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 either through 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:

Persistent Fields and Properties in Entity Classes

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

Persistent Fields

If the entity class uses persistent fields, the Persistence runtime accesses entity class instance variables directly. All fields not annotated javax.persistence.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 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 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 signature for single-valued persistent properties are as follows: 

Type getProperty()
void setProperty(Type type)

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

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

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

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

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 javax.persistence.Id annotation to denote the primary key property or field.

Composite primary keys must correspond to either a single persistent property or field, or 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 javax.persistence.EmbeddedId and javax.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:

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

Primary Key Classes

A primary key class must meet these requirements:

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

public final class LineItemKey implements Serializable {
  public Integer orderId;
  public int itemId;

  public LineItemKey() {}

  public LineItemKey(Integer orderId, int itemId) {
    this.orderId = orderId;
    this.itemId = itemId;
  }

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

  public int hashCode() {
    return (
          (orderId==null?0:orderId.hashCode())
          ^
          ((int) itemId)
        );
  }

  public String toString() {
    return "" + orderId + "-" + itemId;
  }
}

Multiplicity in Entity Relationships

There are four types of multiplicities: one-to-one, one-to-many, many-to-one, and many-to-many.

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 javax.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, Order would have a one-to-many relationship with LineItem. One-to-many relationships use the javax.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, from the perspective of LineItem the relationship to Order is many-to-one. Many-to-one relationships use the javax.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, in college each 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 javax.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 relative field, then we often say that it "knows" about its related object. For example, if Order knows what LineItem instances it has and if LineItem knows what Order it belongs to, then they have a bidirectional relationship.

Bidirectional relationships must follow these rules:

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

Java Persistence query language 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 Order and LineItem, a query could navigate in both directions, because these two entities have a bidirectional relationship.

Cascade Deletes 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, and if the order is deleted, then the line item should also be deleted. This is called a cascade delete relationship.

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<Order> getOrders() { return orders; }