Hibernate Envers - Easy Entity Auditing

If you're using JPA, when coniguring Hibernate (in persistence.xml), add the following event listeners: (this will allow Envers to check if any audited entities were modified)

<persistence-unit ...>
<provider>org.hibernate.ejb.HibernatePersistence</provider>
<class>...</class>
<properties>
   <property name="hibernate.dialect" ... />
   <!-- other hibernate properties -->

   <property name="hibernate.ejb.event.post-insert"
             value="org.hibernate.ejb.event.EJB3PostInsertEventListener,org.hibernate.envers.event.AuditEventListener" />
   <property name="hibernate.ejb.event.post-update"
             value="org.hibernate.ejb.event.EJB3PostUpdateEventListener,org.hibernate.envers.event.AuditEventListener" />
   <property name="hibernate.ejb.event.post-delete"
             value="org.hibernate.ejb.event.EJB3PostDeleteEventListener,org.hibernate.envers.event.AuditEventListener" />
   <property name="hibernate.ejb.event.pre-collection-update"
             value="org.hibernate.envers.event.AuditEventListener" />
   <property name="hibernate.ejb.event.pre-collection-remove"
             value="org.hibernate.envers.event.AuditEventListener" />
   <property name="hibernate.ejb.event.post-collection-recreate"
             value="org.hibernate.envers.event.AuditEventListener" />
</properties>
</persistence-unit>

If you're using Hibernate directly, add the following to hibernate.cfg.xml:

	<hibernate-configuration>
	<session-factory>

	<listener class="org.hibernate.envers.event.AuditEventListener" type="post-insert"/>
	<listener class="org.hibernate.envers.event.AuditEventListener" type="post-update"/>
	<listener class="org.hibernate.envers.event.AuditEventListener" type="post-delete"/>
	<listener class="org.hibernate.envers.event.AuditEventListener" type="pre-collection-update"/>
	<listener class="org.hibernate.envers.event.AuditEventListener" type="pre-collection-remove"/>
	<listener class="org.hibernate.envers.event.AuditEventListener" type="post-collection-recreate"/>


	</session-factory>
	</hibernate-configuration>
    

The EJB3Post...EvenListeners are needed, so that ejb3 entity lifecycle callback methods work (@PostPersist, @PostUpdate, @PostRemove.

Then, annotate your persistent class with @Audited - this will make all properties audited. For example:

import org.hibernate.envers.Audited;

import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.GeneratedValue;
import javax.persistence.Column;

@Entity
@Audited // that's the important part :)
public class Person {
    @Id
    @GeneratedValue
    private int id;

    private String name;

    private String surname;

    @ManyToOne
    private Address address;

    // add getters, setters, constructors, equals and hashCode here
}

And the referenced entity:

@Entity
@Audited
public class Address {
    @Id
    @GeneratedValue
    private int id;

    private String streetName;

    private Integer houseNumber;

    private Integer flatNumber;

    @OneToMany(mappedBy = "address")
    private Set<Person> persons;

    // add getters, setters, constructors, equals and hashCode here
}

And that's it! You create, modify and delete the entites as always. If you look at the generated schema, you will notice that it is unchanged by adding auditing for the Address and Person entities. Also, the data they hold is the same. There are, however, two new tables - Address_AUD and Person_AUD, which store the historical data, whenever you commit a transaction.

Instead of annotating the whole class and auditing all properties, you can annotate only some persistent properties with @Audited. This will cause only these properties to be audited.

You can access the audit (history) of an entity using the AuditReader interface, which you can obtain when having an open EntityManager.

AuditReader reader = AuditReaderFactory.get(entityManager);
Person oldPerson = reader.find(Person.class, personId, revision)

The T find(Class<T> cls, Object primaryKey, Number revision) method returns an entity with the given primary key, with the data it contained at the given revision. If the entity didn't exist at this revision, null is returned. Only the audited properties will be set on the returned entity. The rest will be null.

You can also get a list of revisions at which an entity was modified using the getRevisions method, as well as retrieve the date, at which a revision was created using the getRevisionDate method.

