Specifies custom SQL expressions used to read and write to the column mapped by the annotated persistent attribute in all generated SQL involving the annotated persistent attribute.

• A write expression must contain exactly one JDBC-style '?' placeholder.
• A read expression may not contain JDBC-style placeholders.

For example:

@Column(name="credit_card_num")
@ColumnTransformer(read="decrypt(credit_card_num)"
                   write="encrypt(?)")
String creditCardNumber;

A column transformer write expression transforms the value of a persistent attribute of an entity as it is being written to the database.

• If there is a matching read expression to undo the effect of this transformation, then we're entitled to consider the in-memory state of the Java entity instance as synchronized with the database after a SQL insert or update is executed.
• On the other hand, if there's no matching read expression, or if the read expression does not exactly undo the effect of the transformation, the in-memory state of the Java entity instance should be considered unsynchronized with the database after every SQL insert or update is executed.

In the second scenario, we may ask Hibernate to resynchronize the in-memory state with the database after each insert or update by annotating the persistent attribute @Generated(event={INSERT,UPDATE}, writable=true). This results in a SQL select after every insert or update.