何先生在春季完全可以使用JPA？ 一些疑问

Question

I am studying for the Spring Core certification and I am finding some doubts about some concept related to the JPA use in Spring.

Reading the training course documentation it speack about the JPA EntityManager interface and it say that it do the following things:

• Manages a unit of work and persistent objects therein: the PersistenceContext

• Lifecycle often bound to a Transaction (usually containermanaged)

So what exactly is a unit of work in this context?

Is it something like a group of persistent classes on which is definied a JPA provider (like Hibernate ) and it is definied a transactional types (local vs. JTA)? Or what?

So I know that the EntityManager interface provide a bridje between the OO model and the relational model using the following method that represent some common operation on a DB:

• persist(Object o): ** Adds the entity to the Persistence Context: **SQL: insert into table …

• remove(Object o): ** Removes the entity from the Persistence Context: **SQL: delete from table …

• find(Class entity, Object primaryKey): ** Find by primary key: **SQL: select * from table where id = ?

• Query createQuery(String jpqlString): Create a JPQL query

• flush(): Force changed entity state to be written to database immediately

etcetc

So now I have some dount about this interface. This is only an interface so I need a concrete implementation of it to work. I think that I have not to implement the previous method by myself but I have to use an ORM like Hibernate that provide me an implementation so I don't need to do extrawork. Is it correct?

Tnx

Answer 1

Unit of Work

A unit of work is a design pattern described by Martin Fowler as “ [maintaining] a list of objects affected by a business transaction and coordinates the writing out of changes and the resolution of concurrency problems. ”[PoEAA] In other words, its a series of operations we wish to carry out against the database together. Basically, it is a transaction, though fulfilling a unit of work will often span multiple physical database transactions (see Section 11.1.2, “Long conversations”). So really we are talking about a more abstract notion of a transaction. The term "business transaction" is also sometimes used in lieu of unit of work.

(Better to quote existing documentation instead of rearranging the words, I think it is pretty clear)

And you are right about EntityManager , it is a interface that each JPA provider implements, along with the rest of the spec (JPA itself is a collection of interfaces, not able to do anything without an implementation). Again, another fine quote of the documentation

An EntityManager instance is associated with a persistence context. A persistence context is a set of entity instances in which for any persistent entity identity there is a unique entity instance. Within the persistence context, the entity instances and their lifecycle are managed. The EntityManager API is used to create and remove persistent entity instances, to find entities by their primary key, and to query over entities.

The set of entities that can be managed by a given EntityManager instance is defined by a persistence unit. A persistence unit defines the set of all classes that are related or grouped by the application, and which must be colocated in their mapping to a single database.