Excepciones comunes de Hibernate

Programación

1. Introducción

En este tutorial, discutiremos algunas excepciones comunes que podemos encontrar al trabajar con Hibernate.

Revisaremos su propósito y algunas causas comunes. Además, analizaremos sus soluciones.

2. Descripción general de la excepción de hibernación

Muchas condiciones pueden provocar que se produzcan excepciones al utilizar Hibernate. Estos pueden ser errores de mapeo, problemas de infraestructura, errores de SQL, violaciones de la integridad de los datos, problemas de sesión y errores de transacción.

Estas excepciones se extienden principalmente desde HibernateException . Sin embargo, si usamos Hibernate como proveedor de persistencia JPA, estas excepciones pueden quedar envueltas en PersistenceException .

Ambas clases base se extienden desde RuntimeException . Por lo tanto, están todos sin marcar. Por lo tanto, no es necesario detectarlos o declararlos en todos los lugares donde se usan.

Además, la mayoría de estos son irrecuperables. Como resultado, volver a intentar la operación no ayudaría. Esto significa que tenemos que abandonar la sesión actual al encontrarnos con ellos.

Veamos ahora cada uno de estos, uno a la vez.

3. Errores de mapeo

El mapeo objeto-relacional es un beneficio importante de Hibernate. Específicamente, nos libera de escribir sentencias SQL manualmente.

Al mismo tiempo, nos obliga a especificar el mapeo entre los objetos Java y las tablas de la base de datos. En consecuencia, los especificamos mediante anotaciones o mediante documentos cartográficos. Estas asignaciones se pueden codificar manualmente. Alternativamente, podemos usar herramientas para generarlos.

Al especificar estas asignaciones, podemos cometer errores. Estos podrían estar en la especificación de mapeo. O puede haber una discrepancia entre un objeto Java y la tabla de base de datos correspondiente.

Tales errores de mapeo generan excepciones. Los encontramos con frecuencia durante el desarrollo inicial. Además, es posible que nos encontremos con ellos al migrar cambios entre entornos.

Analicemos estos errores con algunos ejemplos.

3.1. MappingException

Un problema con el mapeo objeto-relacional causa una MappingException a ser lanzado :

public void whenQueryExecutedWithUnmappedEntity_thenMappingException() { thrown.expectCause(isA(MappingException.class)); thrown.expectMessage("Unknown entity: java.lang.String"); Session session = sessionFactory.getCurrentSession(); NativeQuery query = session .createNativeQuery("select name from PRODUCT", String.class); query.getResultList(); }

En el código anterior, el método createNativeQuery intenta asignar el resultado de la consulta al tipo String de Java especificado . Utiliza el mapeo implícito de la clase String de Metamodel para hacer el mapeo.

Sin embargo, la clase String no tiene ningún mapeo especificado. Por lo tanto, Hibernate no sabe cómo asignar la columna de nombre a String y lanza la excepción.

Para obtener un análisis detallado de las posibles causas y soluciones, consulte Hibernate Mapping Exception - Unknown Entity.

Del mismo modo, otros errores también pueden causar esta excepción:

  • Mezcla de anotaciones en campos y métodos
  • No se puede especificar @JoinTable para una asociación @ManyToMany
  • El constructor predeterminado de la clase asignada arroja una excepción durante el procesamiento de la asignación

Además, MappingException tiene algunas subclases que pueden indicar problemas de mapeo específicos:

  • AnnotationException: un problema con una anotación
  • DuplicateMappingException: asignación duplicada para un nombre de clase, tabla o propiedad
  • InvalidMappingException: la asignación no es válida
  • MappingNotFoundException: no se pudo encontrar el recurso de mapeo
  • PropertyNotFoundException: no se pudo encontrar un método getter o setter esperado en una clase

Por lo tanto, si nos encontramos con esta excepción, primero debemos verificar nuestras asignaciones .

