Introduction to Jakarta Persistence

This chapter provides a description of Jakarta Persistence.

Overview of Jakarta Persistence

Jakarta Persistence provides Java developers with an object/relational mapping facility for managing relational data in Java applications. Jakarta Persistence consists of four areas:

Jakarta Persistence
The query language
The Jakarta Persistence Criteria API
Object/relational mapping metadata

Entities

An entity is a lightweight persistence domain object. Typically, an entity represents a table in a relational database, and each entity instance corresponds to a row in that table. The primary programming artifact of an entity is the entity class, although entities can use helper classes.

The persistent state of an entity is represented through either persistent fields or persistent properties. These fields or properties use object/relational mapping annotations to map the entities and entity relationships to the relational data in the underlying data store.

Requirements for Entity Classes

An entity class must follow these requirements.

The class must be annotated with the jakarta.persistence.Entity annotation.
The class must have a public or protected, no-argument constructor. The class may have other constructors.
The class must not be declared final. No methods or persistent instance variables must be declared final.
If an entity instance is passed by value as a detached object, such as through a session bean’s remote business interface, the class must implement the Serializable interface.
Entities may extend both entity and non-entity classes, and non-entity classes may extend entity classes.
Persistent instance variables must be declared private, protected, or package-private and can be accessed directly only by the entity class’s methods. Clients must access the entity’s state through accessor or business methods.

Persistent Fields and Properties in Entity Classes

The persistent state of an entity can be accessed through either the entity’s instance variables or properties. The fields or properties must be of the following Java language types:

Java primitive types
java.lang.String
Other serializable types, including:
- Wrappers of Java primitive types
- java.math.BigInteger
- java.math.BigDecimal
- java.util.Date
- java.util.Calendar
- java.sql.Date
- java.sql.Time
- java.sql.TimeStamp
- User-defined serializable types
- byte[]
- Byte[]
- char[]
- Character[]
Enumerated types
Other entities and/or collections of entities
Embeddable classes

Entities may use persistent fields, persistent properties, or a combination of both. If the mapping annotations are applied to the entity’s instance variables, the entity uses persistent fields. If the mapping annotations are applied to the entity’s getter methods for JavaBeans-style properties, the entity uses persistent properties.

Persistent Fields

If the entity class uses persistent fields, the Persistence runtime accesses entity-class instance variables directly. All fields not annotated jakarta.persistence.Transient or not marked as Java transient will be persisted to the data store. The object/relational mapping annotations must be applied to the instance variables.

Persistent Properties

If the entity uses persistent properties, the entity must follow the method conventions of JavaBeans components. JavaBeans-style properties use getter and setter methods that are typically named after the entity class’s instance variable names. For every persistent property property of type Type of the entity, there is a getter method getProperty and setter method setProperty. If the property is a Boolean, you may use isProperty instead of getProperty. For example, if a Customer entity uses persistent properties and has a private instance variable called firstName, the class defines a getFirstName and setFirstName method for retrieving and setting the state of the firstName instance variable.

The method signatures for single-valued persistent properties are as follows:

Type getProperty()
void setProperty(Type type)

The object/relational mapping annotations for persistent properties must be applied to the getter methods. Mapping annotations cannot be applied to fields or properties annotated @Transient or marked transient.

Using Collections in Entity Fields and Properties

Collection-valued persistent fields and properties must use the supported Java collection interfaces regardless of whether the entity uses persistent fields or properties. The following collection interfaces may be used:

java.util.Collection
java.util.Set
java.util.List
java.util.Map

If the entity class uses persistent fields, the type in the preceding method signatures must be one of these collection types. Generic variants of these collection types may also be used. For example, if it has a persistent property that contains a set of phone numbers, the Customer entity would have the following methods:

Set<PhoneNumber> getPhoneNumbers() { ... }
void setPhoneNumbers(Set<PhoneNumber>) { ... }

If a field or property of an entity consists of a collection of basic types or embeddable classes, use the jakarta.persistence.ElementCollection annotation on the field or property.

The two attributes of @ElementCollection are targetClass and fetch. The targetClass attribute specifies the class name of the basic or embeddable class and is optional if the field or property is defined using Java programming language generics. The optional fetch attribute is used to specify whether the collection should be retrieved lazily or eagerly, using the jakarta.persistence.FetchType constants of either LAZY or EAGER, respectively. By default, the collection will be fetched lazily.

The following entity, Person, has a persistent field, nicknames, which is a collection of String classes that will be fetched eagerly. The targetClass element is not required, because it uses generics to define the field:

@Entity
public class Person {
    ...
    @ElementCollection(fetch=EAGER)
    protected Set<String> nickname = new HashSet();
    ...
}

Collections of entity elements and relationships may be represented by java.util.Map collections. A Map consists of a key and a value.

When using Map elements or relationships, the following rules apply.

The Map key or value may be a basic Java programming language type, an embeddable class, or an entity.
When the Map value is an embeddable class or basic type, use the @ElementCollection annotation.
When the Map value is an entity, use the @OneToMany or @ManyToMany annotation.
Use the Map type on only one side of a bidirectional relationship.

