Column mapping with @Column in Spring Boot - Mini Project: Build & Apply

Column mapping with @Column in Spring Boot

📖 Scenario: You are building a simple Spring Boot application to manage books in a library. Each book has a title and a number of pages. You want to map these fields to specific columns in a database table using the @Column annotation.

🎯 Goal: Create a Spring Boot entity class called Book with fields title and pages. Use the @Column annotation to map title to a database column named book_title and pages to a column named number_of_pages.

📋 What You'll Learn

Create a class named Book annotated with @Entity

Add a private field title of type String

Add a private field pages of type int

Use @Column(name = "book_title") on the title field

Use @Column(name = "number_of_pages") on the pages field

💡 Why This Matters

🌍 Real World

Mapping Java class fields to database columns is essential when working with databases in Spring Boot applications. It ensures your data is stored and retrieved correctly.

💼 Career

Understanding @Column mapping is a basic skill for Java developers working with Spring Boot and JPA for database operations.

1

Create the Book entity class with fields

Create a public class called Book and add two private fields: title of type String and pages of type int.

Spring Boot

public class Book {
    // Your code here
}

Hint

Think of the Book class as a blueprint for book objects. Add the fields inside the class.

2

Add @Entity annotation to the Book class

Add the @Entity annotation above the Book class declaration to mark it as a JPA entity.

Spring Boot

@Entity
public class Book {
    private String title;
    private int pages;
    // Your code here
}

Hint

The @Entity annotation tells Spring Boot this class maps to a database table.

3

Map the title field to book_title column

Add the @Column(name = "book_title") annotation above the title field to map it to the database column named book_title.

Spring Boot

@Entity
public class Book {
    @Column(name = "book_title")
    private String title;
    private int pages;
    // Your code here
}

Hint

The @Column annotation lets you specify the exact database column name for a field.

4

Map the pages field to number_of_pages column

Add the @Column(name = "number_of_pages") annotation above the pages field to map it to the database column named number_of_pages.

Spring Boot

@Entity
public class Book {
    @Column(name = "book_title")
    private String title;
    @Column(name = "number_of_pages")
    private int pages;
    // Your code here
}

Hint

Just like the title, use @Column to map the pages field to the correct column.

Practice

1. What is the main purpose of the @Column annotation in Spring Boot?

easy

A. To define a primary key for the entity

B. To create a new database table automatically

C. To configure the database connection settings

D. To map a Java field to a specific database column

Solution

1. Step 1: Understand the role of @Column

   The @Column annotation is used to link a Java class field to a database column, specifying details like name and constraints.

2. Step 2: Differentiate from other annotations

   Other annotations like @Id define primary keys, and database connection settings are configured elsewhere, not with @Column.

3. Final Answer:

   To map a Java field to a specific database column -> Option D

4. Quick Check:

   @Column maps field to column [OK]

Hint: Remember: @Column links fields to columns directly [OK]

Common Mistakes:

• Confusing @Column with @Id for primary keys
• Thinking @Column creates tables
• Mixing @Column with database connection setup

2. Which of the following is the correct way to specify a column name and length using @Column?

easy

A. @Column(name = "user_name", length = 50)

B. @Column(columnName = "user_name", size = 50)

C. @Column(name = user_name, length = 50)

D. @Column(name = "user_name", maxLength = 50)

Solution

1. Step 1: Check correct attribute names

   The correct attributes for @Column are 'name' for column name and 'length' for string length.

2. Step 2: Validate syntax

   Attributes must be strings in quotes for names; length is an integer. @Column(name = "user_name", length = 50) uses correct syntax and attribute names.

3. Final Answer:

   @Column(name = "user_name", length = 50) -> Option A

4. Quick Check:

   Use name and length attributes correctly [OK]

Hint: Use 'name' and 'length' exactly in @Column [OK]

Common Mistakes:

• Using wrong attribute names like columnName or maxLength
• Forgetting quotes around string values
• Using variables without quotes for name

3. Given the entity field:

@Column(name = "email", unique = true, nullable = false)
private String email;

What will happen if you try to save two entities with the same email?

medium

A. The application will throw a null pointer exception

B. The database will throw a unique constraint violation error

C. The second entity will overwrite the first one silently

D. Both entities will be saved without error

Solution

1. Step 1: Understand unique constraint

   The 'unique = true' attribute enforces that no two rows can have the same value in this column.

2. Step 2: Predict behavior on duplicate insert

   Trying to save a duplicate email will cause the database to reject the insert/update with a unique constraint violation error.

3. Final Answer:

   The database will throw a unique constraint violation error -> Option B

4. Quick Check:

   unique = true prevents duplicates [OK]

Hint: unique=true means no duplicate values allowed [OK]

Common Mistakes:

• Assuming duplicates are allowed with unique=true
• Confusing unique with nullable
• Expecting application to handle duplicates silently

4. Consider this code snippet:

@Column(name = "age", nullable = false)
private Integer age;

What is wrong if the application allows saving an entity with age = null without error?

medium

A. The database column is not set to NOT NULL, so nullable=false is ignored

B. The @Column annotation does not enforce nullability at the database level

C. The field type Integer allows null, so nullable=false has no effect

D. The entity is missing @NotNull validation annotation

Solution

1. Step 1: Understand nullable attribute

   The 'nullable = false' in @Column tells JPA to generate a NOT NULL constraint in the database schema.

2. Step 2: Recognize runtime enforcement

   @Column does not validate null values in Java code; it only affects database schema. If schema is not updated, nulls can be saved.

3. Final Answer:

   The database column is not set to NOT NULL, so nullable=false is ignored -> Option A

4. Quick Check:

   nullable=false affects DB schema, not Java validation [OK]

Hint: @Column nullable=false sets DB constraint, not Java checks [OK]

Common Mistakes:

• Thinking nullable=false validates Java field nulls
• Assuming Integer type disables nulls
• Confusing @Column with validation annotations

5. You want to map a Java field private String phoneNumber; to a database column named phone_num that must be unique and cannot be null, with a maximum length of 15 characters. Which is the correct @Column annotation to use?

hard

A. @Column(name = "phone_num", unique = false, nullable = true, length = 15)

B. @Column(name = "phoneNumber", unique = true, nullable = false, length = 15)

C. @Column(name = "phone_num", unique = true, nullable = false, length = 15)

D. @Column(name = "phone_num", unique = true, nullable = false)

Solution

1. Step 1: Match column name and constraints

   The column name must be 'phone_num', unique true, nullable false, and length 15 as per requirements.

2. Step 2: Check each option

   @Column(name = "phone_num", unique = true, nullable = false, length = 15) matches all requirements exactly. @Column(name = "phoneNumber", unique = true, nullable = false, length = 15) uses wrong column name. @Column(name = "phone_num", unique = false, nullable = true, length = 15) allows null and not unique. @Column(name = "phone_num", unique = true, nullable = false) misses length attribute.

3. Final Answer:

   @Column(name = "phone_num", unique = true, nullable = false, length = 15) -> Option C

4. Quick Check:

   Use correct name, unique, nullable, and length [OK]

Hint: Set all attributes explicitly to match requirements [OK]

Common Mistakes:

• Using Java field name instead of column name
• Forgetting unique or nullable constraints
• Missing length attribute for string columns