JPA Detect changes in Entities automatically

[Markus Schmider]

Is there a automatic way to detect if an object has changed since it has been fetched from the database.
I want to persist a whole object tree. But I must detect which objects of the tree have changes and then set the date and user on them.

[Jelle Klap]

You can use callback methods in your entities, or better yet an entity listener, to fill in audit information at various life cycle stages. In this case you could use listener with a @PreUpdate annotated method. Something like:

package com.javaranch;

import java.util.Date;

public interface Auditable {	
	Date getDateModified();
	void setDateModified(Date dateModified);
}

package com.javaranch;

import java.util.Date;

import javax.persistence.PreUpdate;

public class AuditListener {
	
	@PreUpdate
    public void preUpdate(Auditable entity) {
        entity.setDateModified(new Date());
    }	
}

package com.javaranch;

import java.util.Date;

import javax.persistence.Entity;
import javax.persistence.EntityListeners;
import javax.persistence.Temporal;
import javax.persistence.TemporalType;

@Entity
@EntityListeners({ AuditListener.class })
public class Moose implements Auditable {

    @Temporal(TemporalType.TIMESTAMP)
	private Date dateModified;
	
	@Override
	public Date getDateModified() {
		return dateModified;
	}

	@Override
	public void setDateModified(Date dateModified) {
		this.dateModified = dateModified;
	}    
}

This is just a very minimal example to convey the basic idea. You could add much more audit data to the Auditable interface, and it would be a good idea to move the audit data fields out of the entity and into a seperate base class annotated as @MappedSuperclass. That sort of stuff.

[Markus Schmider]

Thanks for you reply.

I have tried something like that:

public class AuditListener {
	
	private UserService userService;
	
	public AuditListener() {
		super();
		this.userService = UserService.getInstance();
	}

	@PrePersist
	public void auditSave(Object o) {
		setDate(o);
		setUser(o);
	}
	
	@PreUpdate
	public void auditUpdate(Object o){
		setDate(o);
		setUser(o);
	}
	
	private void setDate(Object o) {
		if (o instanceof Audited) {
			Audited audited = (Audited) o;
			Date now = new Date();
			audited.setChangeDate(now);
		}
	}
	
	private void setUser(Object o) {
		if (o instanceof Audited) {
			Audited audited = (Audited) o;
			String user = this.userService.getUser();
			audited.setUserName(user);
		}
	}

}

@Entity
@NamedQueries({
	@NamedQuery(name="Order.findAll", query="SELECT o FROM Order o"),
	@NamedQuery(name="Order.deleteALL", query="DELETE FROM Order o")
})
@Table(name="order_table")
@EntityListeners(AuditListener.class)
public class Order extends Audited implements Serializable {
	
	public final static String FIND_ALL = "Order.findAll";
	public final static String DELETE_ALL = "Order.deleteALL";
	
	private static final long serialVersionUID = -6973644731708915605L;

	@Id @GeneratedValue
	private Long id;
	
	private String remark;
	
	@OneToOne(cascade=CascadeType.MERGE)
	@JoinColumn(name="orderType_fk")
	private OrderType orderType;
	
	@OneToMany(cascade=CascadeType.ALL)
	@JoinColumn(name="order_id")
	private List<Item> items;
	

@Entity
@EntityListeners(AuditListener.class)
public class Item extends Audited implements Serializable {
	
	private static final long serialVersionUID = -3764458400558428426L;
	
	@Id @GeneratedValue
	private Long id;
	private String name;

[Jelle Klap]

That's strange, but it may have to do with the way you've mapped the association between Order and Item, though and who "owns" the association.
I would probably map it something like this:

@Entity
@Table(name="ORDERS")
public class Order {

    // ...	
	@OneToMany(mappedBy = "order", cascade = CascadeType.PERSIST)
	private List<OrderLine> orderLines = new ArrayList<OrderLine>();
    // ...
}

@Entity
@Table(name="ORDER_LINE")
public class OrderLine  {

    // ...
	@OneToOne
	@JoinColumn(name = "ORDER_ID", nullable = false)
	private Order order;

	@OneToOne
	@JoinColumn(name= "ITEM_ID", nullable = false)
	private Item item;
    // ...
}

@Entity
@Table(name = "ITEM")
public class Item {
    // ...
}