JPA entity with @Entity annotation in Spring Boot - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output when saving a JPA entity without an @Id field?

Consider a JPA entity class annotated with @Entity but missing the @Id annotation on any field. What happens when you try to save an instance of this entity using Spring Data JPA?

Spring Boot

import jakarta.persistence.Entity;

@Entity
public class Product {
    private String name;
    private double price;

    // getters and setters
}

A A runtime exception is thrown indicating no identifier is found.

B The entity saves successfully with an auto-generated ID.

C The entity saves but with a null ID value.

D The application fails to start due to missing @Id.

💡 Hint

Every JPA entity must have a field annotated with @Id to identify the primary key.

JPA requires an @Id field to uniquely identify each entity instance. Without it, saving the entity causes a runtime exception because JPA cannot manage the entity's identity.

📝 Syntax

intermediate

Which option correctly defines a JPA entity with a primary key?

Select the code snippet that correctly defines a JPA entity class with an @Entity annotation and a primary key field.

A

import jakarta.persistence.Entity;
import jakarta.persistence.Id;

@Entity
public class User {
    @Id
    public void userId() {}
}

B

import jakarta.persistence.Entity;
import jakarta.persistence.Id;

@Entity
public class User {
    @Id
    private Long userId;
}

C

import jakarta.persistence.Entity;
import jakarta.persistence.Id;

@Entity
public class User {
    private Long userId;
    @Id
}

D

import jakarta.persistence.Entity;

@Entity
public class User {
    private Long userId;
}

💡 Hint

The @Id annotation must be on a field or getter method representing the primary key.

Option B correctly places @Id on a private field. Option B misses @Id. Option B places @Id without a field. Option B places @Id on a method that is not a getter.

❓ state_output

advanced

What is the state of a JPA entity after calling EntityManager.persist()?

Given a new JPA entity instance, what is its state immediately after calling EntityManager.persist(entity)?

A Managed - the entity is now tracked and will be synchronized with the database.

B Transient - the entity is not yet associated with the persistence context.

C Detached - the entity is no longer managed by the persistence context.

D Removed - the entity is marked for deletion from the database.

💡 Hint

Persisting an entity means it becomes managed by the persistence context.

Calling persist() makes the entity managed, so changes to it will be tracked and saved to the database on flush or commit.

🔧 Debug

advanced

Why does this JPA entity cause a runtime error?

Examine the following JPA entity code. Why does it cause a runtime error when the application starts?

Spring Boot

import jakarta.persistence.Entity;
import jakarta.persistence.Id;

@Entity
public class Order {
    @Id
    private Long id;

    private String description;

    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    // Missing no-argument constructor
    public Order(String description) {
        this.description = description;
    }
}

A The entity class is missing the @Table annotation.

B The @Id annotation is incorrectly placed on a private field.

C The entity lacks a no-argument constructor required by JPA.

D The getter and setter methods are missing for the description field.

💡 Hint

JPA requires a public or protected no-argument constructor for entities.

JPA needs a no-argument constructor to instantiate entities via reflection. Without it, a runtime error occurs.

🧠 Conceptual

expert

What happens if two JPA entities share the same @Entity name?

Two different classes are annotated with @Entity(name = "Customer"). What is the effect when both are used in the same Spring Boot application?

A Both entities will be treated as one, merging their fields and causing data corruption.

B The second entity will override the first silently without error.

C The application will run normally, but queries must specify the full class name.

D Hibernate will throw a deployment exception due to duplicate entity names.

💡 Hint

Entity names must be unique within the persistence unit to avoid conflicts.

Hibernate requires unique entity names. Duplicate names cause deployment errors to prevent ambiguity in queries.