Java Data Objects

Fields

Fields contain the state of an instance. JDO provides for the access, management, and storage of an instance’s fields in a datastore. All of Java’s field type categories are supported: primitive types, reference types, and interface types. JDO also supports all of Java’s field modifiers, including private , public , protected , static , transient , final , and volatile . But static and final fields cannot be persistent, as we will discuss later in this chapter.

As we explained earlier, you can have both transient and persistent instances of a persistent class. The individual fields of a persistent class can also be transient or persistent for all of the class’s persistent instances. A field’s type and modifiers determine whether it is persistent or transient, by default. You can override the default persistence of a field in the metadata. We cover transient fields later in this chapter.

You can specify persistence-related information about a field by using the field metadata element. Its required name attribute should have the name of the field in the Java class declaration. It has attributes to control the field’s persistence and the type of its elements if it is a collection. We cover these attributes later in this chapter. If the class uses application identity, one or more fields need to indicate they are a primary-key field; Chapter 10 covers this in detail. Chapter 12 addresses advanced field-management facilities enabled by the remaining field element attributes. ...