For example, using the entities defined above, the following code will generate revision number 1, which will contain two new Person and two new Address entities:

entityManager.getTransaction().begin();

Address address1 = new Address("Privet Drive", 4);
Person person1 = new Person("Harry", "Potter", address1);

Address address2 = new Address("Grimmauld Place", 12);
Person person2 = new Person("Hermione", "Granger", address2);

entityManager.persist(address1);
entityManager.persist(address2);
entityManager.persist(person1);
entityManager.persist(person2);

entityManager.getTransaction().commit();

Now we change some entities. This will generate revision number 2, which will contain modifications of one person entity and two address entities (as the collection of persons living at address2 and address1 changes):

entityManager.getTransaction().begin();

Address address1 = entityManager.find(Address.class, address1.getId());
Person person2 = entityManager.find(Person.class, person2.getId());

// Changing the address's house number
address1.setHouseNumber(5)

// And moving Hermione to Harry
person2.setAddress(address1);

entityManager.getTransaction().commit();

We can retrieve the old versions (the audit) easily:

AuditReader reader = AuditReaderFactory.get(entityManager);

Person person2_rev1 = reader.find(Person.class, person2.getId(), 1);
assert person2_rev1.getAddress().equals(new Address("Grimmauld Place", 12));

Address address1_rev1 = reader.find(Address.class, address1.getId(), 1);
assert address1_rev1.getPersons().getSize() == 1;

// and so on

Envers provides an easy way to log additional data for each revision. You simply need to annotate one entity with @RevisionEntity, and a new instance of this entity will be persisted when a new revision is created (that is, whenever an audited entity is modified). As revisions are global, you can have at most one revisions entity.

Please note that the revision entity must be a mapped Hibernate entity.

This entity must have at least two properties:

You can either add these properties to your entity, or extend org.hibernate.envers.DefaultRevisionEntity, which already has those two properties.

When using a Date, instead of a long/Long for the revision timestamp, take care not to use a mapping of the property which will loose precision (for example, using @Temporal(DATE) is wrong, as it doesn't store the time information, so many of your revisions will appear to happen at exactly the same time). A good choice is a @Temporal(TIMESTAMP).

To fill the entity with additional data, you'll need to implement the org.jboss.envers.RevisionListener interface. Its newRevision method will be called when a new revision is created, before persisting the revision entity. The implementation should be stateless and thread-safe. The listener then has to be attached to the revisions entity by specifying it as a parameter to the @RevisionEntity annotation.

Alternatively, you can use the getCurrentRevision method of the AuditReader interface to obtain the current revision, and fill it with desired information. The method has a persist parameter specifying, if the revision entity should be persisted before returning. If set to true, the revision number will be available in the returned revision entity (as it is normally generated by the database), but the revision entity will be persisted regardless of wheter there are any audited entities changed. If set to false, the revision number will be null, but the revision entity will be persisted only if some audited entities have changed.

A simplest example of a revisions entity, which with each revision associates the username of the user making the change is:

package org.jboss.envers.example;

import org.hibernate.envers.RevisionEntity;
import org.hibernate.envers.DefaultRevisionEntity;

import javax.persistence.Entity;

@Entity
@RevisionEntity(ExampleListener.class)
public class ExampleRevEntity extends DefaultRevisionEntity {
	private String username;

	public String getUsername() { return username; }
	public void setUsername(String username) { this.username = username; }
}

Or, if you don't want to extend any class:

package org.hibernate.envers.example;

import org.hibernate.envers.RevisionNumber;
import org.hibernate.envers.RevisionTimestamp;
import org.hibernate.envers.RevisionEntity;

import javax.persistence.Id;
import javax.persistence.GeneratedValue;
import javax.persistence.Entity;

@Entity
@RevisionEntity(ExampleListener.class)
public class ExampleRevEntity {
    @Id
    @GeneratedValue
    @RevisionNumber
    private int id;

    @RevisionTimestamp
    private long timestamp;

    private String username;

    // Getters, setters, equals, hashCode ...
}

An example listener, which, if used in a JBoss Seam application, stores the currently logged in user username:

package org.hibernate.envers.example;

import org.hibernate.envers.RevisionListener;
import org.jboss.seam.security.Identity;
import org.jboss.seam.Component;

public class ExampleListener implements RevisionListener {
    public void newRevision(Object revisionEntity) {
        ExampleRevEntity exampleRevEntity = (ExampleRevEntity) revisionEntity;
        Identity identity = (Identity) Component.getInstance("org.jboss.seam.security.identity");

        exampleRevEntity.setUsername(identity.getUsername());
    }
}

Having an "empty" revision entity - that is, with no additional properties except the two mandatory ones - is also an easy way to change the names of the table and of the properties in the revisions table automatically generated by Envers.

In case there is no entity annotated with @RevisionEntity, a default table will be generated, with the name REVINFO.