3.2. AnnotationException

Para comprender la AnnotationException, creemos una entidad sin una anotación de identificador en ningún campo o propiedad:

@Entity public class EntityWithNoId { private int id; public int getId() { return id; } // standard setter }

Dado que Hibernate espera que cada entidad tenga un identificador , obtendremos una AnnotationException cuando usemos la entidad:

public void givenEntityWithoutId_whenSessionFactoryCreated_thenAnnotationException() { thrown.expect(AnnotationException.class); thrown.expectMessage("No identifier specified for entity"); Configuration cfg = getConfiguration(); cfg.addAnnotatedClass(EntityWithNoId.class); cfg.buildSessionFactory(); }

Además, algunas otras causas probables son:

  • Generador de secuencia desconocido utilizado en la anotación @GeneratedValue
  • @ Anotación temporal utilizada con una clase de fecha / hora de Java 8
  • Falta la entidad de destino o no existe para @ManyToOne o @OneToMany
  • Clases de colección sin procesar utilizadas con anotaciones de relación @OneToMany o @ManyToMany
  • Clases concretas utilizadas con las anotaciones de colección @OneToMany , @ManyToMany o @ElementCollection ya que Hibernate espera las interfaces de colección

Para resolver esta excepción, primero debemos verificar la anotación específica mencionada en el mensaje de error.

4. Errores de gestión de esquemas

La gestión automática del esquema de la base de datos es otro beneficio de Hibernate. Por ejemplo, puede generar declaraciones DDL para crear o validar objetos de base de datos.

Para usar esta función, necesitamos configurar la propiedad hibernate.hbm2ddl.auto de manera apropiada.

Si hay problemas al realizar la administración de esquemas, obtenemos una excepción. Examinemos estos errores.

4.1. SchemaManagementException

Cualquier problema relacionado con la infraestructura al realizar la gestión de esquemas provoca una SchemaManagementException .

Para demostrarlo, indiquemos a Hibernate que valide el esquema de la base de datos:

public void givenMissingTable_whenSchemaValidated_thenSchemaManagementException() { thrown.expect(SchemaManagementException.class); thrown.expectMessage("Schema-validation: missing table"); Configuration cfg = getConfiguration(); cfg.setProperty(AvailableSettings.HBM2DDL_AUTO, "validate"); cfg.addAnnotatedClass(Product.class); cfg.buildSessionFactory(); }

Dado que la tabla correspondiente a Producto no está presente en la base de datos, obtenemos la excepción de validación de esquema mientras construimos S essionFactory .

Además, hay otros escenarios posibles para esta excepción:

  • incapaz de conectarse a la base de datos para realizar tareas de gestión de esquemas
  • the schema is not present in the database

4.2. CommandAcceptanceException

Any problem executing a DDL corresponding to a specific schema management command can cause a CommandAcceptanceException.

As an example, let's specify the wrong dialect while setting up the SessionFactory:

public void whenWrongDialectSpecified_thenCommandAcceptanceException() { thrown.expect(SchemaManagementException.class); thrown.expectCause(isA(CommandAcceptanceException.class)); thrown.expectMessage("Halting on error : Error executing DDL"); Configuration cfg = getConfiguration(); cfg.setProperty(AvailableSettings.DIALECT, "org.hibernate.dialect.MySQLDialect"); cfg.setProperty(AvailableSettings.HBM2DDL_AUTO, "update"); cfg.setProperty(AvailableSettings.HBM2DDL_HALT_ON_ERROR,"true"); cfg.getProperties() .put(AvailableSettings.HBM2DDL_HALT_ON_ERROR, true); cfg.addAnnotatedClass(Product.class); cfg.buildSessionFactory(); }

Here, we've specified the wrong dialect: MySQLDialect. Also, we're instructing Hibernate to update the schema objects. Consequently, the DDL statements executed by Hibernate to update the H2 database will fail and we'll get an exception.

By default, Hibernate silently logs this exception and moves on. When we later use the SessionFactory, we get the exception.

To ensure that an exception is thrown on this error, we've set the property HBM2DDL_HALT_ON_ERROR to true.

Similarly, these are some other common causes for this error:

  • There is a mismatch in column names between mapping and the database
  • Two classes are mapped to the same table
  • The name used for a class or table is a reserved word in the database, like USER, for example
  • The user used to connect to the database does not have the required privilege

5. SQL Execution Errors

When we insert, update, delete or query data using Hibernate, it executes DML statements against the database using JDBC. This API raises an SQLException if the operation results in errors or warnings.

