JPA Level 1 caching
JPA has 2 levels of caching. The first level of caching is the persistence context.
The JPA Entity Manager maintains a set of Managed Entities in the Persistence Context.
The Entity Manager guarantees that within a single Persistence Context,
for any particular database row, there will be only one object
instance. However the same entity could be managed in another User's
transaction, so you should use either optimistic or pessimistic
locking as explained in JPA 2.0 Concurrency and locking
The code below shows that a find on a managed entity with the same id
and class as another in the same persistence context , will return the
The diagram below shows the life cycle of an Entity in relation to the Persistent Context.
The code below illustrates the life cycle of an Entity. A reference to
a container managed EntityManager is injected using the persistence
context annotation. A new order entity is created and the entity has
the state of new. Persist is called, making this a managed entity.
because it is a stateless session bean it is by default using container
managed transactions , when this transaction commits , the order is
made persistent in the database. When the orderline entity is returned
at the end of the transaction it is a detached entity.
The Persistence Context can be either Transaction Scoped-- the
Persistence Context 'lives' for the length of the transaction, or
Extended-- the Persistence Context spans multiple transactions. With a
Transaction scoped Persistence Context, Entities are "Detached" at the
end of a transaction.
As shown below, to persist the changes on a detached entity, you call
the EntityManager's merge() operation, which returns an updated managed
entity, the entity updates will be persisted to the database at the end
of the transaction.
An Extended Persistence Context spans multiple transactions, and the
set of Entities in the Persistence Context stay Managed. This can be
useful in a work flow scenario where a "conversation" with a user spans
The code below shows an example of a Stateful Session EJB with an
Extended Persistence Context in a use case scenario to add line Items
to an Order. After the Order is persisted in the createOrder method, it
remains managed until the EJB remove method is called. In the
addLineItem method , the Order Entity can be updated because it is
managed, and the updates will be persisted at the end of the
The example below contrasts updating the Order using a transaction
scoped Persistence Context verses an extended Persistence context. With
the transaction scoped persistence context, an Entity Manager find must
be done to look up the Order, this returns a Managed Entity which can
be updated. With the Extended Persistence Context the find is not
necessary. The performance advantage of not doing a database read to
look up the Entity, must be weighed against the disadvantages of memory
consumption for caching, and the risk of cached entities being updated by another
transaction. Depending on the application and the risk of
contention among concurrent transactions this may or may not give
better performance / scalability.
JPA second level (L2) caching
JPA second level (L2) caching shares entity state across various persistence contexts.
JPA 1.0 did not specify support of a second level cache, however, most
of the persistence providers provided support for second level
cache(s). JPA 2.0 specifies support for basic cache operations with the
new Cache API, which is accessible from the EntityManagerFactory, shown
If L2 caching is enabled, entities not found in persistence context, will be loaded from L2 cache, if found.
The advantages of L2 caching are:
- avoids database access for already loaded entities
- faster for reading frequently accessed unmodified entities
The disadvantages of L2 caching are:
- memory consumption for large amount of objects
- Stale data for updated objects
- Concurrency for write (optimistic lock exception, or pessimistic lock)
- Bad scalability for frequent or concurrently updated entities
You should configure L2 caching for entities that are:
- read often
- modified infrequently
- Not critical if stale
You should protect any data that can be concurrently modified with a locking strategy:
- Must handle optimistic lock failures on flush/commit
- configure expiration, refresh policy to minimize lock failures
The Query cache is useful for queries that are run frequently with the same parameters, for not modified tables.
The EclipseLink JPA persistence provider caching Architecture
The EclipseLink caching Architecture is shown below.
Support for second level cache in EclipseLink is turned on by default,
entities read are L2 cached.
You can disable the L2 cache. EclipseLink caches entities in L2,
Hibernate caches entity id and state in L2. You can configure caching
by Entity type or Persistence Unit with the following configuration
- Cache isolation, type, size, expiration, coordination, invalidation,refreshing
- Coordination (cluster-messaging)
- Messaging: JMS, RMI, RMI-IIOP, …
- Mode: SYNC, SYNC+NEW, INVALIDATE, NONE
The example below shows configuring the L2 cache for an entity using the @Cache annotation
The Hibernate JPA persistence provider caching Architecture
The Hibernate JPA persistence provider caching architecture is different than EclipseLink:
it is not configured by default, it does not cache enities just id and
state, and you can plug in different L2 caches. The diagram below
shows the different L2 cache types that you can plug into Hibernate.
The configuration of the cache depends on the type of caching plugged
in. The example below shows configuring the hibernate L2 cache for an
entity using the @Cache annotation