Chapter 39. Entity Manager CRUD Methods

Author author = new Author();

author.setFirstName("dr");

author.setLastName("seuss");

author.setSubject("children");

author.setPublishDate(new Date());


logger.debug("creating author: {}", author);

em.persist(author);

logger.debug("created author: {}", author);

-creating author:ejava.examples.daoex.bo.Author@17e7691, id=0, fn=dr, ln=seuss, subject=children, pdate=Sun Sep 16 10:14:32 EDT 2012, version=0
-created author:ejava.examples.daoex.bo.Author@17e7691, id=50, fn=dr, ln=seuss, subject=children, pdate=Sun Sep 16 10:14:32 EDT 2012, version=0

Inserts entity into database
Actual insert time depends on transaction active and FlushMode

New or removed entity enters managed state

All further changes are watched and will update database



logger.debug("em.contains(author)={}", em.contains(author));

em.persist(author);

logger.debug("created author: {}", author);        

logger.debug("em.contains(author)={}", em.contains(author));



-em.contains(author)=false

-created author:...

-em.contains(author)=true

Existing entity is ignored
- Cascades will still occur for CascadeType.PERSIST and ALL relationships
```
        em.persist(author);

        ...

        em.persist(author);
```
Calling persist applies cascades to new relationships
It is quite legal to call persist() with an already managed entity. This is how one can get cascade actions to be applied to new relationships formed after the entity is originally managed.

Detached entities will be rejected

Entities with an identity but not associated with a persistence context



        Author author = new Author(1);

        author.setFirstName("dr");

...

        logger.debug("creating author: {}", author);

        logger.debug("em.contains(author)={}", em.contains(author));

        try {

            em.persist(author);

            fail("did not detect detached entity");

        } catch (PersistenceException ex) {

            logger.debug("caught expected exception:" + ex.getLocalizedMessage(), ex);

        }

        logger.debug("em.contains(author)={}", em.contains(author));

-creating author:ejava.examples.daoex.bo.Author@ad339b, id=1, ...
-em.contains(author)=false
-caught expected exception:javax.persistence.PersistenceException: org.hibernate.PersistentObjectException: 
        detached entity passed to persist: ejava.examples.daoex.bo.Author
-em.contains(author)=false

39.2. find()



<T> T find(Class<T> entityClass, Object primaryKey);

<T> T find(Class<T> entityClass, Object primaryKey, Map<String, Object> properties);

Searches for the entity by primary key value
Returns managed entity if found, else null
Properties permitted for vendor-specific values and ignored if not understood

39.3. getReference()



<T> T getReference(Class<T> entityClass, Object primaryKey);

Searches for the entity by primary key
Returns a LAZY load reference if found
Throws EntityNotFoundException if not exist

39.4. merge()



//create initial author

Author author = new Author();

...

em.persist(author);


//create detached author with changes

Author author2 = new Author(author.getId());

author2.setFirstName("updated " + author.getFirstName());

...

//merge changes 

Author tmp = em.merge(author2);

em.getTransaction().begin();

em.getTransaction().commit();

...

//verify results

assertFalse("author2 is managed", em.contains(author2));

assertTrue("tmp Author is not managed", em.contains(tmp));

assertSame("merged result not existing managed", author, tmp);

...

//verify changes were made to the DB

Author author3 = em.find(Author.class, author.getId());

assertEquals("updated " + firstName, author3.getFirstName());

Update database with state of detached object
State of detached object copied into managed entity or new instance created
Merging a managed entity is ignored. CascadeType.MERGE and ALL relationships are propagated
Merging a removed entity instance results in an IllegalArgumentException
Includes automatic checking of @Version property

Could be replaced with manual merge



public Author update(Author author) {

    Author dbAuthor = em.find(Author.class, author.getId());

    dbAuthor.setFirstName(author.getFirstName());

    dbAuthor.setLastName(author.getLastName());

    dbAuthor.setSubject(author.getSubject());

    dbAuthor.setPublishDate(author.getPublishDate());

    return dbAuthor;

}

Note

The above manual merge will successfully merge a transient entity -- but what errors and conditions does it not account for?

39.5. remove()



void remove(Object entity);

Managed entity is removed from database
Actual delete time depends on transaction active and FlushMode
- Extended Persistence Context - queues delete until transaction active

New entities are ignored but CascadeType.REMOVE and ALL relationships are cascaded



logger.debug("em.contains(author)={}", em.contains(author));

em.remove(author);

logger.debug("em.contains(author)={}", em.contains(author));

-em.contains(author)=false
-HHH000114: Handling transient entity in delete processing
-em.contains(author)=false

Detached entities are rejected



Author author = new Author(1);

...

try {

    em.remove(author);

    fail("did not reject removal of detached object");

} catch (IllegalArgumentException ex) {

    logger.debug("caught expected exception:" + ex);

}

-caught expected exception:java.lang.IllegalArgumentException: 
        Removing a detached instance ejava.examples.daoex.bo.Author#1

Removed entities are ignored



Author author = new Author();

...

logger.debug("peristed: {}", author);


logger.debug("em.contains(author)={}", em.contains(author));

em.remove(author);

logger.debug("em.contains(author)={}", em.contains(author));


//entity managers will ignore the removal of a removed entity

em.remove(author);

logger.debug("em.contains(author)={}", em.contains(author));

-peristed:ejava.examples.daoex.bo.Author@6c5356, id=50, ...
-em.contains(author)=true
-em.contains(author)=false
-em.contains(author)=false