Hibernate converts this exception into JDBCException or one of its suitable subclasses:

  • ConstraintViolationException
  • DataException
  • JDBCConnectionException
  • LockAcquisitionException
  • PessimisticLockException
  • QueryTimeoutException
  • SQLGrammarException
  • GenericJDBCException

Let's discuss common errors.

5.1. JDBCException

JDBCException is always caused by a particular SQL statement. We can call the getSQL method to get the offending SQL statement.

Furthermore, we can retrieve the underlying SQLException with the getSQLException method.

5.2. SQLGrammarException

SQLGrammarException indicates that the SQL sent to the database was invalid. It could be due to a syntax error or an invalid object reference.

For example, a missing table can result in this error while querying data:

public void givenMissingTable_whenQueryExecuted_thenSQLGrammarException() { thrown.expect(isA(PersistenceException.class)); thrown.expectCause(isA(SQLGrammarException.class)); thrown.expectMessage("SQLGrammarException: could not prepare statement"); Session session = sessionFactory.getCurrentSession(); NativeQuery query = session.createNativeQuery( "select * from NON_EXISTING_TABLE", Product.class); query.getResultList(); }

Also, we can get this error while saving data if the table is missing:

public void givenMissingTable_whenEntitySaved_thenSQLGrammarException() { thrown.expect(isA(PersistenceException.class)); thrown.expectCause(isA(SQLGrammarException.class)); thrown .expectMessage("SQLGrammarException: could not prepare statement"); Configuration cfg = getConfiguration(); cfg.addAnnotatedClass(Product.class); SessionFactory sessionFactory = cfg.buildSessionFactory(); Session session = null; Transaction transaction = null; try { session = sessionFactory.openSession(); transaction = session.beginTransaction(); Product product = new Product(); product.setId(1); product.setName("Product 1"); session.save(product); transaction.commit(); } catch (Exception e) { rollbackTransactionQuietly(transaction); throw (e); } finally { closeSessionQuietly(session); closeSessionFactoryQuietly(sessionFactory); } }

Some other possible causes are:

  • The naming strategy used doesn't map the classes to the correct tables
  • The column specified in @JoinColumn doesn't exist

5.3. ConstraintViolationException

A ConstraintViolationException indicates that the requested DML operation caused an integrity constraint to be violated. We can get the name of this constraint by calling the getConstraintName method.

A common cause of this exception is trying to save duplicate records:

public void whenDuplicateIdSaved_thenConstraintViolationException() { thrown.expect(isA(PersistenceException.class)); thrown.expectCause(isA(ConstraintViolationException.class)); thrown.expectMessage( "ConstraintViolationException: could not execute statement"); Session session = null; Transaction transaction = null; for (int i = 1; i <= 2; i++) { try { session = sessionFactory.openSession(); transaction = session.beginTransaction(); Product product = new Product(); product.setId(1); product.setName("Product " + i); session.save(product); transaction.commit(); } catch (Exception e) { rollbackTransactionQuietly(transaction); throw (e); } finally { closeSessionQuietly(session); } } }

Also, saving a null value to a NOT NULL column in the database can raise this error.

In order to resolve this error, we should perform all validations in the business layer. Furthermore, database constraints should not be used to do application validations.

5.4. DataException

DataException indicates that the evaluation of an SQL statement resulted in some illegal operation, type mismatch or incorrect cardinality.

For instance, using character data against a numeric column can cause this error:

public void givenQueryWithDataTypeMismatch_WhenQueryExecuted_thenDataException() { thrown.expectCause(isA(DataException.class)); thrown.expectMessage( "org.hibernate.exception.DataException: could not prepare statement"); Session session = sessionFactory.getCurrentSession(); NativeQuery query = session.createNativeQuery( "select * from PRODUCT where", Product.class); query.getResultList(); }

To fix this error, we should ensure that the data types and length match between the application code and the database.

5.5. JDBCConnectionException

A JDBCConectionException indicates problems communicating with the database.

For example, a database or network going down can cause this exception to be thrown.

Additionally, an incorrect database setup can cause this exception. One such case is the database connection being closed by the server because it was idle for a long time. This can happen if we're using connection pooling and the idle timeout setting on the pool is more than the connection timeout value in the database.

To solve this problem, we should first ensure that the database host is present and that it's up. Then, we should verify that the correct authentication is used for the database connection. Finally, we should check that the timeout value is correctly set on the connection pool.

