JPA - Basic Persistable Types

In this tutorial we will go through basic Java data types that JPA supports as persistable fields/properties of an entity class. These types are supported without any mandatory mapping annotations.

java.lang.String

It is persisted as VARCHAR(n)

Primitives and Primitives wrappers

java.math.* types

Note that if Java type maps to a column of SQL type 'DECIMAL(n,p)', then we can control precision/scale of the resultant columns by using the annotation @Column on the target field and by setting desired values of its elements 'precision' and/or 'scale'.

java.util.Date and java.util.Calendar

In above case, @Temporal annotation should be used for an explicit SQL type conversion.

java.sql.* date types

Array of Simple types and other serializable types

Arrays of all Java primitives and primitives wrapper can be persisted without any extra configuration. Also arrays of String, java.util.Date, java.util.calendar, java.sql.Date, java.sql.Time and java.sql.Timestamp can also be persisted in the same manner. In short, any type which is serializable can be persisted. All these types are converted to SQL type VARBINARY except for char[] and Character[] which are converted to VARCHAR. VARBINARY is just like BLOB except that VARBINARY supports only limited size.

As stated above that all types which are serializable can be mapped to database. That includes a user defined type that implements serializable. Here's an example.

Note that most of the time it is not be desirable to serialize objects in database, that's because we will not be able to perform granular operations on the data. For example for updating a single field of a serialized object, we have to load the whole object which won't be good for performance. Also as opposed to persisting data in relational structure, serializing all objects in database will not allow the various parts of application and tools to access and use data in different ways without scarifying the productivity and performance.

Using @Basic annotation

The Basic annotation can be applied to a persistent property or instance variable of any of the above types. Following its definition:

    package javax.persistence;
     ....
    @Target({METHOD, FIELD})
    @Retention(RUNTIME)
    public @interface Basic {
        FetchType fetch() default EAGER;
        boolean optional() default true;
    }

As seen above, this annotation can be used to specify whether the attribute is to be loaded lazily and whether it's optional (nullable).

The use of @Basic is optional. If the Basic annotation is not specified, the default attributes of the Basic annotation will apply.

@Basic vs @Column

The attributes of @Basic annotation are applied to entities during runtime, whereas the attributes of @Column annotation are applied to database columns. For example @Basic#optional specifies whether the entity field can be null or not, whereas @Column#nullable specifies whether database column can be null or not.