2.12.1. Entity immutability

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If a specific entity is immutable, it is good practice to mark it with the `@Immutable` annotation.

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<div class="title">Example 193. Immutable entity</div>
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<pre class="prettyprint highlight">`@Entity(name = "Event")
@Immutable
public static class Event {

    @Id
    private Long id;

    private Date createdOn;

    private String message;

    //Getters and setters are omitted for brevity

}`</pre>
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Internally, Hibernate is going to perform several optimizations, such as:

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• reducing memory footprint since there is no need to retain the dehydrated state for the dirty checking mechanism

• speeding-up the Persistence Context flushing phase since immutable entities can skip the dirty checking process </div>

  Considering the following entity is persisted in the database:

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  Example 194. Persisting an immutable entity

  `doInJPA( this::entityManagerFactory, entityManager -> {

  Event event = new Event();
  event.setId( 1L );
  event.setCreatedOn( new Date( ) );
  event.setMessage( "Hibernate User Guide rocks!" );
  
  entityManager.persist( event );
  

  } );`</pre> </div> </div> </div> </div>

  When loading the entity and trying to change its state, Hibernate will skip any modification, therefore no SQL UPDATE statement is executed.

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  Example 195. The immutable entity ignores any update

  `doInJPA( this::entityManagerFactory, entityManager -> {

  Event event = entityManager.find( Event.class, 1L );
  log.info( "Change event message" );
  event.setMessage( "Hibernate User Guide" );
  

  } ); doInJPA( this::entityManagerFactory, entityManager -> {

  Event event = entityManager.find( Event.class, 1L );
  assertEquals("Hibernate User Guide rocks!", event.getMessage());
  

  } );</pre> </div> </div> <div class="listingblock"> <div class="content"> <pre class="prettyprint highlight">SELECT e.id AS id10_0,

     e.createdOn AS createdO2_0_0_,
     e.message AS message3_0_0_
  

  FROM event e WHERE e.id = 1

  -- Change event message

  SELECT e.id AS id10_0,

     e.createdOn AS createdO2_0_0_,
     e.message AS message3_0_0_
  

  FROM event e WHERE e.id = 1`</pre> </div> </div> </div> </div> </div>