5. Additional Configuration Annotations

1. @Transient

About

@Transient is a JPA annotation used to mark a field or property of an entity that should not be persisted (saved) in the database. In other words, JPA ignores this field when reading or writing the entity to the database.

The field will still exist inside the Java object and can be used for temporary, calculated, or helper data — but it will not be mapped to any database column.

Why is @Transient Needed?

• Temporary Fields: Sometimes you want to maintain a field only for business logic inside your application but don't want it to be stored in the database.

• Computed Values: You might want a property that is calculated dynamically from other fields and doesn't need database storage.

• Helper Variables: Fields needed for UI, caching, intermediate results, or computations.

• DTO Enrichment: When enriching an entity before sending it over the network without affecting database structure.

Without @Transient, JPA would assume that every field should be persisted, leading to errors if no matching database column exists.

Where is @Transient Used?

• Inside Entity classes (@Entity-annotated classes).

• On fields or getter methods (depending on whether field access or property access is used).

• When you have non-persistent data inside an entity.

Syntax of @Transient

@Entity
public class Employee {

    @Id
    private Long id;

    private String name;

    @Transient
    private int calculatedAge; // Not persisted

    // Getter and Setter methods
}

In this example, calculatedAge is present in the Java object but not saved to or retrieved from the database.

Example: @Transient on Getter Method

If we use property-based access (annotations on getters instead of fields):

@Entity
public class Product {

    @Id
    private Long id;

    private String name;

    private double price;

    @Transient
    public double getDiscountedPrice() {
        return price * 0.9; // 10% discount
    }
}

Here, JPA calls the getter when needed but never persists the discounted price.

How It Works

• During entity persistence (save), JPA ignores the @Transient field.

• During entity loading (read), JPA does not populate the @Transient field.

• You are responsible for manually setting or computing the transient field if needed at runtime.

JPA @Transient vs Java transient keyword

Aspect	@Transient (JPA)	transient (Java keyword)
Purpose	Ignore field in database persistence	Ignore field during Java object serialization
Context	JPA/Hibernate ORM mapping	Java Serializable interface
Effect	Field not mapped to a database column	Field not included in a serialized byte stream
Usage	Annotate field with @Transient	Use transient keyword before field declaration

We can even use both together if you want the field to be neither persisted to the database nor serialized to disk.

@Transient
private transient int tempValue;

2. @ElementCollection

About

In JPA, @ElementCollection is used when an entity needs to store a collection of simple types (like String, Integer, etc.) or Embeddable classes, without making them separate entities. Instead of mapping each element to an entity (@Entity), the collection elements are stored in a separate table, automatically managed by JPA.

• Simple Type Collection → Collection of primitive or basic types (e.g., List<String>).

• Embeddable Type Collection → Collection of value objects (@Embeddable).

Why @ElementCollection is Needed?

• When we have multiple simple values associated with a single entity.

• When we want to avoid creating a full-blown entity for small, dependent data.

• To keep modelling clean for lightweight data that doesn't need identity (@Id).

Instead of creating separate entities and foreign keys, JPA handles the additional table automatically.

Where is @ElementCollection Used?

• On fields representing collections (usually List, Set, or Map).

• Inside an entity class (@Entity).

• For non-entity elements (simple types or @Embeddable types).

How It Works ?

• JPA creates a separate table to store the collection elements.

• It links the records back to the owning entity using a foreign key.

• The table is managed automatically: insert, update, delete as part of the parent entity's lifecycle.

Syntax

Example: Collection of Basic Types

@Entity
public class Employee {

    @Id
    private Long id;

    private String name;

    @ElementCollection
    @CollectionTable(name = "employee_skills", 
                     joinColumns = @JoinColumn(name = "employee_id"))
    @Column(name = "skill")
    private List<String> skills = new ArrayList<>();

    // Getter and Setter methods
}

• skills is a list of String.

• A table called employee_skills will be created with:

  • employee_id (foreign key)

  • skill (the actual skill name)

Table Mapping Behind the Scenes

Employee Table:

id (PK)	name
1	John Doe

Employee_Skills Table:

employee_id (FK)	skill
1	Java
1	Spring Boot
1	Hibernate

Important Supporting Annotations

Annotation	Purpose
@CollectionTable	Defines the table name and join columns for the collection.
@Column	Defines the name of the column that stores the element value itself.

If we don't specify @CollectionTable, JPA automatically generates a table name and foreign key.

@ElementCollection with @Embeddable

We can also store complex value objects instead of just simple types.

