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Hibernate / EclipseLink Interview questions

What is persistence.xml used for?

persistence.xml is the standard JPA configuration file, located under META-INF/, that defines one or more persistence units. It tells the JPA runtime which provider to use, how to connect to the database, which entity classes belong to the unit, and any additional provider-specific properties.

<persistence-unit name="myPU" transaction-type="RESOURCE_LOCAL">
    <provider>org.eclipse.persistence.jpa.PersistenceProvider</provider>
    <class>com.example.Employee</class>
    <properties>
        <property name="jakarta.persistence.jdbc.url" value="jdbc:h2:mem:testdb"/>
        <property name="eclipselink.ddl-generation" value="create-tables"/>
        <property name="eclipselink.logging.level" value="FINE"/>
    </properties>
</persistence-unit>

The eclipselink.* properties shown here are provider-specific extensions, not part of the JPA standard; they only take effect because EclipseLink is the configured provider and recognizes those property names.

persistence.xml is expected to live under which directory?
Properties prefixed with eclipselink. in persistence.xml are:

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What is EclipseLink? What is JPA (Jakarta Persistence API)? What is the relationship between EclipseLink and Jakarta Persistence? What is a Persistence Unit in EclipseLink? What is persistence.xml used for? What is an EntityManager in EclipseLink? What is an EntityManagerFactory? What is a JPA entity? What are EclipseLink's core components? What is EclipseLink MOXy? What is class weaving in EclipseLink? What is the EclipseLink shared cache (L2 cache)? What is a fetch group in EclipseLink? What is optimistic locking in EclipseLink and JPA? What is the @Convert annotation used for? What is a Customizer in EclipseLink? What is JPQL? What are EclipseLink query hints? What is DDL generation in EclipseLink? What is a UnitOfWork in EclipseLink's native API? What is EclipseLink's native Session API? What is static weaving in EclipseLink? What are EclipseLink descriptors? What is EclipseLink DBWS? What is EclipseLink SDO? Explain the lifecycle of a JPA entity managed by EclipseLink? Why is class weaving important for lazy loading in EclipseLink? How does EclipseLink's shared cache differ from Hibernate's second-level cache? What is the difference between static weaving and dynamic weaving? How do you configure batch fetching using EclipseLink query hints? When should you use pessimistic locking instead of optimistic locking? How do you troubleshoot lazy loading failures caused by missing weaving in EclipseLink? What is the difference between EAGER and LAZY fetch strategies, and how does EclipseLink implement lazy loading without a live session? How does EclipseLink implement change tracking for detecting entity modifications? Explain the internal working of EclipseLink's cache coordination? What is the difference between EclipseLink and Hibernate? How do you implement multitenancy in EclipseLink? Why use fetch groups instead of always fetching the full entity? What is the difference between attribute change tracking and deferred change detection? How does EclipseLink cache query results, and what invalidation strategies are available? When would you choose isolated cache over shared cache for a multitenant entity? How do you configure a Customizer to add native EclipseLink mappings not expressible via annotations? What is the difference between JPQL and the EclipseLink native Expression/Criteria API? Explain the internal working of EclipseLink's Unit of Work commit process? How do you optimize EclipseLink performance for a high-throughput application? What is the difference between EclipseLink's @Convert and JPA's standard AttributeConverter? How does EclipseLink decide when to use join fetch versus batch fetch? Why should you use @Version for optimistic locking instead of manual version checks? What is the difference between EntityManager persistence context types: transaction-scoped vs extended? How do you troubleshoot stale data issues caused by EclipseLink's shared cache?
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