If the key type of a Map is a Java programming language basic type, use the annotation jakarta.persistence.MapKeyColumn to set the column mapping for the key. By default, the name attribute of @MapKeyColumn is of the form RELATIONSHIP-FIELD/PROPERTY-NAME_KEY. For example, if the referencing relationship field name is image, the default name attribute is IMAGE_KEY.

If the key type of a Map is an entity, use the jakarta.persistence.MapKeyJoinColumn annotation. If the multiple columns are needed to set the mapping, use the annotation jakarta.persistence.MapKeyJoinColumns to include multiple @MapKeyJoinColumn annotations. If no @MapKeyJoinColumn is present, the mapping column name is by default set to RELATIONSHIP-FIELD/PROPERTY-NAME_KEY. For example, if the relationship field name is employee, the default name attribute is EMPLOYEE_KEY.

If Java programming language generic types are not used in the relationship field or property, the key class must be explicitly set using the jakarta.persistence.MapKeyClass annotation.

If the Map key is the primary key or a persistent field or property of the entity that is the Map value, use the jakarta.persistence.MapKey annotation. The @MapKeyClass and @MapKey annotations cannot be used on the same field or property.

If the Map value is a Java programming language basic type or an embeddable class, it will be mapped as a collection table in the underlying database. If generic types are not used, the @ElementCollection annotation’s targetClass attribute must be set to the type of the Map value.

If the Map value is an entity and part of a many-to-many or one-to-many unidirectional relationship, it will be mapped as a join table in the underlying database. A unidirectional one-to-many relationship that uses a Map may also be mapped using the @JoinColumn annotation.

If the entity is part of a one-to-many/many-to-one bidirectional relationship, it will be mapped in the table of the entity that represents the value of the Map. If generic types are not used, the targetEntity attribute of the @OneToMany and @ManyToMany annotations must be set to the type of the Map value.

Validating Persistent Fields and Properties

Jakarta Bean Validation provides a mechanism for validating application data. Bean Validation is integrated into the Jakarta EE containers, allowing the same validation logic to be used in any of the tiers of an enterprise application.

Bean Validation constraints may be applied to persistent entity classes, embeddable classes, and mapped superclasses. By default, the Persistence provider will automatically perform validation on entities with persistent fields or properties annotated with Bean Validation constraints immediately after the PrePersist, PreUpdate, and PreRemove lifecycle events.

Bean Validation constraints are annotations applied to the fields or properties of Java programming language classes. Bean Validation provides a set of constraints as well as an API for defining custom constraints. Custom constraints can be specific combinations of the default constraints, or new constraints that don’t use the default constraints. Each constraint is associated with at least one validator class that validates the value of the constrained field or property. Custom constraint developers must also provide a validator class for the constraint.

Bean Validation constraints are applied to the persistent fields or properties of persistent classes. When adding Bean Validation constraints, use the same access strategy as the persistent class. That is, if the persistent class uses field access, apply the Bean Validation constraint annotations on the class’s fields. If the class uses property access, apply the constraints on the getter methods.

Built In Jakarta Bean Validation Constraints lists Bean Validation’s built-in constraints, defined in the jakarta.validation.constraints package.

All the built-in constraints listed in Built In Jakarta Bean Validation Constraints have a corresponding annotation, ConstraintName.List, for grouping multiple constraints of the same type on the same field or property. For example, the following persistent field has two @Pattern constraints:

@Pattern.List({
    @Pattern(regexp="..."),
    @Pattern(regexp="...")
})

The following entity class, Contact, has Bean Validation constraints applied to its persistent fields:

@Entity
public class Contact implements Serializable {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;
    @NotNull
    protected String firstName;
    @NotNull
    protected String lastName;
    @Pattern(regexp = "[a-z0-9!#$%&'*+/=?^_`{|}~-]+(?:\\."
            + "[a-z0-9!#$%&'*+/=?^_`{|}~-]+)*@"
            + "(?:[a-z0-9](?:[a-z0-9-]*[a-z0-9])?\\.)+[a-z0-9]"
            + "(?:[a-z0-9-]*[a-z0-9])?",
            message = "{invalid.email}")
    protected String email;
    @Pattern(regexp = "^\\(?(\\d{3})\\)?[- ]?(\\d{3})[- ]?(\\d{4})$",
            message = "{invalid.phonenumber}")
    protected String mobilePhone;
    @Pattern(regexp = "^\\(?(\\d{3})\\)?[- ]?(\\d{3})[- ]?(\\d{4})$",
            message = "{invalid.phonenumber}")
    protected String homePhone;
    @Temporal(jakarta.persistence.TemporalType.DATE)
    @Past
    protected Date birthday;
    ...
}

The @NotNull annotation on the firstName and lastName fields specifies that those fields are now required. If a new Contact instance is created where firstName or lastName have not been initialized, Bean Validation will throw a validation error. Similarly, if a previously created instance of Contact has been modified so that firstName or lastName are null, a validation error will be thrown.

The email field has a @Pattern constraint applied to it, with a complicated regular expression that matches most valid email addresses. If the value of email doesn’t match this regular expression, a validation error will be thrown.

The homePhone and mobilePhone fields have the same @Pattern constraints. The regular expression matches 10 digit telephone numbers in the United States and Canada of the form (xxx) xxx-xxxx.