@Embeddable
public class Address {
    private String street;
    private String city;
    private String zipCode;
    // Getter and Setter
}

@Entity
public class Customer {

    @Id
    private Long id;

    private String name;

    @ElementCollection
    @CollectionTable(name = "customer_addresses", 
                     joinColumns = @JoinColumn(name = "customer_id"))
    private List<Address> addresses = new ArrayList<>();

    // Getter and Setter methods
}

Here:

• A customer_addresses table will be created.

• It will have columns for customer_id, street, city, zipCode.

How It Is Managed

• Insertion: When we save the parent entity, the collection elements are inserted automatically.

• Deletion: When we delete or update, the corresponding collection records are handled automatically.

• No Separate Identity: Elements are part of the parent entity, they are not independent entities.

Characteristics of @ElementCollection

Aspect	Details
Purpose	To map collections of simple types or embeddable types.
Database Behavior	A separate table is created automatically to store the collection elements.
Lifecycle	Tied to the parent entity’s lifecycle.
Identity	Elements have no primary key; the parent's foreign key manages the relationship.
Fetching	Default is LAZY fetching (loaded only when accessed).
Operations	Insert, update, delete are cascaded from the owning entity automatically.

FetchType in @ElementCollection

• By default, FetchType.LAZY is applied.

• We can manually make it FetchType.EAGER if you want the collection to load immediately.

@ElementCollection(fetch = FetchType.EAGER)
private List<String> skills;

Use EAGER cautiously — it might lead to performance overhead if the collection is large.

3. @Lob

About

• In JPA, @Lob is used to map large objects to database columns.

• It marks a persistent property or field to be stored as either:

  • BLOB (Binary Large Object) → for binary data like images, files, etc.

  • CLOB (Character Large Object) → for large text content.

JPA automatically decides whether to use a BLOB or CLOB based on the Java type.

Why @Lob is Needed?

• To store large amounts of data (binary or text) in the database without size limitations of standard columns.

• To map fields like:

  • Long textual content (articles, descriptions, logs).

  • Large files (images, videos, PDFs, audio, etc.).

Where is @Lob Used?

• On entity fields of type:

  • String (for large text → CLOB)

  • byte[] (for binary data → BLOB)

  • Serializable objects (if using Java object serialization)

• Inside a class annotated with @Entity.

How It Works ?

• If the field type is String, JPA maps it as a CLOB.

• If the field type is byte[], JPA maps it as a BLOB.

• The corresponding database column is treated as large object storage.

Syntax of @Lob

Example: Storing Large Text (CLOB)

@Entity
public class Article {

    @Id
    private Long id;

    private String title;

    @Lob
    @Column(name = "content", columnDefinition = "CLOB")
    private String content;

    // Getter and Setter
}

• content is a large text field.

• It will be stored as a CLOB in the database.

Example: Storing Binary Data (BLOB)

@Entity
public class Document {

    @Id
    private Long id;

    private String name;

    @Lob
    @Column(name = "file_data", columnDefinition = "BLOB")
    private byte[] fileData;

    // Getter and Setter
}

• fileData is a byte array.

• It will be stored as a BLOB in the database.

Example: Serializable Object

We can serialize Java objects into a BLOB.

@Entity
public class Report {

    @Id
    private Long id;

    private String title;

    @Lob
    private Serializable reportData;

    // Getter and Setter
}

• reportData must be a Serializable object.

• Stored as a serialized byte stream in the database.

Important Points

• CLOB: Large character/text storage — Used for things like articles, logs, documentation.

• BLOB: Large binary storage — Used for things like images, files, attachments.

• Lazy Fetching: LOB fields are usually fetched lazily (FetchType.LAZY) by JPA providers to avoid memory overload.

• Size Limitation: BLOB/CLOB allows storing very large amounts of data, beyond the limit of VARCHAR or similar standard types.

• Database Support: Actual behavior can vary slightly depending on the database (Oracle, MySQL, PostgreSQL, etc.).

FetchType Behavior for @Lob

• According to the JPA specification, @Lob annotated fields are recommended to be lazily fetched.

• But, it depends on the provider (Hibernate, EclipseLink, etc.).

• We can explicitly specify:

@Lob
@Basic(fetch = FetchType.LAZY)
private byte[] image;

Best Practices for @Lob

Best Practice	Details
Use Lazy Fetching	To avoid loading large data when not needed.
Separate Large Data	If possible, keep LOB data in a separate table to avoid heavy load during regular queries.
Explicit Column Types	Use @Column(columnDefinition = "CLOB" or "BLOB") if you want to tightly control DDL generation.
Optimize Database Storage	Ensure database tablespace is designed to handle large LOBs properly.
Handle Streams	If LOB data is very large, prefer streaming APIs instead of loading everything into memory at once.