                CustomEntityTrackingRevisionListener.java

public class CustomEntityTrackingRevisionListener
             implements EntityTrackingRevisionListener {
    @Override
    public void entityChanged(Class entityClass, String entityName,
                              Serializable entityId, RevisionType revisionType,
                              Object revisionEntity) {
        String type = entityClass.getName();
        ((CustomTrackingRevisionEntity)revisionEntity).addModifiedEntityType(type);
    }

    @Override
    public void newRevision(Object revisionEntity) {
    }
}

                CustomTrackingRevisionEntity.java

@Entity
@RevisionEntity(CustomEntityTrackingRevisionListener.class)
public class CustomTrackingRevisionEntity {
    @Id
    @GeneratedValue
    @RevisionNumber
    private int customId;

    @RevisionTimestamp
    private long customTimestamp;

    @OneToMany(mappedBy="revision", cascade={CascadeType.PERSIST, CascadeType.REMOVE})
    private Set<ModifiedEntityTypeEntity> modifiedEntityTypes =
                                              new HashSet<ModifiedEntityTypeEntity>();

    public void addModifiedEntityType(String entityClassName) {
        modifiedEntityTypes.add(new ModifiedEntityTypeEntity(this, entityClassName));
    }

    ...
}

                ModifiedEntityTypeEntity.java

@Entity
public class ModifiedEntityTypeEntity {
    @Id
    @GeneratedValue
    private Integer id;

    @ManyToOne
    private CustomTrackingRevisionEntity revision;

    private String entityClassName;

    ...
}

CustomTrackingRevisionEntity revEntity =
    getAuditReader().findRevision(CustomTrackingRevisionEntity.class, revisionNumber);
Set<ModifiedEntityTypeEntity> modifiedEntityTypes = revEntity.getModifiedEntityTypes()

You can think of historic data as having two dimension. The first - horizontal - is the state of the database at a given revision. Thus, you can query for entities as they were at revision N. The second - vertical - are the revisions, at which entities changed. Hence, you can query for revisions, in which a given entity changed.

The queries in Envers are similar to Hibernate Criteria, so if you are common with them, using Envers queries will be much easier.

The main limitation of the current queries implementation is that you cannot traverse relations. You can only specify constraints on the ids of the related entities, and only on the "owning" side of the relation. This however will be changed in future releases.

Please note, that queries on the audited data will be in many cases much slower than corresponding queries on "live" data, as they involve correlated subselects.

In the future, queries will be improved both in terms of speed and possibilities, when using the valid-time audit strategy, that is when storing both start and end revisions for entities. See Chapter 3, Configuration.

AuditQuery query = getAuditReader().createQuery().forEntitiesAtRevision(MyEntity.class, revisionNumber);

query.add(AuditEntity.property("name").eq("John"));

query.add(AuditEntity.property("address").eq(relatedEntityInstance));
// or
query.add(AuditEntity.relatedId("address").eq(relatedEntityId));

List personsAtAddress = getAuditReader().createQuery()
    .forEntitiesAtRevision(Person.class, 12)
    .addOrder(AuditEntity.property("surname").desc())
    .add(AuditEntity.relatedId("address").eq(addressId))
    .setFirstResult(4)
    .setMaxResults(2)
    .getResultList();

AuditQuery query = getAuditReader().createQuery()
    .forRevisionsOfEntity(MyEntity.class, false, true);

Number revision = (Number) getAuditReader().createQuery()
    .forRevisionsOfEntity(MyEntity.class, false, true)
    .setProjection(AuditEntity.revisionNumber().min())
    .add(AuditEntity.id().eq(entityId))
    .add(AuditEntity.revisionNumber().gt(42))
    .getSingleResult();

Number revision = (Number) getAuditReader().createQuery()
    .forRevisionsOfEntity(MyEntity.class, false, true)
    // We are only interested in the first revision
    .setProjection(AuditEntity.revisionNumber().min())
    .add(AuditEntity.property("actualDate").minimize()
        .add(AuditEntity.property("actualDate").ge(givenDate))
        .add(AuditEntity.id().eq(givenEntityId)))
    .getSingleResult();

If you'd like to generate the database schema file with the Hibernate Tools Ant task, you'll probably notice that the generated file doesn't contain definitions of audit tables. To generate also the audit tables, you simply need to use org.hibernate.tool.ant.EnversHibernateToolTask instead of the usual org.hibernate.tool.ant.HibernateToolTask. The former class extends the latter, and only adds generation of the version entities. So you can use the task just as you used to.

For example:

<target name="schemaexport" depends="build-demo"
  description="Exports a generated schema to DB and file">
  <taskdef name="hibernatetool"
    classname="org.hibernate.tool.ant.EnversHibernateToolTask"
    classpathref="build.demo.classpath"/>

  <hibernatetool destdir=".">
    <classpath>
      <fileset refid="lib.hibernate" />
      <path location="${build.demo.dir}" />
      <path location="${build.main.dir}" />
    </classpath>
    <jpaconfiguration persistenceunit="ConsolePU" />
    <hbm2ddl
      drop="false"
      create="true"
      export="false"
      outputfilename="versioning-ddl.sql"
      delimiter=";"
      format="true"/>
  </hibernatetool>
</target>

