Hibernate JPA @Synchronize Example

@Synchronize

The @Synchronize annotation is usually used in conjunction with the @Subselect annotation to specify the list of database tables used by the @Subselect SQL query.

With this information in place, Hibernate will properly trigger an entity flush whenever a query targeting the @Subselect entity is to be executed while the Persistence Context has scheduled some insert/update/delete actions against the database tables used by the @Subselect SQL query.

Therefore, the @Synchronize annotation prevents the derived entity from returning stale data when executing entity queries against the @Subselect entity.

Synchronize Ensures that auto-flush happens correctly and that queries against the derived entity do not return stale data. Mostly used with Subselect.

@Entity(name = "Client")
@Table(name = "client")
public static class Client {

@Id
private Long id;

@Column(name = "first_name")
private String firstName;

@Column(name = "last_name")
private String lastName;

//Getters and setters omitted for brevity

}

@Entity(name = "Account")
@Table(name = "account")
public static class Account {

@Id
private Long id;

@ManyToOne
private Client client;

private String description;

//Getters and setters omitted for brevity

}

@Entity(name = "AccountTransaction")
@Table(name = "account_transaction")
public static class AccountTransaction {

@Id
@GeneratedValue
private Long id;

@ManyToOne
private Account account;

private Integer cents;

private String description;

//Getters and setters omitted for brevity

}

@Entity(name = "AccountSummary")
@Subselect(
"select " +
" a.id as id, " +
" concat(concat(c.first_name, ' '), c.last_name) as clientName, " +
" sum(atr.cents) as balance " +
"from account a " +
"join client c on c.id = a.client_id " +
"join account_transaction atr on a.id = atr.account_id " +
"group by a.id, concat(concat(c.first_name, ' '), c.last_name)"
)
@Synchronize( {"client", "account", "account_transaction"} )
public static class AccountSummary {

@Id
private Long id;

private String clientName;

private int balance;

//Getters and setters omitted for brevity

}
In the example above, the Account entity does not retain any balance since every account operation is registered as an AccountTransaction. To find the Account balance, we need to query the AccountSummary which shares the same identifier with the Account entity.

However, the AccountSummary is not mapped to a physical table, but to an SQL query.

So, if we have the following AccountTransaction record, the AccountSummary balance will match the proper amount of money in this Account.