Hibernate Annotations are a way to define the Object-Relational Mapping (ORM) mapping between Java classes and database tables, columns, and relationships using Java annotations. Annotations provide a more concise and readable way to configure Hibernate mappings compared to XML configuration files.
- Hibernate supports a wide range of annotations for various mapping scenarios. Here are some commonly used Hibernate annotations:
@Entity:
- Used to annotate a Java class to indicate that it’s a persistent entity. Each entity class corresponds to a database table.
@Entity
public class Product {
// ...
}
@Table:
- Specifies the name of the database table associated with the annotated entity class. You can also specify additional attributes such as the schema name.
@Entity
@Table(name = "products")
public class Product {
// ...
}
@Id:
- Marks a field or property as the primary key of the entity. It’s often used in conjunction with one of the generation strategy annotations like @GeneratedValue.
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;
@GeneratedValue:
- Specifies the strategy for generating primary key values. Common strategies include GenerationType.IDENTITY, GenerationType.SEQUENCE, and GenerationType.AUTO.
@GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;
@Column:
- Defines the mapping between an entity attribute and a database column. You can specify attributes like column name, length, nullable, etc.
@Column(name = "product_name", length = 100,
nullable = false)
private String name;
- name attribute permits the name of the column to be explicitly specified.
- length attribute permits the size of the column used to map a value particularly for a String value.
- ullable attribute permits the column to be marked NOT NULL when the schema is generated.>/li>
- unique attribute permits the column to be marked as containing only unique values.
@OneToMany and @ManyToOne:
- Used to define one-to-many and many-to-one relationships between entities. These annotations help establish associations between tables.
@OneToMany(mappedBy = "category")
private Set<Product> products;
@ManyToOne
@JoinColumn(name = "category_id")
private Category category;
@ManyToMany:
- Indicates a many-to-many relationship between entities. It requires a join table to map the relationship.
@ManyToMany
@JoinTable(
name = "user_role",
joinColumns = @JoinColumn(name = "user_id"),
inverseJoinColumns = @JoinColumn(name = "role_id")
)
private Set<Role> roles;
@JoinColumn:
- Specifies the mapping of a join column for relationships. It is often used in conjunction with @ManyToOne or @OneToOne.
@ManyToOne
@JoinColumn(name = "customer_id")
private Customer customer;
@Enumerated:
- Maps an enumerated (enum) type to a database column. You can specify whether the enum should be stored as a string or an integer in the database.
@Enumerated(EnumType.STRING)
private Status status;
@Temporal:
- Used to map java.util.Date or java.util.Calendar fields to a database date or timestamp column.
@Temporal(TemporalType.DATE)
private Date birthDate;
