| First, we must define some terms and provide some distinctions that are essential for understanding JDO. The term "object" often refers to either a class or an instance of a class, which can be confusing sometimes. Therefore, we will use the terms "instance" and "class" instead of "object," because it will be essential for you to understand which we are discussing. 4.1.1 Kinds of Classes When using JDO, every class falls into one of the following two categories: - Persistent class
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A persistent class can have its instances stored in the datastore. To be persistent, a class must be specified in a metadata file and enhanced. The JDO specification refers to these as persistence-capable classes. - Transient class
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A transient class cannot have its instances stored in the datastore. Transient classes are not listed in a metadata file. Furthermore, classes can be distinguished by their use of the JDO API: - JDO-aware class
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A JDO-aware class makes direct use of the JDO API. For example, it can perform a JDO query to retrieve instances from the datastore, or make specific instances persistent. - JDO-transparent class
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A JDO-transparent class does not make direct use of the JDO API. Whether a class is JDO-aware or JDO-transparent is unrelated to whether it is persistent. For example, the persistent classes Movie , Actor , and Role that we introduced in Chapter 1 are JDO-transparent, because they never made an explicit call to the JDO API. On the other hand, the MediaManiaApp class is JDO-aware, because it uses the JDO API directly: it creates a PersistenceManager and uses it to execute transactions. MediaManiaApp is not persistent. 4.1.2 Kinds of Instances JDO supports several kinds of instances. The names we introduce in this section are used throughout the book to refer to these different kinds of instances. In particular, we use specific terminology to differentiate a transient instance of a transient class from a transient instance of a persistent class. All JDO implementations support the first three kinds of instances listed here; the last two are optional: - Instance of a transient class
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All instances of a transient class are transient. For the most part, however, we focus on instances of persistent classes. - Transient instance
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A transient instance is an instance of a persistent class that is not associated with the datastore. It is simply an instance you create in your application that is never made persistent and is used independent of the datastore. - Persistent instance
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A persistent instance is an instance of a persistent class whose behavior is linked to a transactional datastore. Its fields are watched by the JDO implementation and saved to or restored from the datastore, as appropriate. The datastore manages the state of its persistent fields and information identifying its class. - Transient transactional instance
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A transient transactional instance is transient and is not represented in the datastore. But it is transactional, and its state is rolled back if a transactional rollback occurs. For JDO to manage a transient transactional instance, you need to enhance its class. Transient transactional instances are covered in Chapter 13. - Persistent nontransactional instance
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A persistent-nontransactional instance is persistent, but it is not managed as part of a transaction. Persistent nontransactional instances are discussed in Chapter 14. Table 4-1 illustrates these different kinds of instances, based on their persistence and transactional behavior. Table 4-1. Kinds of instances | Behavior | Instance of atransient class | Transient instance | Persistent instance | | Transactional | | Transient transactional instance | Persistent instance | | Nontransactional | Instance of a transient class | Transient instance | Persistent nontransactional instance |  |
