mapped superclass, version, and summary
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Gavin
Gavin King
parent
7b5f66c292
commit
349ecee462
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@ -103,7 +103,43 @@ It is in principle possible to mix field and property access using explicit `@Ac
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We don't recommend doing this.
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====
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Every entity must have an identifier attribute.
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An entity class like `Book`, which does not extend any other entity class is called a _root entity_.
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Every root entity must declare an identifier attribute.
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[[entity-inheritance]]
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=== Entity class inheritance
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An entity class may `extend` another entity class.
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[source,java]
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----
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@Entity
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class AudioBook extends Book {
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AudioBook() {}
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...
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}
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----
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A subclass entity inherits every persistent attribute of every entity it extends.
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A root entity may also extend another class and inherit mapped attributes from the other class.
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But in this case, the class which declares the mapped attributes must be annotated `@MappedSuperclass`.
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[source,java]
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----
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@MappedSuperclass
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class Versioned {
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...
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}
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@Entity
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class Book extends Versioned {
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...
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}
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----
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Every entity must have an identifier attribute, but only a root entity class may declare an attribute annotated `@Id`.
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Subclass entities always inherit the identifier attribute of the root entity.
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[[identifier-attributes]]
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=== Identifier attributes
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@ -389,6 +425,20 @@ Either way, we may now use `BookId` to obtain instances of `Book`:
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Book book = session.find(Book.class, new BookId(isbn, printing));
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----
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[[version-attributes]]
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=== Version attributes
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An entity may have an attribute which is used by Hibernate for optimistic lock checking.
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A version attribute is usually of type `Integer` or `LocalDateTime`, though several other types are also allowed.
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[source,java]
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----
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@Version
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LocalDateTime lastUpdated;
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----
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The `@Id` and `@Version` attributes we've already seen are just specialized examples of _basic attributes_.
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[[basic-attributes]]
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=== Basic attributes
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@ -1057,7 +1107,8 @@ Unfortunately, JPA does not define a standard way to map SQL arrays, but here's
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----
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@Entity
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class Event {
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@Id @GeneratedValue long id;
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@Id @GeneratedValue
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Long id;
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...
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@Array(length=7)
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DayOfWeek[] daysOfWeek; // stored as a SQL ARRAY type
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@ -1090,7 +1141,8 @@ JPA _does_ define a standard way to map a collection to a table:
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----
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@Entity
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class Event {
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@Id @GeneratedValue long id;
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@Id @GeneratedValue
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Long id;
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...
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@ElementCollection
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DayOfWeek[] daysOfWeek; // stored in a dedicated table
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@ -1118,6 +1170,77 @@ Most likely, we'll realize that we need to add a separate entity after all.
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There's much more we could say about "element collections", but we won't say it, because we don't want to hand you the gun that you'll shoot your foot with.
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====
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[[entities-summary]]
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=== Summary of annotations
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Let's pause to remember the annotations we've met so far.
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.Declaring entities and embeddable types
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[cols="2,4,1"]
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|===
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| Annotation | Purpose | JPA-standard
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| `@Entity` | Declare an entity class | ✓
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| `@MappedSuperclass` | Declare a non-entity class with mapped attributes inherited by an entity | ✓
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| `@Embeddable` | Declare an embeddable type | ✓
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| `@Converter` | Register an `AttributeConverter` | ✓
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|===
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.Declaring basic attributes
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[cols="2,4,1"]
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|===
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| Annotation | Purpose | JPA-standard
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| `@Id` | Declare a basic-typed identifier attribute | ✓
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| `@Version` | Declare a version attribute | ✓
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| `@Basic` | (Optional) Declare a basic attribute | ✓
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| `@EmbeddedId` | Declare an embeddable-typed identifier attribute | ✓
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| `@Embedded` | (Optional) Declare an embeddable-typed attribute | ✓
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| `@Enumerated` | (Optional) Declare an `enum`-typed attribute and specify how it is encoded | ✓
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| `@Array` | (Optional) Declare that an attribute maps to a SQL `ARRAY`, and specify the length | ✗
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| `@ElementCollection` | Declare that a collection is mapped to a dedicated table | ✓
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|===
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.Converters and compositional basic types
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[cols="2,4,1"]
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|===
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| Annotation | Purpose | JPA-standard
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| `@Convert` | Apply a converter to an attribute | ✓
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| `@JavaType` | Explicitly specify an implementation of `JavaType` for a basic attribute | ✗
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| `@JdbcType` | Explicitly specify an implementation of `JdbcType` for a basic attribute | ✗
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| `@JdbcTypeCode` | Explicitly specify a JDBC type code used to determine the `JdbcType` for a basic attribute | ✗
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| `@JavaTypeRegistration` | Register a `JavaType` for a given Java type | ✗
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| `@JdbcTypeRegistration` | Register a `JdbcType` for a given JDBC type code | ✗
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|===
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.System-generated identifiers
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[cols="2,4,1"]
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|===
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| Annotation | Purpose | JPA-standard
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| `@GeneratedValue` | Specify that an identifier is system-generated | ✓
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| `@SequenceGenerator` | Define an id generated backed by on a database sequence | ✓
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| `@TableGenerator` | Define an id generated backed by a database table | ✓
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| `@IdGeneratorType` | Declare an annotation that associates a custom `Generator` with each `@Id` attribute it annotates | ✗
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| `@ValueGenerationType` | Declare an annotation that associates a custom `Generator` with each `@Basic` attribute it annotates | ✗
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|===
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.Declaring entity associations
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[cols="2,4,1"]
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|===
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| Annotation | Purpose | JPA-standard
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| `@ManyToOne` | Declare the single-valued side of a many-to-one association (the owning side) | ✓
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| `@OneToMany` | Declare the many-valued side of a many-to-one association (the unowned side) | ✓
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| `@ManyToMany` | Declare either side of a one-to-one association | ✓
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| `@OneToOne` | Declare either side of a one-to-one association | ✓
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| `@MapsId` | Declare that the owning side of a `@OneToOne` association maps the primary key column | ✓
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|===
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Phew!
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That's already a lot of annotations, and we have not even started with the annotations for O/R mapping!
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[[equals-and-hash]]
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=== `equals()` and `hashCode()`
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