Explain Formula annotation in Hibernate.

@Formula annotation is used to calculate a given entity attribute using an SQL query expression. It defines a formula (derived value) which is a SQL fragment that acts as a @Column alternative in most cases. It represents read-only state.

@Entity
@Table(name="area")
public class Area implements Serializable {
	
	private static final long serialVersionUID = 1L;

	@Id
	@Column(name="id")
	private int id;
	
	@Column(name="length")
	private int length;
	
	@Column(name="width") 
	private int width;
	
	@Formula(" length * width ")
	public long area;
	
	...
	}
	

In the example, area property value is calculated using the length and the width properties.

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