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Article

Developing a Master-Detail View – Part 3

Create and manage entities

The JPA ORM Generation wizard allows modification of existing O/R mappings and the creation of new entity associations.

The tasks you will complete in this step are:

Create a new one-to-one association between CUSTOMER and CUSTOMERID
Oracle Enterprise Pack for Eclipse also supports the creation of new entity associations in case your database lacks foreign key definitions (such as for performance reasons). It can create Simple Associations (one to one, one to many, many to one) between two tables and Many to Many Associations through an intermediate table.

  1. In the Project Explorer, right-click the project oepe-jpa-tutorial and select JPA > Generate Entities from Tables... Click Next.

  1. Select the database tables from the Select Tables dialog as shown below. Select all the database tables except CONTACT and click Next.

  1. In the Table associations dialog, click the Create New Association button.

  1. In the Create New Association dialog, select the CUSTOMER table for Table 1 and the CUSTOMERID table for Table 2 (as shown below) and click Next. This creates a simple one-to-one association between the CUSTOMER and CUSTOMERID tables.

  1. In the Join Columns dialog, specify the join columns between tables. Select the CUSTOMERID column for the CUSTOMER table and the CUSTOMERIDcolumn for the CUSTOMERID table.

Click Next.

  1. Select One to One to specify one customer per customerid.
    Click Finish.

Since the new associations (shown in brown) are not observed in the database schema, they will be annotated programmatically in the respective entity beans.

  1. Click Finish.

Review Generated Classes with Annotations

Review the following annotations in the class Customer and CustomerId.

  • One-to-One Properties: A one-to-one property designates a relationship in which an entity A references a single entity B, and no other As can reference the same B. This is a one-to-one relationship between A and B.

The following shows the one-to-one mapping association between Customer and CustomerId in the Customer class.

@OneToOne(fetch=FetchType.EAGER)
@JoinColumn(name="CUSTOMERID", referencedColumnName="CUSTOMERID", nullable=false, insertable=false, updatable=false)
public Customerid getCustomeridBean() {
return this.customeridBean;
}
public void setCustomeridBean(Customerid customeridBean) {
this.customeridBean = customeridBean;
}

The following shows the one-to-one mapping association between CustomerId and Customer in the CustomerId class.

@OneToOne(mappedBy="customeridBean", fetch=FetchType.EAGER)
public Customer getCustomer() {
return this.customer;
}
public void setCustomer(Customer customer) {
this.customer = customer;
}

Add Annotations to an Existing Java Class
In this step, you will create a Java class Contact and use Oracle Enterprise Pack for Eclipse to annotate existing Java classes (POJOs) to make them into JPA entity beans. This way, you will follow a "top-down" development scenario.

  1. Select File > New > Class.
  2. Under the package oracle.beans, create a Java class Contact, which implements the java.io.Serializable interface. Select Constructors from superclass to add a no argument constructor.

  1. Add the following set of private variables of type String to the Contact class. Note: This step defines properties that map to columns in the CONTACT database table.

private String contactId;
private String address;
private String city;
private String phone;

  1. Add getter and setter methods for each property by right-clicking with source view and selecting Source > Generate Getters and Setters. In the Generate Getters and Setters dialog, select all properties and click OK.
  2. Add the following property.

private static final long serialVersionUID = 1L;

  1. Override the equals( ) and hashCode( ) methods, because you are going to use Contact instances in Set.

public boolean equals(Object other) {
if ( (this == other ) ) return true;
if ( !(other instanceof Contact) ) return false;
Contact castOther = (Contact) other;
if( this.getContactId( ).equals(castOther.getContactId( )) ) {
return true;
} else {
return false;
}
}
public int hashCode() {
return this.getContactId( ).hashCode( );
}

  1. Save the Contact class. Now you have the object representation for CONTACT persistence data. Next, you will annotate the Contact object.
  2. In the Project Explorer view, right-click on the project oepe-jpa-tutorial. Choose JPA > Generate Entity from Java Class and then click Next.

  1. Click Browse to select a Java class. Type Contact to view and select the com.bea.beans.Contact object to be mapped.
    Click OK.

  1. When you choose Property, Oracle Enterprise Pack for Eclipse will annotate your class's accessor methods; if you choose field, Oracle Enterprise Pack for Eclipse annotates your class's fields. Set Entity access to Property and click Next.

  1. In the Class Mapping Properties dialog, confirm that the database Table Selection field has the value CONTACT and the Primary Key Property field has the value contactId. If these fields do not show the required values, select them. Then click Next.

13.  In the bean Property Mapping dialog, you can verify the mappings between fields and database columns that are being annotated in the Contact java class. Note that you can select a property mapping and click Edit to modify a selected property mapping. In this tutorial, you do not need to change any mapping information.

Click Finish.

  1. Edit the annotated class Contact to add a bidirectional many-to-one mapping association with Customer.
    • Import the following javax.persistence classes for adding a new many-to-one column join.

import javax.persistence.JoinColumn;
import javax.persistence.ManyToOne;

    • Add a property customer of type Customer.

private Customer customer;

    • Add the following annotation to the Contact entity for adding a many-to-one association with Customer. The @JoinColumn annotation defines the attributes Column Name, which is the name of the column to which the property is bound, and the Referenced Column, which is the name of the primary key column being joined to.

@ManyToOne()
@JoinColumn(name="CUSTOMERID", referencedColumnName="CUSTOMERID")
public Customer getCustomer() {
return this.customer;
}
public void setCustomer(Customer customer) {
this.customer = customer;
}

    • Save the class Contact.
  1. For the above many-to-one customer property in the Contact entity, there should be a one-to-many contact property in the Customer entity.
    • Open the class Customer.
    • Add a property contacts of type java.util.Set

private java.util.Set<Contact> contacts;

    • Add the following annotation to the Customer entity for adding a one-to-many association with Contact. The @OneToMany annotation defines the attribute mappedBy, which is the name of the many-to-one field in the Contact entity that maps this bidirectional relation.

@OneToMany(mappedBy="customer")
public java.util.Set<Contact> getContacts() {
return this.contacts;
}
public void setContacts(java.util.Set<Contact> contacts) {
this.contacts = contacts;
}

    • Add the following parameters to the @JoinColumn annotation in the Customer entity to ensure that only one writable mapping exists for CUSTOMER.CUSTOMERID: unique=true, nullable=false, updatable=false, insertable=false. Only one may be defined as writable; all others must be specified read-only.

//bi-directional one-to-one association to Customerid
@OneToOne
@JoinColumn(name="CUSTOMERID", unique=true, nullable=false, updatable=false, insertable=false)
public Customerid getCustomeridBean() {
return this.customeridBean;
}

    • Save the class Customer.

In Part 4, I will show how to manage persistent entities with the JPA persistence entity editor.

More Stories By Pieter Humphrey

Pieter Humphrey has been at Oracle (by way of BEA Systems) for 7+ years, working in development, marketing, sales, and developer relations to advance Java technology in the enterprise. He ran the dev2dev community at BEA and continues to with work with Oracle OTN and Eclipse Foundation communities. His current role is focused on application development tools like Oracle Enterprise Eclipse Pack, Oracle Workshop for WebLogic, JDeveloper, TopLink and the ADF framework, and is a frequent speaker at industry events and workshops.

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