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Tags JPA, Oracle, Tip
It is possible to use both annotations and descriptors in orm.xml to define entity mappings with JPA. The debate on which way to go has become very heated, but in my opinion there is a strong case for both.
- Annotations are best for metadata which you would consider closely bound to the code. This promotes clarity, as elements which are closely bound together are also in close proximity, promoting clarity and maintainability.
- XML is best for metadata which may change independantly of the code, and provides the ability to isolate platform specific issues from the code, for example to enhance persistence provider independance.
The OO design principle “Separate what changes from what stays the same” comes to mind clearly here.
I found the following simple example helpful in this context. The ID for the entity is defined via annotations in the code. However, I have defined the ID generator seperately in orm.xml. The generator name is fixed and referred to by the @GeneratedValue annotation, but the actual generator is in orm.xml and may be specified either as a table generator (which is database independant), or as a sequence generator which allows me to take advantage of Oracle’s sequences for primary key generation. Either generator may be defined in orm.xml, but the code does not change. This gives database independance, whilst still allowing leverage of the enhanced features of a specific database platform. In the example below, for illustration, orm.xml contains both generators and one has been commented out.
Note that to enable sequence generation in Oracle for this example to work, the correct properties must be set in persistence.xml, as by default Eclipselink will use table based ID generation even if you turn sequence generation on. The way to do this is detailed in this post here.
Class UserInfo
package uk.co.salientsoft.jpatest.domain;
import java.io.Serializable;
import javax.persistence.*;
@Entity
public class UserInfo implements Serializable {
@Id
@GeneratedValue(generator="UserID")
private long userID;
private String userName;
private static final long serialVersionUID = 1L;
public UserInfo() {
super();
}
public long getUserID() {
return this.userID;
}
public void setUserID(long userID) {
this.userID = userID;
}
public String getUserName() {
return this.userName;
}
public void setUserName(String userName) {
this.userName = userName;
}
}
orm.xml
<?xml version="1.0" encoding="UTF-8"?> <entity-mappings version="1.0" xmlns="http://java.sun.com/xml/ns/persistence/orm" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/persistence/orm http://java.sun.com/xml/ns/persistence/orm_1_0.xsd"> <!--<table-generator name="UserID" />--> <sequence-generator name="UserID" sequence-name="UserID"/> </entity-mappings>
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