Will generate the following schema:

    create table Address (
        id integer generated by default as identity (start with 1),
        flatNumber integer,
        houseNumber integer,
        streetName varchar(255),
        primary key (id)
    );

    create table Address_AUD (
        id integer not null,
        REV integer not null,
        flatNumber integer,
        houseNumber integer,
        streetName varchar(255),
        REVTYPE tinyint,
        primary key (id, REV)
    );

    create table Person (
        id integer generated by default as identity (start with 1),
        name varchar(255),
        surname varchar(255),
        address_id integer,
        primary key (id)
    );

    create table Person_AUD (
        id integer not null,
        REV integer not null,
        name varchar(255),
        surname varchar(255),
        REVTYPE tinyint,
        address_id integer,
        primary key (id, REV)
    );

    create table REVINFO (
        REV integer generated by default as identity (start with 1),
        REVTSTMP bigint,
        primary key (REV)
    );

    alter table Person
        add constraint FK8E488775E4C3EA63
        foreign key (address_id)
        references Address;
    

For each audited entity (that is, for each entity containing at least one audited field), an audit table is created. By default, the audit table's name is created by adding a "_AUD" suffix to the original name, but this can be overriden by specifing a different suffix/prefix (see Chapter 3, Configuration) or on a per-entity basis using the @AuditTable annotation.

The audit table has the following fields:

The primary key of the audit table is the combination of the original id of the entity and the revision number - there can be at most one historic entry for a given entity instance at a given revision.

The current entity data is stored in the original table and in the audit table. This is a duplication of data, however as this solution makes the query system much more powerful, and as memory is cheap, hopefully this won't be a major drawback for the users. A row in the audit table with entity id ID, revision N and data D means: entity with id ID has data D from revision N upwards. Hence, if we want to find an entity at revision M, we have to search for a row in the audit table, which has the revision number smaller or equal to M, but as large as possible. If no such row is found, or a row with a "deleted" marker is found, it means that the entity didn't exist at that revision.

The "revision type" field can currently have three values: 0, 1, 2, which means, respectively, ADD, MOD and DEL. A row with a revision of type DEL will only contain the id of the entity and no data (all fields NULL), as it only serves as a marker saying "this entity was deleted at that revision".

Additionaly, there is a "REVINFO" table generated, which contains only two fields: the revision id and revision timestamp. A row is inserted into this table on each new revision, that is, on each commit of a transaction, which changes audited data. The name of this table can be configured, as well as additional content stored, using the @RevisionEntity annotation, see Chapter 4, Logging data for revisions.

While global revisions are a good way to provide correct auditing of relations, some people have pointed out that this may be a bottleneck in systems, where data is very often modified. One viable solution is to introduce an option to have an entity "locally revisioned", that is revisions would be created for it independently. This wouldn't enable correct versioning of relations, but wouldn't also require the "REVINFO" table. Another possibility if to have "revisioning groups", that is groups of entities which share revision numbering. Each such group would have to consist of one or more strongly connected component of the graph induced by relations between entities. Your opinions on the subject are very welcome on the forum! :)

With the inclusion of Envers as a Hibernate module, some of the public API and configuration defaults changed. In general, "versioning" is renamed to "auditing" (to avoid confusion with the annotation used for indicating an optimistic locking field - @Version).

Because of changing some configuration defaults, there should be no more problems using Envers out-of-the-box with Oracle and other databases, which don't allow tables and field names to start with "_".

<persistence-unit ...>
<provider>org.hibernate.ejb.HibernatePersistence</provider>
<class>...</class>
<properties>
   <property name="hibernate.dialect" ... />
   <!-- other hibernate properties -->

   <!-- Envers listeners -->

   <property name="org.hibernate.envers.auditTableSuffix" value="_versions" />
   <property name="org.hibernate.envers.revisionFieldName" value="_revision" />
   <property name="org.hibernate.envers.revisionTypeFieldName" value="_rev_type" />
   <!-- other envers properties -->
</properties>
</persistence-unit>

package org.hibernate.envers.example;

import org.hibernate.envers.RevisionNumber;
import org.hibernate.envers.RevisionTimestamp;
import org.hibernate.envers.RevisionEntity;

import javax.persistence.Id;
import javax.persistence.GeneratedValue;
import javax.persistence.Entity;
import javax.persistence.Column;
import javax.persistence.Table;

@Entity
@RevisionEntity
@Table(name="_revisions_info")
public class ExampleRevEntity {
    @Id
    @GeneratedValue
    @RevisionNumber
    @Column(name="revision_id")
    private int id;

    @RevisionTimestamp
    @Column(name="revision_timestamp")
    private long timestamp;

    // Getters, setters, equals, hashCode ...
}

Some useful links: