Java Data Objects

Java Classes and Metadata

You can make most of your classes persistent in a JDO environment. JDO has the ability to make plain ordinary Java objects (POJOs) persistent. This includes classes that represent the entities in your application domain, utility classes that model other data, and abstractions you need to support your application’s functionality. Your classes can also use all of Java’s class and field modifiers, including: private , public , protected , static , transient , abstract , final , synchronized , and volatile . In some cases, as we will explore later in this chapter, some of these modifiers cannot be used with persistent fields.

The persistent state of a persistent class is represented entirely by the values of its Java fields. If you have a class that has some state that needs to be preserved and it depends on inaccessible or remote objects (e.g., it extends java.net.SocketImpl or uses Java Native Interface (JNI)), you cannot make the class persistent. You also cannot have a persistent nonstatic inner class, because the state of the inner class instance depends on the state of its enclosing instance.

With a few exceptions, system-defined classes (those defined in java.lang, java.io, java.net, etc.) cannot be persistent. They are also not allowed to be the type of a persistent field. This includes classes such as System , Thread , Socket , and File . We list the system classes that are supported in Table 4-2 later in this chapter. You may be using an implementation ...