5.6. QueryTimeoutException

When a database query times out, we get this exception. We can also see it due to other errors, such as the tablespace becoming full.

This is one of the few recoverable errors, which means that we can retry the statement in the same transaction.

To fix this issue, we can increase the query timeout for long-running queries in multiple ways:

  • Set the timeout element in a @NamedQuery or @NamedNativeQuery annotation
  • Invoke the setHint method of the Query interface
  • Call the setTimeout method of the Transaction interface
  • Invoke the setTimeout method of the Query interface

6. Session-State-Related Errors

Let's now look into errors due to Hibernate session usage errors.

6.1. NonUniqueObjectException

Hibernate doesn't allow two objects with the same identifier in a single session.

If we try to associate two instances of the same Java class with the same identifier in a single session, we get a NonUniqueObjectException. We can get the name and identifier of the entity by calling the getEntityName() and getIdentifier() methods.

To reproduce this error, let's try to save two instances of Product with the same id with a session:

public void givenSessionContainingAnId_whenIdAssociatedAgain_thenNonUniqueObjectException() { thrown.expect(isA(NonUniqueObjectException.class)); thrown.expectMessage( "A different object with the same identifier value was already associated with the session"); Session session = null; Transaction transaction = null; try { session = sessionFactory.openSession(); transaction = session.beginTransaction(); Product product = new Product(); product.setId(1); product.setName("Product 1"); session.save(product); product = new Product(); product.setId(1); product.setName("Product 2"); session.save(product); transaction.commit(); } catch (Exception e) { rollbackTransactionQuietly(transaction); throw (e); } finally { closeSessionQuietly(session); } }

We'll get a NonUniqueObjectException, as expected.

This exception occurs frequently while reattaching a detached object with a session by calling the update method. If the session has another instance with the same identifier loaded, then we get this error. In order to fix this, we can use the merge method to reattach the detached object.

6.2. StaleStateException

Hibernate throws StaleStateExceptions when the version number or timestamp check fails. It indicates that the session contained stale data.

Sometimes this gets wrapped into an OptimisticLockException.

This error usually happens while using long-running transactions with versioning.

In addition, it can also happen while trying to update or delete an entity if the corresponding database row doesn't exist:

public void whenUpdatingNonExistingObject_thenStaleStateException() { thrown.expect(isA(OptimisticLockException.class)); thrown.expectMessage( "Batch update returned unexpected row count from update"); thrown.expectCause(isA(StaleStateException.class)); Session session = null; Transaction transaction = null; try { session = sessionFactory.openSession(); transaction = session.beginTransaction(); Product product = new Product(); product.setId(15); product.setName("Product1"); session.update(product); transaction.commit(); } catch (Exception e) { rollbackTransactionQuietly(transaction); throw (e); } finally { closeSessionQuietly(session); } }

Some other possible scenarios are:

  • we did not specify a proper unsaved-value strategy for the entity
  • two users tried to delete the same row at almost the same time
  • we manually set a value in the autogenerated ID or version field

7. Lazy Initialization Errors

We usually configure associations to be loaded lazily in order to improve application performance. The associations are fetched only when they're first used.

However, Hibernate requires an active session to fetch data. If the session is already closed when we try to access an uninitialized association, we get an exception.

Let's look into this exception and the various ways to fix it.

7.1. LazyInitializationException

LazyInitializationException indicates an attempt to load uninitialized data outside an active session. We can get this error in many scenarios.

First, we can get this exception while accessing a lazy relationship in the presentation layer. The reason is that the entity was partially loaded in the business layer and the session was closed.

Secondly, we can get this error with Spring Data if we use the getOne method. This method lazily fetches the instance.

There are many ways to solve this exception.

First of all, we can make all relationships eagerly loaded. But, this would impact the application performance because we'll be loading data that won't be used.

Secondly, we can keep the session open until the view is rendered. This is known as the “Open Session in View” and it's an anti-pattern. We should avoid this as it has several disadvantages.

Thirdly, we can open another session and reattach the entity in order to fetch the relationships. We can do so by using the merge method on the session.

Finally, we can initialize the required associations in the business layers. We'll discuss this in the next section.

7.2. Initializing Relevant Lazy Relationships in the Business Layer

There are many ways to initialize lazy relationships.