The birthday field is annotated with the @Past constraint, which ensures that the value of birthday must be in the past.

Primary Keys in Entities

Each entity has a unique object identifier. A customer entity, for example, might be identified by a customer number. The unique identifier, or primary key, enables clients to locate a particular entity instance. Every entity must have a primary key. An entity may have either a simple or a composite primary key.

Simple primary keys use the jakarta.persistence.Id annotation to denote the primary key property or field.

Composite primary keys are used when a primary key consists of more than one attribute, which corresponds to a set of single persistent properties or fields. Composite primary keys must be defined in a primary key class. Composite primary keys are denoted using the jakarta.persistence.EmbeddedId and jakarta.persistence.IdClass annotations.

The primary key, or the property or field of a composite primary key, must be one of the following Java language types:

Java primitive types
Java primitive wrapper types
java.lang.String
java.util.Date (the temporal type should be DATE)
java.sql.Date
java.math.BigDecimal
java.math.BigInteger

Floating-point types should never be used in primary keys. If you use a generated primary key, only integral types will be portable.

A primary key class must meet these requirements.

The access control modifier of the class must be public.
The properties of the primary key class must be public or protected if property-based access is used.
The class must have a public default constructor.
The class must implement the hashCode() and equals(Object other) methods.
The class must be serializable.
A composite primary key must be represented and mapped to multiple fields or properties of the entity class or must be represented and mapped as an embeddable class.
If the class is mapped to multiple fields or properties of the entity class, the names and types of the primary key fields or properties in the primary key class must match those of the entity class.

The following primary key class is a composite key, and the customerOrder and itemId fields together uniquely identify an entity:

public final class LineItemKey implements Serializable {
    private Integer customerOrder;
    private int itemId;

    public LineItemKey() {}

    public LineItemKey(Integer order, int itemId) {
        this.setCustomerOrder(order);
        this.setItemId(itemId);
    }

    @Override
    public int hashCode() {
        return ((this.getCustomerOrder() == null
                ? 0 : this.getCustomerOrder().hashCode())
                ^ ((int) this.getItemId()));
    }

    @Override
    public boolean equals(Object otherOb) {
        if (this == otherOb) {
            return true;
        }
        if (!(otherOb instanceof LineItemKey)) {
            return false;
        }
        LineItemKey other = (LineItemKey) otherOb;
        return ((this.getCustomerOrder() == null
                ? other.getCustomerOrder() == null : this.getCustomerOrder()
                .equals(other.getCustomerOrder()))
                && (this.getItemId() == other.getItemId()));
    }

    @Override
    public String toString() {
        return "" + getCustomerOrder() + "-" + getItemId();
    }
    /* Getters and setters */
}

Multiplicity in Entity Relationships

Multiplicities are of the following types.

One-to-one: Each entity instance is related to a single instance of another entity. For example, to model a physical warehouse in which each storage bin contains a single widget, StorageBin and Widget would have a one-to-one relationship. One-to-one relationships use the jakarta.persistence.OneToOne annotation on the corresponding persistent property or field.
One-to-many: An entity instance can be related to multiple instances of the other entities. A sales order, for example, can have multiple line items. In the order application, CustomerOrder would have a one-to-many relationship with LineItem. One-to-many relationships use the jakarta.persistence.OneToMany annotation on the corresponding persistent property or field.
Many-to-one: Multiple instances of an entity can be related to a single instance of the other entity. This multiplicity is the opposite of a one-to-many relationship. In the example just mentioned, the relationship to CustomerOrder from the perspective of LineItem is many-to-one. Many-to-one relationships use the jakarta.persistence.ManyToOne annotation on the corresponding persistent property or field.
Many-to-many: The entity instances can be related to multiple instances of each other. For example, each college course has many students, and every student may take several courses. Therefore, in an enrollment application, Course and Student would have a many-to-many relationship. Many-to-many relationships use the jakarta.persistence.ManyToMany annotation on the corresponding persistent property or field.

Direction in Entity Relationships

The direction of a relationship can be either bidirectional or unidirectional. A bidirectional relationship has both an owning side and an inverse side. A unidirectional relationship has only an owning side. The owning side of a relationship determines how the Persistence runtime makes updates to the relationship in the database.

Bidirectional Relationships

In a bidirectional relationship, each entity has a relationship field or property that refers to the other entity. Through the relationship field or property, an entity class’s code can access its related object. If an entity has a related field, the entity is said to "know" about its related object. For example, if CustomerOrder knows what LineItem instances it has and if LineItem knows what CustomerOrder it belongs to, they have a bidirectional relationship.

Bidirectional relationships must follow these rules.

The inverse side of a bidirectional relationship must refer to its owning side by using the mappedBy element of the @OneToOne, @OneToMany, or @ManyToMany annotation. The mappedBy element designates the property or field in the entity that is the owner of the relationship.
The many side of many-to-one bidirectional relationships must not define the mappedBy element. The many side is always the owning side of the relationship.
For one-to-one bidirectional relationships, the owning side corresponds to the side that contains the corresponding foreign key.
For many-to-many bidirectional relationships, either side may be the owning side.

