make start on basic types
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Gavin
Christian Beikov
parent
e678018b6b
commit
f66d5cd3d0
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@ -96,7 +96,7 @@ We don't recommend doing this.
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Every entity must have an identifier attribute.
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[identifier-attributes]
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[[identifier-attributes]]
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=== Identifier attributes
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An identifier attribute is usually a field:
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@ -139,7 +139,7 @@ Identifier values may be:
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- assigned by the application, that is, by your Java code, or
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- generated and assigned by Hibernate.
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[generated-identifiers]
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[[generated-identifiers]]
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=== Generated identifiers
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An identifier is often system-generated, in which case it should be annotated `@GeneratedValue`:
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@ -250,7 +250,7 @@ Not every id maps to a (system-generated) surrogate key.
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Primary keys which are meaningful to the user of the system are called _natural keys_.
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Of particular interest are natural keys which comprise more than one database column, and such natural keys are called _composite keys_.
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[composite-identifiers]
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[[composite-identifiers]]
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=== Composite identifiers
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If your database uses composite keys, you'll need more than one identifier attribute.
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@ -364,3 +364,140 @@ Either way, we may now use `BookId` to obtain instances of `Book`:
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----
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Book book = session.find(Book.class, new BookId(isbn, printing));
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----
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[[basic-attributes]]
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=== Basic attributes
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A _basic_ attribute of an entity is a field or property which maps to a single column of the associated database table.
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The JPA specification defines a quite limited set of basic types:
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|====
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| Classification | Package | Types
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| Primitive types | | `boolean`, `int`, `double`, etc
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| Primitive wrappers | `java.lang` | `Boolean`, `Integer`, `Double`, etc
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| Strings | `java.lang` | `String`
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| Arbitrary-precision numeric types | `java.math` | `BigInteger`, `BigDecimal`
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| Date/time types | `java.time` | `LocalDate`, `LocalTime`, `LocalDateTime`, `OffsetDateTime`, `Instant`
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| Deprecated date/time types | `java.util` | `Date`, `Calendar`
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| Deprecated JDBC date/time types | `java.sql` | `Date`, `Time`, `Timestamp`
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| Binary and character arrays | | `byte[]`, `char[]`
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| UUIDs | `java.util` | `UUID`
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| Enumerated types | | Any `enum`
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| Serializable types | | Any type which implements `java.io.Serializable`
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|====
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[IMPORTANT]
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.Please don't use `Date`!
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====
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We're begging you to use types from the `java.time` package instead of anything which inherits `java.util.Date`.
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====
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The `@Basic` annotation explicitly specifies that an attribute is basic, but it's often not needed, since attributes are assumed basic by default.
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On the other hand, if an attribute cannot be null, use of `@Basic(optional=false)` is highly recommended.
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[source,java]
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----
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@Basic(optional=false) String firstName;
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@Basic(optional=false) String lastName;
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String middleName; // may be null
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----
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[TIP]
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.`optional` vs `nullable` in JPA
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====
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There are two ways to mark a mapped column `not null` in JPA:
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- using `@Basic(optional=false)`, or
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- using `@Column(nullable=false)`.
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You might wonder what the difference is.
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Well, it's perhaps not obvious to a casual user of the JPA annotations, but they actually come in two "layers":
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- annotations like `@Entity`, `@Id`, and `@Basic` belong to the _logical_ layer—they specify the semantics of your Java domain model, whereas
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- annotations like `@Table` and `@Column` belong to the _mapping_ layer—they specify how elements of the domain model map to objects in the relational database.
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Information may be inferred from the logical layer down to the mapping layer, but is never inferred in the opposite direction.
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Now, the `@Column` annotation belongs to the _mapping_ layer, and so its `nullable` member only affects schema generation (resulting in a `not null` constraint in the generated DDL).
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The `@Basic` annotation belongs to the logical layer, and so an attribute marked `optional=false` is checked by Hibernate before it even writes an entity to the database.
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Note that:
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- `optional=false` implies `nullable=false`, but
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- `nullable=false` _does not_ imply `optional=false`.
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Therefore, we recommend `@Basic(optional=false)` in preference to `@Column(nullable=false)` in most circumstances.
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====
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JPA provides the `AttributeConverter` interface, and the `@Converter` annotation to convert any Java type to one of the types listed above, or perform whatever other sort of pre- and post-processing you need on a basic attribute before writing and reading it to or from the database.
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Converters substantially widen the set of attribute types that can be handled.
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For example, the following converter will be automatically applied to any attribute of type `BitSet`, and takes care of persisting the `BitSet` to a column of type `varbinary`:
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[source,java]
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----
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@Converter(autoApply = true)
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public static class BitSetConverter implements AttributeConverter<BitSet,byte[]> {
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@Override
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public byte[] convertToDatabaseColumn(BitSet bitSet) {
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return bitSet.toByteArray();
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}
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@Override
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public BitSet convertToEntityAttribute(byte[] bytes) {
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return BitSet.valueOf(bytes);
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}
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}
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----
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On the other hand, if you don't set `autoapply=true`, then you must explicitly apply the converter using the `@Convert` annotation:
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[source,java]
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----
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@Convert(converter = BitSetConverter.class)
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@Basic(optional = false)
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BitSet bitset;
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----
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All this is nice, but it probably won't surprise you that Hibernate goes beyond what is required by JPA.
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=== Compositional basic types
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=== `equals()` and `hashCode()`
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Entity classes should override `equals()` and `hashCode()`. People new to
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Hibernate or JPA are often confused by exactly which fields should be
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included in the `hashCode()`, so please keep the following principles in
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mind:
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- You should not include mutable fields in the hashcode, since that would
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require rehashing any collection containing the entity whenever the field
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is mutated.
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- It's not completely wrong to include a generated identifier (surrogate
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key) in the hashcode, but since the identifier is not generated until
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the entity instance is made persistent, you must take great care to not
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add it to any hashed collection before the identifier is generated. We
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therefore advise against including any database-generated field in the
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hashcode.
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It's OK to include any immutable, non-generated field in the hashcode.
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TIP: We therefore recommend identifying a _natural key_ for each entity,
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that is, a combination of fields that uniquely identifies an instance of
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the entity, from the perspective of the data model of the program. The
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business key should correspond to a unique constraint on the database,
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and to the fields which are included in `equals()` and `hashCode()`.
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That said, an implementation of `equals()` and `hashCode()` based on the
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generated identifier of the entity can work _if you're careful_.
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IMPORTANT: If you can't identify a natural key, it might be a sign that
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you need to think more carefully about some aspect of your data model.
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If an entity doesn't have a meaningful unique key, then it's impossible
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to say what event or object it represents in the "real world" outside
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your program.
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Note that even when you've identified a natural key, we still recommend
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the use of a generated surrogate key in foreign keys, since this makes
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your data model _much_ easier to change.
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