JPA and Hibernate Interview Guide

Entity Mapping

Example entity:

@Entity
@Data
public class Employee {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    private String name;

    private String salary;

    @Version
    private Long version;
}

Annotation Reference

Annotation	Purpose
@Entity	Maps the class to a database table
@Id	Primary key
@GeneratedValue	Auto-generates the primary key
@Version	Enables optimistic locking

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How @Version Works

Optimistic locking prevents lost updates.

sequenceDiagram
    participant UserA
    participant DB
    participant UserB

    UserA->>DB: Read Employee (version=0)
    UserB->>DB: Read Employee (version=0)

    UserA->>DB: Update Employee
    DB-->>UserA: version becomes 1

    UserB->>DB: Update Employee(version=0)
    DB-->>UserB: OptimisticLockException

Each successful update increments the version automatically.

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Recommended Salary Type

Instead of:

private String salary;

Prefer:

private BigDecimal salary;

Corresponding SQL:

salary DECIMAL(10,2)

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Automatic Schema Generation

Spring Boot can create or update tables automatically.

spring.jpa.hibernate.ddl-auto=update

Common Values

Value	Purpose
none	No schema generation
validate	Validate schema only
update	Update existing schema
create	Drop and recreate tables
create-drop	Recreate and remove on shutdown

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Interview Questions

What is optimistic locking?

Optimistic locking uses a version column to detect concurrent updates. If another transaction has already modified the row, Hibernate throws an OptimisticLockException.

Why use @Version?

• Prevents lost updates
• Supports concurrent users
• Requires no explicit database locking

Why is ddl-auto=update discouraged in production?

Schema changes should be managed using migration tools such as Flyway or Liquibase instead of automatic updates.

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Best Practices

• Use BigDecimal for monetary values.
• Use @Version for concurrent updates.
• Prefer Flyway or Liquibase for schema migrations.
• Avoid ddl-auto=create in production.
• Use meaningful SQL migration scripts.

For running Spring Boot against a Dockerized MySQL (service names vs localhost, the “Unable to determine Dialect” error, and table setup SQL), see the Spring Boot + Docker + MySQL guide.

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Key Takeaways

• @Version enables optimistic locking.
• Hibernate increments the version automatically.
• ddl-auto=update is suitable for development only.
• Use migration tools for production schema management.