Unidirectional Relationships

In a unidirectional relationship, only one entity has a relationship field or property that refers to the other. For example, LineItem would have a relationship field that identifies Product, but Product would not have a relationship field or property for LineItem. In other words, LineItem knows about Product, but Product doesn’t know which LineItem instances refer to it.

Queries and Relationship Direction

Jakarta Persistence query language and Criteria API queries often navigate across relationships. The direction of a relationship determines whether a query can navigate from one entity to another. For example, a query can navigate from LineItem to Product but cannot navigate in the opposite direction. For CustomerOrder and LineItem, a query could navigate in both directions because these two entities have a bidirectional relationship.

Cascade Operations and Relationships

Entities that use relationships often have dependencies on the existence of the other entity in the relationship. For example, a line item is part of an order; if the order is deleted, the line item also should be deleted. This is called a cascade delete relationship.

The jakarta.persistence.CascadeType enumerated type defines the cascade operations that are applied in the cascade element of the relationship annotations. Cascade Operations for Entities lists the cascade operations for entities.

Cascade Operations for Entities
Cascade Operation	Description
`ALL`	All cascade operations will be applied to the parent entity’s related entity. `All` is equivalent to specifying `cascade={DETACH, MERGE, PERSIST, REFRESH, REMOVE}`
`DETACH`	If the parent entity is detached from the persistence context, the related entity will also be detached.
`MERGE`	If the parent entity is merged into the persistence context, the related entity will also be merged.
`PERSIST`	If the parent entity is persisted into the persistence context, the related entity will also be persisted.
`REFRESH`	If the parent entity is refreshed in the current persistence context, the related entity will also be refreshed.
`REMOVE`	If the parent entity is removed from the current persistence context, the related entity will also be removed.

Cascade delete relationships are specified using the cascade=REMOVE element specification for @OneToOne and @OneToMany relationships. For example:

@OneToMany(cascade=REMOVE, mappedBy="customer")
public Set<CustomerOrder> getOrders() { return orders; }

Orphan Removal in Relationships

When a target entity in a one-to-one or one-to-many relationship is removed from the relationship, it is often desirable to cascade the remove operation to the target entity. Such target entities are considered "orphans," and the orphanRemoval attribute can be used to specify that orphaned entities should be removed. For example, if an order has many line items and one of them is removed from the order, the removed line item is considered an orphan. If orphanRemoval is set to true, the line item entity will be deleted when the line item is removed from the order.

The orphanRemoval attribute in @OneToMany and @oneToOne takes a Boolean value and is by default false.

The following example will cascade the remove operation to the orphaned order entity when the customer entity is deleted:

@OneToMany(mappedBy="customer", orphanRemoval="true")
public List<CustomerOrder> getOrders() { ... }

Embeddable Classes in Entities

Embeddable classes are used to represent the state of an entity but don’t have a persistent identity of their own, unlike entity classes. Instances of an embeddable class share the identity of the entity that owns it. Embeddable classes exist only as the state of another entity. An entity may have single-valued or collection-valued embeddable class attributes.

Embeddable classes have the same rules as entity classes but are annotated with the jakarta.persistence.Embeddable annotation instead of @Entity.

The following embeddable class, ZipCode, has the fields zip and plusFour:

@Embeddable
public class ZipCode {
    String zip;
    String plusFour;
    ...
}

This embeddable class is used by the Address entity:

@Entity
public class Address {
    @Id
    protected long id
    String street1;
    String street2;
    String city;
    String province;
    @Embedded
    ZipCode zipCode;
    String country;
    ...
}

Entities that own embeddable classes as part of their persistent state may annotate the field or property with the jakarta.persistence.Embedded annotation but are not required to do so.

Embeddable classes may themselves use other embeddable classes to represent their state. They may also contain collections of basic Java programming language types or other embeddable classes. Embeddable classes may also contain relationships to other entities or collections of entities. If the embeddable class has such a relationship, the relationship is from the target entity or collection of entities to the entity that owns the embeddable class.

Entity Inheritance

Entities support class inheritance, polymorphic associations, and polymorphic queries. Entity classes can extend non-entity classes, and non-entity classes can extend entity classes. Entity classes can be both abstract and concrete.

The roster example application demonstrates entity inheritance, as described in Entity Inheritance in the roster Application.

Abstract Entities

An abstract class may be declared an entity by decorating the class with @Entity. Abstract entities are like concrete entities but cannot be instantiated.

Abstract entities can be queried just like concrete entities. If an abstract entity is the target of a query, the query operates on all the concrete subclasses of the abstract entity:

@Entity
public abstract class Employee {
    @Id
    protected Integer employeeId;
    ...
}
@Entity
public class FullTimeEmployee extends Employee {
    protected Integer salary;
    ...
}
@Entity
public class PartTimeEmployee extends Employee {
    protected Float hourlyWage;
}

Mapped Superclasses

Entities may inherit from superclasses that contain persistent state and mapping information but are not entities. That is, the superclass is not decorated with the @Entity annotation and is not mapped as an entity by the Jakarta Persistence provider. These superclasses are most often used when you have state and mapping information common to multiple entity classes.

