Hibernate对象映射类型

Hibernate understands both the Java and JDBC representations of application data. The ability to read and write object data to a database is called marshalling, and is the function of a Hibernate type. A type is an implementation of the org.hibernate.type.Type interface. A Hibernate type describes various aspects of behavior of the Java type such as how to check for equality and how to clone values.

Usage of the word type

A Hibernate type is neither a Java type nor a SQL datatype. It provides information about both of these.

When you encounter the term type in regards to Hibernate, it may refer to the Java type, the JDBC type, or the Hibernate type, depending on context.

1. Value types

1.1. Basic types

Basic value types usually map a single database value, or column, to a single, non-aggregated Java type. Hibernate provides a number of built-in basic types, which follow the natural mappings recommended in the JDBC specifications. You can override these mappings and provide and use alternative mappings. These topics are discussed further on.

Table 1.1. Basic Type Mappings

1.2. National Character Types

National Character types, which is a new feature since JDBC 4.0 API, now available in hibernate type system.
National Language Support enables you retrieve data or insert data into a database in any character
set that the underlying database supports.

Depending on your environment, you might want to set the configuration option hibernate.use_nationalized_character_data
to true and having all string or clob based attributes having this national character support automatically.
There is nothing else to be changed, and you don‘t have to use any hibernate specific mapping, so it is portable
( though the national character support feature is not required and may not work on other JPA provider impl ).

The other way of using this feature is having the @Nationalized
annotation on the attribute
that should be nationalized. This only works on string based
attributes, including string, char, char array and clob.

                @Entity( name="NationalizedEntity")
                public static class NationalizedEntity {
                    @Id
                    private Integer id;

                    @Nationalized
                    private String nvarcharAtt;

                    @Lob
                    @Nationalized
                    private String materializedNclobAtt;

                    @Lob
                    @Nationalized
                    private NClob nclobAtt;

                    @Nationalized
                    private Character ncharacterAtt;

                    @Nationalized
                    private Character[] ncharArrAtt;

                    @Type(type = "ntext")
                    private String nlongvarcharcharAtt;
                }

Table 1.2. National Character Type Mappings

1.3. Composite types

Composite types, or embedded types, as they are called by the Java
Persistence API, have traditionally been called components in Hibernate. All of these
terms mean the same thing.

Components represent aggregations of values into a single Java type. An example is an
Address class, which aggregates street, city, state, and postal code. A composite type
behaves in a similar way to an entity. They are each classes written specifically for an application. They may
both include references to other application-specific classes, as well as to collections and simple JDK
types. The only distinguishing factors are that a component does not have its own lifecycle or define an
identifier.

1.4. Collection types

A collection type refers to the data type itself, not its contents.

A Collection denotes a one-to-one or one-to-many relationship between tables of a database.

Refer to the chapter on Collections for more information on collections.

2. Entity Types

Entities are application-specific classes which correlate to rows in a table, using a unique identifier. Because
of the requirement for a unique identifier, ntities exist independently and define their own lifecycle. As an
example, deleting a Membership should not delete the User or the Group. For more information, see the chapter on
Persistent Classes.

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