One option is to initialize them by invoking the corresponding methods on the entity. In this case, Hibernate will issue multiple database queries causing degraded performance. We refer to it as the “N+1 SELECT” problem.

Secondly, we can use Fetch Join to get the data in a single query. However, we need to write custom code to achieve this.

Finally, we can use entity graphs to define all the attributes to be fetched. We can use the annotations @NamedEntityGraph, @NamedAttributeNode, and @NamedEntitySubgraph to declaratively define the entity graph. We can also define them programmatically with the JPA API. Then, we retrieve the entire graph in a single call by specifying it in the fetch operation.

8. Transaction Issues

Transactions define units of work and isolation between concurrent activities. We can demarcate them in two different ways. First, we can define them declaratively using annotations. Second, we can manage them programmatically using the Hibernate Transaction API.

Furthermore, Hibernate delegates the transaction management to a transaction manager. If a transaction could not be started, committed or rolled back due to any reason, Hibernate throws an exception.

We usually get a TransactionException or an IllegalArgumentException depending on the transaction manager.

As an illustration, let's try to commit a transaction which has been marked for rollback:

public void givenTxnMarkedRollbackOnly_whenCommitted_thenTransactionException() { thrown.expect(isA(TransactionException.class)); thrown.expectMessage( "Transaction was marked for rollback only; cannot commit"); Session session = null; Transaction transaction = null; try { session = sessionFactory.openSession(); transaction = session.beginTransaction(); Product product = new Product(); product.setId(15); product.setName("Product1"); session.save(product); transaction.setRollbackOnly(); transaction.commit(); } catch (Exception e) { rollbackTransactionQuietly(transaction); throw (e); } finally { closeSessionQuietly(session); } }

Similarly, other errors can also cause an exception:

  • Mixing declarative and programmatic transactions
  • Attempting to start a transaction when another one is already active in the session
  • Trying to commit or rollback without starting a transaction
  • Trying to commit or rollback a transaction multiple times

9. Concurrency Issues

Hibernate supports two locking strategies to prevent database inconsistency due to concurrent transactions – optimistic and pessimistic. Both of them raise an exception in case of a locking conflict.

To support high concurrency and high scalability, we typically use optimistic concurrency control with version checking. This uses version numbers or timestamps to detect conflicting updates.

OptimisticLockingException is thrown to indicate an optimistic locking conflict. For instance, we get this error if we perform two updates or deletes of the same entity without refreshing it after the first operation:

public void whenDeletingADeletedObject_thenOptimisticLockException() { thrown.expect(isA(OptimisticLockException.class)); thrown.expectMessage( "Batch update returned unexpected row count from update"); thrown.expectCause(isA(StaleStateException.class)); Session session = null; Transaction transaction = null; try { session = sessionFactory.openSession(); transaction = session.beginTransaction(); Product product = new Product(); product.setId(12); product.setName("Product 12"); session.save(product1); transaction.commit(); session.close(); session = sessionFactory.openSession(); transaction = session.beginTransaction(); product = session.get(Product.class, 12); session.createNativeQuery("delete from Product where id=12") .executeUpdate(); // We need to refresh to fix the error. // session.refresh(product); session.delete(product); transaction.commit(); } catch (Exception e) { rollbackTransactionQuietly(transaction); throw (e); } finally { closeSessionQuietly(session); } }

Asimismo, también podemos obtener este error si dos usuarios intentan actualizar la misma entidad casi al mismo tiempo. En este caso, el primero puede tener éxito y el segundo genera este error.

Por lo tanto, no podemos evitar completamente este error sin introducir un bloqueo pesimista . Sin embargo, podemos minimizar la probabilidad de que ocurra haciendo lo siguiente:

  • Mantenga las operaciones de actualización lo más breves posible
  • Actualice las representaciones de entidades en el cliente con la mayor frecuencia posible.
  • No almacene en caché la entidad ni ningún objeto de valor que la represente
  • Siempre actualice la representación de la entidad en el cliente después de la actualización

10. Conclusión

En este artículo, analizamos algunas excepciones comunes encontradas al usar Hibernate. Además, investigamos sus probables causas y resoluciones.

Como de costumbre, el código fuente completo se puede encontrar en GitHub.