Mapped superclasses are specified by decorating the class with the annotation jakarta.persistence.MappedSuperclass:

@MappedSuperclass
public class Employee {
    @Id
    protected Integer employeeId;
    ...
}
@Entity
public class FullTimeEmployee extends Employee {
    protected Integer salary;
    ...
}
@Entity
public class PartTimeEmployee extends Employee {
    protected Float hourlyWage;
    ...
}

Mapped superclasses cannot be queried and cannot be used in EntityManager or Query operations. You must use entity subclasses of the mapped superclass in EntityManager or Query operations. Mapped superclasses can’t be targets of entity relationships. Mapped superclasses can be abstract or concrete.

Mapped superclasses do not have any corresponding tables in the underlying datastore. Entities that inherit from the mapped superclass define the table mappings. For instance, in the preceding code sample, the underlying tables would be FULLTIMEEMPLOYEE and PARTTIMEEMPLOYEE, but there is no EMPLOYEE table.

Non-Entity Superclasses

Entities may have non-entity superclasses, and these superclasses can be either abstract or concrete. The state of non-entity superclasses is nonpersistent, and any state inherited from the non-entity superclass by an entity class is nonpersistent. Non-entity superclasses may not be used in EntityManager or Query operations. Any mapping or relationship annotations in non-entity superclasses are ignored.

Entity Inheritance Mapping Strategies

You can configure how the Jakarta Persistence provider maps inherited entities to the underlying datastore by decorating the root class of the hierarchy with the annotation jakarta.persistence.Inheritance. The following mapping strategies are used to map the entity data to the underlying database:

A single table per class hierarchy
A table per concrete entity class
A "join" strategy, whereby fields or properties that are specific to a subclass are mapped to a different table than the fields or properties that are common to the parent class

The strategy is configured by setting the strategy element of @Inheritance to one of the options defined in the jakarta.persistence.InheritanceType enumerated type:

public enum InheritanceType {
    SINGLE_TABLE,
    JOINED,
    TABLE_PER_CLASS
};

The default strategy, InheritanceType.SINGLE_TABLE, is used if the @Inheritance annotation is not specified on the root class of the entity hierarchy.

The Single Table per Class Hierarchy Strategy

With this strategy, which corresponds to the default InheritanceType.SINGLE_TABLE, all classes in the hierarchy are mapped to a single table in the database. This table has a discriminator column containing a value that identifies the subclass to which the instance represented by the row belongs.

The discriminator column, whose elements are shown in @DiscriminatorColumn Elements, can be specified by using the jakarta.persistence.DiscriminatorColumn annotation on the root of the entity class hierarchy.

@DiscriminatorColumn Elements
Type	Name	Description
`String`	`name`	The name of the column to be used as the discriminator column. The default is `DTYPE`. This element is optional.
`DiscriminatorType`	`discriminatorType`	The type of the column to be used as a discriminator column. The default is `DiscriminatorType.STRING`. This element is optional.
`String`	`columnDefinition`	The SQL fragment to use when creating the discriminator column. The default is generated by the Persistence provider and is implementation-specific. This element is optional.
`String`	`length`	The column length for `String`-based discriminator types. This element is ignored for non-`String` discriminator types. The default is 31. This element is optional.

The jakarta.persistence.DiscriminatorType enumerated type is used to set the type of the discriminator column in the database by setting the discriminatorType element of @DiscriminatorColumn to one of the defined types. DiscriminatorType is defined as follows:

public enum DiscriminatorType {
    STRING,
    CHAR,
    INTEGER
};

If @DiscriminatorColumn is not specified on the root of the entity hierarchy and a discriminator column is required, the Persistence provider assumes a default column name of DTYPE and column type of DiscriminatorType.STRING.

The jakarta.persistence.DiscriminatorValue annotation may be used to set the value entered into the discriminator column for each entity in a class hierarchy. You may decorate only concrete entity classes with @DiscriminatorValue.

If @DiscriminatorValue is not specified on an entity in a class hierarchy that uses a discriminator column, the Persistence provider will provide a default, implementation-specific value. If the discriminatorType element of @DiscriminatorColumn is DiscriminatorType.STRING, the default value is the name of the entity.

This strategy provides good support for polymorphic relationships between entities and queries that cover the entire entity class hierarchy. However, this strategy requires the columns that contain the state of subclasses to be nullable.

The Table per Concrete Class Strategy

In this strategy, which corresponds to InheritanceType.TABLE_PER_CLASS, each concrete class is mapped to a separate table in the database. All fields or properties in the class, including inherited fields or properties, are mapped to columns in the class’s table in the database.

This strategy provides poor support for polymorphic relationships and usually requires either SQL UNION queries or separate SQL queries for each subclass for queries that cover the entire entity class hierarchy.

Support for this strategy is optional and may not be supported by all Jakarta Persistence providers. The default Jakarta Persistence provider in GlassFish Server does not support this strategy.

The Joined Subclass Strategy

In this strategy, which corresponds to InheritanceType.JOINED, the root of the class hierarchy is represented by a single table, and each subclass has a separate table that contains only those fields specific to that subclass. That is, the subclass table does not contain columns for inherited fields or properties. The subclass table also has a column or columns that represent its primary key, which is a foreign key to the primary key of the superclass table.

