JPA 面试题与答案

问题 1. What is JPA?

JPA (Java Persistence API) is a Java EE and Java SE specification that describes a management system for saving java objects to relational database tables in a convenient form.

Java itself does not contain JPA implementations, however, there are many implementations of this specification from different companies (open and not). This is not the only way to save java objects in databases (ORM systems), but one of the most popular in the Java world.

问题 2. What is JPA and its features?

问题 3. Why to use JPA?

• JPA is the standard, and standards are good!
• Using JPA does not tie you to Hibernate.
• JPA gives you most of the features of plain old Hibernate, except:
• No criteria queries in JPA 2.0. Criteria query is a neat feature of Hibernate that constructs queries using Java-based combinators instead of the alternate query language, getting the benefit of IntelliSense and Eclipse's refactoring tools.
• JPA doesn't have Hibernate's DeleteOrphan cascade type.
• Delete Orphan is a useful annotation that directs Hibernate to delete entities in a collection if the parent is deleted, preventing orphaning.
• JPA doesn't have an equivalent to Hibernate's ScrollableResults.

问题 4. What is ORM in JPA?

Mapping between database tables and java objects called ORM (Object Relational Mapping). JPA (Java Persistence API) provides and ORM facility for managing relational tables in Java applications. It is a specification and few implementations are like Hibernate, JDO, EJB, Toplink. By using JPA can be fetched data, insert, update etc.

问题 5. What is an EntityManager?

• Entity manager javax.persistence.EntityManager provides the operations from and to the database, e.g. find objects, persists them, remove objects from the database, etc.
• Entities which are managed by an EntityManager will automatically propagate these changes to the database (if this happens within a commit statement). These objects are known as persistent object. 
• If the Entity Manager is closed (via close()) then the managed entities are in a detached state. These are known as the detached objects. If you want synchronize them again with the database, the a Entity Manager provides the merge() method. Once merged, the object(s) becomes perstent objects again.
• The EntityManager is the API of the persistence context, and an EntityManager can be injected directly in to a DAO without requiring a JPA Template. The Spring Container is capable of acting as a JPA container and of injecting the EntityManager by honoring the @PersistenceContext (both as field-level and a method-level annotation).