public Date getSubmitDate() {
return submitDate;
}

public void setSubmitDate(Date submitDate) {
this.submitDate = submitDate;
}

public Date getPaidDate() {
return paidDate;
}

public void setPaidDate(Date paidDate) {
this.paidDate = paidDate;
}

public Long getTotalClaimed() {
return totalClaimed;
}

public void setTotalClaimed(Long totalClaimed) {
this.totalClaimed = totalClaimed;
}

public Long getTotalFromCurrentAccount() {
return totalFromCurrentAccount;
}

public void setTotalFromCurrentAccount(Long totalFromCurrentAccount) {
this.totalFromCurrentAccount = totalFromCurrentAccount;
}
}

Right, that's the dull part done. Now we can get to the fun part - using this POJO to auto-create our database. That's in the next post in this series...