EntityManagers perform several actions that affect the lifecycle state of entity instances.

public void persist(Object entity);

Transitions new instances to managed. On the next flush or commit, the newly persisted instances will be inserted into the datastore.

For a given entity A, the persist method behaves as follows:

This action can only be used in the context of an active transaction.

public void remove(Object entity);

Transitions managed instances to removed. The instances will be deleted from the datastore on the next flush or commit. Accessing a removed entity has undefined results.

For a given entity A, the remove method behaves as follows:

This action can only be used in the context of an active transaction.

public void refresh(Object entity);

Use the refresh action to make sure the persistent state of an instance is synchronized with the values in the datastore. refresh is intended for long-running optimistic transactions in which there is a danger of seeing stale data.

For a given entity A, the refresh method behaves as follows:

public Object merge(Object entity);

A common use case for an application running in a servlet or application server is to "detach" objects from all server resources, modify them, and then "attach" them again. For example, a servlet might store persistent data in a user session between a modification based on a series of web forms. Between each form request, the web container might decide to serialize the session, requiring that the stored persistent state be disassociated from any other resources. Similarly, a client/server application might transfer persistent objects to a client via serialization, allow the client to modify their state, and then have the client return the modified data in order to be saved. This is sometimes referred to as the data transfer object or value object pattern, and it allows fine-grained manipulation of data objects without incurring the overhead of multiple remote method invocations.

JPA provides support for this pattern by automatically detaching entities when they are serialized or when a persistence context ends (see Section 3, “ Persistence Context ” for an exploration of persistence contexts). The JPA merge API re-attaches detached entities. This allows you to detach a persistent instance, modify the detached instance offline, and merge the instance back into an EntityManager (either the same one that detached the instance, or a new one). The changes will then be applied to the existing instance from the datastore.

A detached entity maintains its persistent identity, but cannot load additional state from the datastore. Accessing any persistent field or property that was not loaded at the time of detachment has undefined results. Also, be sure not to alter the version or identity fields of detached instances if you plan on merging them later.

The merge method returns a managed copy of the given detached entity. Changes made to the persistent state of the detached entity are applied to this managed instance. Because merging involves changing persistent state, you can only merge within a transaction.

If you attempt to merge an instance whose representation has changed in the datastore since detachment, the merge operation will throw an exception, or the transaction in which you perform the merge will fail on commit, just as if a normal optimistic conflict were detected.

For a given entity A, the merge method behaves as follows:

The following diagram illustrates the lifecycle of an entity with respect to the APIs presented in this section.