This strategy provides good support for polymorphic relationships but requires one or more join operations to be performed when instantiating entity subclasses. This may result in poor performance for extensive class hierarchies. Similarly, queries that cover the entire class hierarchy require join operations between the subclass tables, resulting in decreased performance.

Some Jakarta Persistence providers, including the default provider in GlassFish Server, require a discriminator column that corresponds to the root entity when using the joined subclass strategy. If you are not using automatic table creation in your application, make sure that the database table is set up correctly for the discriminator column defaults, or use the @DiscriminatorColumn annotation to match your database schema. For information on discriminator columns, see The Single Table per Class Hierarchy Strategy.

Managing Entities

Entities are managed by the entity manager, which is represented by jakarta.persistence.EntityManager instances. Each EntityManager instance is associated with a persistence context: a set of managed entity instances that exist in a particular data store. A persistence context defines the scope under which particular entity instances are created, persisted, and removed. The EntityManager interface defines the methods that are used to interact with the persistence context.

The EntityManager Interface

The EntityManager API creates and removes persistent entity instances, finds entities by the entity’s primary key, and allows queries to be run on entities.

Container-Managed Entity Managers

With a container-managed entity manager, an EntityManager instance’s persistence context is automatically propagated by the container to all application components that use the EntityManager instance within a single Jakarta transaction.

Jakarta transactions usually involve calls across application components. To complete a Jakarta transaction, these components usually need access to a single persistence context. This occurs when an EntityManager is injected into the application components by means of the jakarta.persistence.PersistenceContext annotation. The persistence context is automatically propagated with the current Jakarta transaction, and EntityManager references that are mapped to the same persistence unit provide access to the persistence context within that transaction. By automatically propagating the persistence context, application components don’t need to pass references to EntityManager instances to each other in order to make changes within a single transaction. The Jakarta EE container manages the lifecycle of container-managed entity managers.

To obtain an EntityManager instance, inject the entity manager into the application component:

@PersistenceContext
EntityManager em;

Application-Managed Entity Managers

With an application-managed entity manager, on the other hand, the persistence context is not propagated to application components, and the lifecycle of EntityManager instances is managed by the application.

Application-managed entity managers are used when applications need to access a persistence context that is not propagated with the Jakarta transaction across EntityManager instances in a particular persistence unit. In this case, each EntityManager creates a new, isolated persistence context. The EntityManager and its associated persistence context are created and destroyed explicitly by the application. They are also used when directly injecting EntityManager instances can’t be done because EntityManager instances are not thread-safe. EntityManagerFactory instances are thread-safe.

Applications create EntityManager instances in this case by using the createEntityManager method of jakarta.persistence.EntityManagerFactory.

To obtain an EntityManager instance, you first must obtain an EntityManagerFactory instance by injecting it into the application component by means of the jakarta.persistence.PersistenceUnit annotation:

@PersistenceUnit
EntityManagerFactory emf;

Then obtain an EntityManager from the EntityManagerFactory instance:

EntityManager em = emf.createEntityManager();

Application-managed entity managers don’t automatically propagate the Jakarta transaction context. Such applications need to manually gain access to the Jakarta transaction manager and add transaction demarcation information when performing entity operations. The jakarta.transaction.UserTransaction interface defines methods to begin, commit, and roll back transactions. Inject an instance of UserTransaction by creating an instance variable annotated with @Resource:

@Resource
UserTransaction utx;

To begin a transaction, call the UserTransaction.begin method. When all the entity operations are complete, call the UserTransaction.commit method to commit the transaction. The UserTransaction.rollback method is used to roll back the current transaction.

The following example shows how to manage transactions in an application that uses an application-managed entity manager:

@PersistenceUnit
EntityManagerFactory emf;
EntityManager em;
@Resource
UserTransaction utx;
...
em = emf.createEntityManager();
try {
    utx.begin();
    em.persist(SomeEntity);
    em.merge(AnotherEntity);
    em.remove(ThirdEntity);
    utx.commit();
} catch (Exception e) {
    utx.rollback();
}

Finding Entities Using the EntityManager

The EntityManager.find method is used to look up entities in the data store by the entity’s primary key:

@PersistenceContext
EntityManager em;
public void enterOrder(int custID, CustomerOrder newOrder) {
    Customer cust = em.find(Customer.class, custID);
    cust.getOrders().add(newOrder);
    newOrder.setCustomer(cust);
}

Managing an Entity Instance’s Lifecycle

You manage entity instances by invoking operations on the entity by means of an EntityManager instance. Entity instances are in one of four states: new, managed, detached, or removed.

New entity instances have no persistent identity and are not yet associated with a persistence context.
Managed entity instances have a persistent identity and are associated with a persistence context.
Detached entity instances have a persistent identity and are not currently associated with a persistence context.
Removed entity instances have a persistent identity, are associated with a persistent context, and are scheduled for removal from the data store.

