====== Hibernate @ValueGenerationType ======
  * [[java:hibernate|Hibernate]]에서 필드의 값을 자동으로 생성하는 기법
  * [[https://docs.jboss.org/hibernate/orm/6.0/javadocs/org/hibernate/annotations/ValueGenerationType.html|ValueGenerationType (Hibernate JavaDocs)]] 는 메타 어노테이션으로 다른 어노테이션을 값을 자동생성하는 어노테이션으로 지정해준다.
  * [[https://docs.jboss.org/hibernate/orm/6.0/javadocs/org/hibernate/tuple/AnnotationValueGeneration.html|AnnotationValueGeneration]] 를 구현하여 실제로 값을 생성해주고, 이렇게 생성된 값이 필드의 값으로 자동으로 들어간다.
  * 이를 기반으로 하여 [[https://docs.jboss.org/hibernate/orm/6.0/javadocs/org/hibernate/tuple/CreationTimestampGeneration.html|CreationTimestampGeneration]] [[https://docs.jboss.org/hibernate/orm/6.0/javadocs/org/hibernate/tuple/UpdateTimestampGeneration.html|UpdateTimestampGeneration]] 등이 만들어졌다.
  * see [[java:hibernate:generated|Hibernate @Generated]]

===== Example =====
<code java>
@Entity(name = "Event")
public static class Event {

	@Id
	@GeneratedValue
	private Long id;

	@Column(name = "`timestamp`")
	@FunctionCreationTimestamp
	private Date timestamp;

	//Constructors, getters, and setters are omitted for brevity
}

@ValueGenerationType(generatedBy = FunctionCreationValueGeneration.class)
@Retention(RetentionPolicy.RUNTIME)
public @interface FunctionCreationTimestamp {}

// DB를 통해 자동생성 지원
public static class FunctionCreationValueGeneration
		implements AnnotationValueGeneration<FunctionCreationTimestamp> {

	@Override
	public void initialize(FunctionCreationTimestamp annotation, Class<?> propertyType) {
	}

	/**
	 * Generate value on INSERT
	 * @return when to generate the value
	 */
	public GenerationTiming getGenerationTiming() {
		return GenerationTiming.INSERT;
	}

	/**
	 * Returns null because the value is generated by the database.
	 * @return null
	 */
	public ValueGenerator<?> getValueGenerator() {
		return null;
	}

	/**
	 * Returns true because the value is generated by the database.
	 * @return true
	 */
	public boolean referenceColumnInSql() {
		return true;
	}

	/**
	 * Returns the database-generated value
	 * @return database-generated value
	 */
	public String getDatabaseGeneratedReferencedColumnValue() {
		return "current_timestamp";
	}
}
</code>
쿼리가 아래처럼 실행된다.
<code sql>
INSERT INTO Event ("timestamp", id)
VALUES (current_timestamp, 1)
</code>

<code java>
// In memory 상에서 자동생성
public static class FunctionCreationValueGeneration
		implements AnnotationValueGeneration<FunctionCreationTimestamp> {

	@Override
	public void initialize(FunctionCreationTimestamp annotation, Class<?> propertyType) {
	}

	/**
	 * Generate value on INSERT
	 * @return when to generate the value
	 */
	public GenerationTiming getGenerationTiming() {
		return GenerationTiming.INSERT;
	}

	/**
	 * Returns the in-memory generated value
	 * @return {@code true}
	 */
	public ValueGenerator<?> getValueGenerator() {
		return (session, owner) -> new Date( );
	}

	/**
	 * Returns false because the value is generated by the database.
	 * @return false
	 */
	public boolean referenceColumnInSql() {
		return false;
	}

	/**
	 * Returns null because the value is generated in-memory.
	 * @return null
	 */
	public String getDatabaseGeneratedReferencedColumnValue() {
		return null;
	}
}
</code>
쿼리가 다음과 같이 날라간다.
<code sql>
INSERT INTO Event ("timestamp", id)
VALUES ('Tue Mar 01 10:58:18 EET 2016', 1)
</code>