OpenJPA

The tutorial from which I learnt openJPA was mentioning about Apache Derby. I have written a small intro about Apache Derby in the following link ;)

Data is the most important part of all programs. We have been using databases and SQL statements for years. However, better approaches exists! Especially object-oriented approaches for data persistence is very useful.

Some examples: EJB, JDO, Hibernate, Toplink... (The last 2 are object-relational mapping tool)

Java persistence API = JPA (standard)

first introduced as a part of EJB 3.0.

openJPA is another solution for data persistency. There are 3 artifacts:

1. entity class
2. persistence.xml (in src/META-INF)
3. A class through which you will insert, update or find an entity.

• Firstly, we should download openJPA from the following link:

http://openjpa.apache.org/downloads.html

• Then add the jars to the project's build-path.

Let's say we have a DB table CUSTOMER:

NAME	PK?	TYPE	NULL?
CUST_ID	Y	INTEGER	NOT NULL
FIRST_NAME		VARCHAR(50)	NOT NULL
LAST_NAME		VARCHAR(50)
STREET		VARCHAR(50)
APPT		VARCHAR(20)	NOT NULL
CITY		VARCHAR(25)
ZIP_CODE		VARCHAR(10)	NOT NULL
CUST_TYPE		VARCHAR(10)	NOT NULL
LAST_UPDATED_TIME		TIMESTAMP	NOT NULL

Now we should have an entity class like this:

package entity;

import javax.persistence.*;

import java.io.Serializable;

import java.util.Date;

/*

 * CUSTOMER ENTITY CLASS

 */

@EntityListeners({CustListner.class})

@Entity(name = "CUSTOMER") //Name of the entity

public class Customer implements Serializable{

@Id //signifies the primary key

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

@GeneratedValue(strategy = GenerationType.AUTO)

private long custId;

@Column(name = "FIRST_NAME", nullable = false,length = 50)

private String firstName;

@Column(name = "LAST_NAME", length = 50)

private String lastName;

@Embedded

private Address address = new Address();

@Column(name = "CUST_TYPE", length = 10)

private String custType;

@Version

@Column(name = "LAST_UPDATED_TIME")

private Date updatedTime;

//Getters and setters

public long getCustId() {

return custId;

}

public void setCustId(long custId) {

this.custId = custId;

}

public String getFirstName() {

return firstName;

}

public void setFirstName(String firstName) {

this.firstName = firstName;

}

public String getLastName() {

return lastName;

}

public void setLastName(String lastName) {

this.lastName = lastName;

}

public Date getUpdatedTime() {

return updatedTime;

}

public void setUpdatedTime(Date updatedTime) {

this.updatedTime = updatedTime;

}

public Address getAddress() {

return address;

}

public void setAddress(Address address) {

this.address = address;

}

public String getCustType() {

return custType;

}

public void setCustType(String custType) {

this.custType = custType;

}

// ToString()

public String toString() {

      StringBuffer sb = new StringBuffer();

      sb.append("custId : " + custId);

      sb.append("   First Name : " + firstName);

      sb.append("   Last Name : " + lastName);

      sb.append("   customer type : " + custType);

      return sb.toString();

   }

}

And here is the sample persistence.xml 

<properties>

            <property name="openjpa.ConnectionDriverName" 

                value="org.apache.derby.jdbc.EmbeddedDriver"/>

      <property name="openjpa.ConnectionURL" 

                value="jdbc:derby:C:/Users/tr1t6321/ipeks"/>

            <property name="openjpa.ConnectionUserName"  value=""/>

            <property name="openjpa.ConnectionPassword" value=""/>

            <property name="openjpa.Log" value="DefaultLevel=TRACE"/>

        </properties>

!!! persistence.xml should be in src/META-INF.


......Inheritance.....
1.  @Table(name="CUSTOMER")
@Entity(name = "CUSTOMER") //Name of the entity
@Inheritance(strategy=InheritanceType.SINGLE_TABLE)
@DiscriminatorColumn(name="CUST_TYPE", discriminatorType=DiscriminatorType.STRING,length=10)
@DiscriminatorValue("RETAIL")

2. @Table(name="CUSTOMER")

@Entity(name = "CUSTOMERJOINED") //Name of the entity

@Inheritance(strategy=InheritanceType.JOINED)

@DiscriminatorColumn(name="CUST_TYPE", discriminatorType=DiscriminatorType.STRING,length=10)

@DiscriminatorValue("RETAIL")

3. @Table(name="CUSTOMER")

@Entity(name = "CUSTOMERTABLE") //Name of the entity

@Inheritance(strategy=InheritanceType.TABLE_PER_CLASS)

Let's insert some customer....

public static void main(String[] args) {
EntityManagerFactory entityManagerFactory =  Persistence.createEntityManagerFactory("testjpa");

EntityManager em = entityManagerFactory.createEntityManager();
EntityTransaction userTransaction = em.getTransaction();

userTransaction.begin();
//inserting Customer
Customer customer = new Customer();
customer.setFirstName("Charles");
customer.setLastName("Dickens");
customer.setCustType("RETAIL");
customer.getAddress().setStreet("10 Downing Street");
customer.getAddress().setAppt("1");
customer.getAddress().setCity("NewYork");
customer.getAddress().setZipCode("12345");
em.persist(customer);
userTransaction.commit();

OpenJPAEntityManager oem = OpenJPAPersistence.cast(em);
Object objId = oem.getObjectId(customer);
Customer cust = em.find(Customer.class, objId);

em.close();
entityManagerFactory.close();
System.out.println("Customer info: " + cust);

}

Some crucial points:

!!! While running your java project with openjpa; put the following argument to

Run > Run configurations> arguments

VM box: -javaagent:D:/apache-openjpa-2.3.0/openjpa-2.3.0.jar

Note: codes are from http://www.javaworld.com/article/2077817/java-se/understanding-jpa-part-1-the-object-oriented-paradigm-of-data-persistence.html. Thanks.