Persisting Entity Instances

New entity instances become managed and persistent either by invoking the persist method or by a cascading persist operation invoked from related entities that have the cascade=PERSIST or cascade=ALL elements set in the relationship annotation. This means that the entity’s data is stored to the database when the transaction associated with the persist operation is completed. If the entity is already managed, the persist operation is ignored, although the persist operation will cascade to related entities that have the cascade element set to PERSIST or ALL in the relationship annotation. If persist is called on a removed entity instance, the entity becomes managed. If the entity is detached, either persist will throw an IllegalArgumentException, or the transaction commit will fail. The following method performs a persist operation:

@PersistenceContext
EntityManager em;
...
public LineItem createLineItem(CustomerOrder order, Product product,
        int quantity) {
    LineItem li = new LineItem(order, product, quantity);
    order.getLineItems().add(li);
    em.persist(li);
    return li;
}

The persist operation is propagated to all entities related to the calling entity that have the cascade element set to ALL or PERSIST in the relationship annotation:

@OneToMany(cascade=ALL, mappedBy="order")
public Collection<LineItem> getLineItems() {
    return lineItems;
}

Removing Entity Instances

Managed entity instances are removed by invoking the remove method or by a cascading remove operation invoked from related entities that have the cascade=REMOVE or cascade=ALL elements set in the relationship annotation. If the remove method is invoked on a new entity, the remove operation is ignored, although remove will cascade to related entities that have the cascade element set to REMOVE or ALL in the relationship annotation. If remove is invoked on a detached entity, either remove will throw an IllegalArgumentException, or the transaction commit will fail. If invoked on an already removed entity, remove will be ignored. The entity’s data will be removed from the data store when the transaction is completed or as a result of the flush operation.

In the following example, all LineItem entities associated with the order are also removed, as CustomerOrder.getLineItems has cascade=ALL set in the relationship annotation:

public void removeOrder(Integer orderId) {
    try {
        CustomerOrder order = em.find(CustomerOrder.class, orderId);
        em.remove(order);
    }...
}

Synchronizing Entity Data to the Database

The state of persistent entities is synchronized to the database when the transaction with which the entity is associated commits. If a managed entity is in a bidirectional relationship with another managed entity, the data will be persisted, based on the owning side of the relationship.

To force synchronization of the managed entity to the data store, invoke the flush method of the EntityManager instance. If the entity is related to another entity and the relationship annotation has the cascade element set to PERSIST or ALL, the related entity’s data will be synchronized with the data store when flush is called.

If the entity is removed, calling flush will remove the entity data from the data store.

Persistence Units

A persistence unit defines a set of all entity classes that are managed by EntityManager instances in an application. This set of entity classes represents the data contained within a single data store.

Persistence units are defined by the persistence.xml configuration file. The following is an example persistence.xml file:

<persistence>
    <persistence-unit name="OrderManagement">
        <description>This unit manages orders and customers.
            It does not rely on any vendor-specific features and can
            therefore be deployed to any persistence provider.
        </description>
        <jta-data-source>jdbc/MyOrderDB</jta-data-source>
        <jar-file>MyOrderApp.jar</jar-file>
        <class>com.widgets.CustomerOrder</class>
        <class>com.widgets.Customer</class>
    </persistence-unit>
</persistence>

This file defines a persistence unit named OrderManagement, which uses a Jakarta Transactions aware data source, jdbc/MyOrderDB. The jar-file and class elements specify managed persistence classes: entity classes, embeddable classes, and mapped superclasses. The jar-file element specifies JAR files that are visible to the packaged persistence unit that contain managed persistence classes, whereas the class element explicitly names managed persistence classes.

The jta-data-source (for Jakarta Transactions aware data sources) and non-jta-data-source (for non Jakarta Transactions aware data sources) elements specify the global JNDI name of the data source to be used by the container.

The JAR file or directory whose META-INF directory contains persistence.xml is called the root of the persistence unit. The scope of the persistence unit is determined by the persistence unit’s root. Each persistence unit must be identified with a name that is unique to the persistence unit’s scope.

Persistent units can be packaged as part of a WAR or enterprise bean JAR file or can be packaged as a JAR file that can then be included in an WAR or EAR file.

If you package the persistent unit as a set of classes in an enterprise bean JAR file, persistence.xml should be put in the enterprise bean JAR’s META-INF directory.
If you package the persistence unit as a set of classes in a WAR file, persistence.xml should be located in the WAR file’s WEB-INF/classes/META-INF directory.

If you package the persistence unit in a JAR file that will be included in a WAR or EAR file, the JAR file should be located in either

The WEB-INF/lib directory of a WAR

Or the EAR file’s library directory

In the Java Persistence API 1.0, JAR files could be located at the root of an EAR file as the root of the persistence unit. This is no longer supported. Portable applications should use the EAR file’s library directory as the root of the persistence unit.

Querying Entities

Jakarta Persistence provides the following methods for querying entities.

The Jakarta Persistence query language (JPQL) is a simple, string-based language similar to SQL used to query entities and their relationships. See The Jakarta Persistence Query Language for more information.
The Criteria API is used to create typesafe queries using Java programming language APIs to query for entities and their relationships. See Using the Criteria API to Create Queries for more information.

Both JPQL and the Criteria API have advantages and disadvantages.

Just a few lines long, JPQL queries are typically more concise and more readable than Criteria queries. Developers familiar with SQL will find it easy to learn the syntax of JPQL. JPQL named queries can be defined in the entity class using a Java programming language annotation or in the application’s deployment descriptor. JPQL queries are not typesafe, however, and require a cast when retrieving the query result from the entity manager. This means that type-casting errors may not be caught at compile time. JPQL queries don’t support open-ended parameters.

Criteria queries allow you to define the query in the business tier of the application. Although this is also possible using JPQL dynamic queries, Criteria queries provide better performance because JPQL dynamic queries must be parsed each time they are called. Criteria queries are typesafe and therefore don’t require casting, as JPQL queries do. The Criteria API is just another Java programming language API and doesn’t require developers to learn the syntax of another query language. Criteria queries are typically more verbose than JPQL queries and require the developer to create several objects and perform operations on those objects before submitting the query to the entity manager.

Database Schema Creation

The persistence provider can be configured to automatically create the database tables, load data into the tables, and remove the tables during application deployment using standard properties in the application’s deployment descriptor. These tasks are typically used during the development phase of a release, not against a production database.

The following is an example of a persistence.xml deployment descriptor that specifies that the provider should drop all database artifacts using a provided script, create the artifacts with a provided script, and load data from a provided script when the application is deployed:

<?xml version="1.0" encoding="UTF-8"?>
<persistence version="3.0" xmlns="https://jakarta.ee/xml/ns/persistence"
 xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
 xsi:schemaLocation="https://jakarta.ee/xml/ns/persistence
 https://jakarta.ee/xml/ns/persistence/persistence_3_0.xsd">
  <persistence-unit name="examplePU" transaction-type="JTA">
    <jta-data-source>java:global/ExampleDataSource</jta-data-source>
    <properties>
        <property name="jakarta.persistence.schema-generation.database.action"
                  value="drop-and-create"/>
        <property name="jakarta.persistence.schema-generation.create-source"
                  value="script"/>
        <property name="jakarta.persistence.schema-generation.create-script-source"
                  value="META-INF/sql/create.sql" />
        <property name="jakarta.persistence.sql-load-script-source"
                  value="META-INF/sql/data.sql" />
        <property name="jakarta.persistence.schema-generation.drop-source"
                  value="script" />
        <property name="jakarta.persistence.schema-generation.drop-script-source"
                  value="META-INF/sql/drop.sql" />
    </properties>
  </persistence-unit>
</persistence>

Configuring an Application to Create or Drop Database Tables

The jakarta.persistence.schema-generation.database.action property is used to specify the action taken by the persistence provider when an application is deployed. If the property is not set, the persistence provider will not create or drop any database artifacts.

Schema Creation Actions
Setting	Description
`none`	No schema creation or deletion will take place.
`create`	The provider will create the database artifacts on application deployment. The artifacts will remain unchanged after application redeployment.
`drop-and-create`	Any artifacts in the database will be deleted, and the provider will create the database artifacts on deployment.
`drop`	Any artifacts in the database will be deleted on application deployment.

In this example, the persistence provider will delete any remaining database artifacts and then create the artifacts when the application is deployed:

<property name="jakarta.persistence.schema-generation.database.action"
           value="drop-and-create"/>

By default, the object/relational metadata in the persistence unit is used to create the database artifacts. You may also supply scripts used by the provider to create and delete the database artifacts. The jakarta.persistence.schema-generation.create-source and jakarta.persistence.schema-generation.drop-source properties control how the provider will create or delete the database artifacts.

Settings for Create and Delete Source Properties
Setting	Description
`metadata`	Use the object/relational metadata in the application to create or delete the database artifacts.
`script`	Use a provided script for creating or deleting the database artifacts.
`metadata-then-script`	Use a combination of object/relational metadata, then a user-provided script to create or delete the database artifacts.
`script-then-metadata`	Use a combination of a user-provided script, then the object/relational metadata to create and delete the database artifacts.

In this example, the persistence provider will use a script packaged within the application to create the database artifacts:

<property name="jakarta.persistence.schema-generation.create-source"
           value="script"/>

If you specify a script in create-source or drop-source, specify the location of the script using the jakarta.persistence.schema-generation.create-script-source or jakarta.persistence.schema-generation.drop-script-source property. The location of the script is relative to the root of the persistence unit:

<property name="jakarta.persistence.schema-generation.create-script-source"
           value="META-INF/sql/create.sql" />

In the above example, the create-script-source is set to a SQL file called create.sql in the META-INF/sql directory relative to root of the persistence unit.

Loading Data Using SQL Scripts

If you want to populate the database tables with data before the application loads, specify the location of a load script in the jakarta.persistence.sql-load-script-source property. The location specified in this property is relative to the root of the persistence unit.

In this example, the load script is a file called data.sql in the META-INF/sql directory relative to the root of the persistence unit:

<property name="jakarta.persistence.sql-load-script-source"
          value="META-INF/sql/data.sql" />

Further Information about Persistence

For more information about Jakarta Persistence, see

Jakarta Persistence 3.0 API specification:
https://jakarta.ee/specifications/persistence/3.0/
EclipseLink, the Jakarta Persistence implementation in GlassFish Server:
https://www.eclipse.org/eclipselink/
EclipseLink wiki documentation:
https://wiki.eclipse.org/EclipseLink