Understanding EclipseLink,
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The following sections describe the concepts that are common to Object-Relational and Object-XML descriptors.
A descriptor stores all the information describing how an instance of a particular object class can be represented in a data source.
EclipseLink descriptors may contain the following information:
The persistent Java class it describes and the corresponding data source (database tables or XML complex type interaction)
A collection of mappings, which describe how the attributes and relationships for that class are represented in the data source
The primary key information (or equivalent) of the data source
A list of query keys (or aliases) for field names
Information for sequence numbers
A set of optional properties for tailoring the behavior of the descriptor, including support for caching refresh options, identity maps, optimistic locking, the event manager, and the query manager
There is a descriptor type for each data source type that EclipseLink supports. In some cases, multiple descriptor types are valid for the same data source type. The type of descriptor you use determines the type of mappings that you can define.
Inheritance describes how a derived (child) class inherits the characteristics of its superclass (parent). You can use descriptors to describe the inheritance relationships between classes in relational and XML projects.
In the descriptor for a child class, you can override mappings that have been specified in the descriptor for a parent class, or map attributes that have not been mapped at all in the parent class descriptor.
Figure 6-1 illustrates the Vehicle object model–a typical Java inheritance hierarchy. The root class Vehicle contains two branch classes: FueledVehicle and NonFueledVehicle. Each branch class contains a leaf class: Car and Bicycle, respectively.
EclipseLink recognizes the following three types of classes in an inheritance hierarchy:
The root class stores information about all instantiable classes in its subclass hierarchy. By default, queries performed on the root class return instances of the root class and its instantiable subclasses. However, the root class can be configured so queries on it return only instances of itself, without instances of its subclasses.
For example, the Vehicle class in Figure 6-1 is a root class.
Branch classes have a persistent superclass and also have subclasses. By default, queries performed on the branch class return instances of the branch class and any of its subclasses. However, as with the root class, the branch class can be configured so queries on it return only instances of itself without instances of its subclasses.
For example, the FueledVehicle class in Figure 6-1 is a branch class.
Leaf classes have a persistent superclass in the hierarchy but do not have subclasses. Queries performed on the leaf class can only return instances of the leaf class.
For example, the Car class in Figure 6-1 is a leaf class.
In the descriptor for a child class, you can override mappings that have been specified in the descriptor for a parent class, or map attributes that have not been mapped at all in the parent class descriptor.
This section includes information on the following topics:
When configuring inheritance, you configure the root class descriptor with the means of determining which subclasses it should instantiate.
You can do this in one of the following ways:
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Note: All leaf classes in the hierarchy must have a class indicator and they must have the same type of class indicator (field or class extraction method). |
You can use a persistent attribute of a class to indicate which subclass should be instantiated. For example, in a relational descriptor, you can use a class indicator field in the root class table. The indicator field should not have an associated direct mapping unless it is set to read-only.
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Note: If the indicator field is part of the primary key, define a write-only transformation mapping for the indicator field. |
You can use strings or numbers as values in the class indicator field.
The root class descriptor must specify how the value in the class indicator field translates into the class to be instantiated.
One approach is to configure the root class descriptor with a class indicator dictionary: a collection of key-values that associates a simple key, stored in the class indicator field, with a class to instantiate. Table 6-1 illustrates the class indicator dictionary for the Vehicle class' subclasses, as shown in Figure 6-1.
Table 6-1 Class Indicator Dictionary for the Vehicle Class
| Key | Value |
|---|---|
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F |
FueledVehicle |
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N |
NonFueledVehicle |
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C |
Car |
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B |
Bicycle |
Another approach is to simply use the class name itself as the value stored in the class indicator field. This avoids having to define unique indicators for each class at the expense of a slightly larger key value (depending on the length of your class names).
You can define a Java method to compute the class indicator based on any available information in the object's data source record. Such a method is called a class extraction method.
Using a class extraction method, you do not need to include an explicit class indicator field in your data model and you can handle relationships that are too complex to describe using class indicator fields.
A class extraction method must have the following characteristics:
it must be defined on the root descriptor's class;
it must be static;
it must take a Record as an argument;
it must return the java.lang.Class object to use for the Record passed in.
You may also need to define only-instances and with-all-subclasses expressions. If you use a class extraction method, then you must provide EclipseLink with expressions to correctly filter sibling instances for all classes that share a common table.
For example, Table 6-2 lists the rows in the EMPLOYEE table. The Employee class is the base class. Director, Manager, Programmer, and TechWriter classes each derive from the Employee class. However, in your application, instances of Manager, Programmer, and TechWriter classes must be represented as Employee instances and instances of Director must be represented as Director instances. Because there is not a one-to-one correspondence between class and JOB_TYPE field value, the JOB_TYPE field alone cannot serve as a class indicator field (see Using Class Indicator Fields). To resolve this issue, you could use the class extraction method, shown in Example 6-1.
Table 6-2 EMPLOYEE Table
| ID | NAME | JOB_TYPE | JOB_TITLE |
|---|---|---|---|
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732 |
Bob Jones |
1 |
Manager |
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733 |
Sarah Smith |
3 |
Technical Writer |
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734 |
Ben Ng |
2 |
Director |
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735 |
Sally Johnson |
3 |
Programmer |
Example 6-1 Class Extraction Method
...
// If the JOB_TYPE field value in record equals 2, return the Director class.
// Return the Employee class for all other JOB_TYPE field values
public static Class getClassFromRecord(Record record) {
if (record.get("JOB_TYPE").equals(new Integer(2)) {
return Director.class;
}
else {
return Employee.class;
}
}
When configuring inheritance using a class extraction method, EclipseLink does not generate SQL for queries on the root class.
For relational projects, EclipseLink assumes that all of the classes in an inheritance hierarchy have the same primary key, as set in the root descriptor.
In a relational project, you can map your inheritance hierarchy to a single table or to multiple tables.
You can designate relational descriptors as aggregates. XML descriptors are always composites (see Descriptors and Aggregation).
When configuring inheritance for a relational aggregate descriptor, all the descriptors in the inheritance tree must be aggregates. The descriptors for aggregate and non-aggregate classes cannot exist in the same inheritance tree.
When configuring inheritance for an XML descriptor, because all XML descriptors are composites, descriptor type does not restrict inheritance.
Two objects—a source (parent or owning) object and a target (child or owned) object—are related by aggregation if there is a strict one-to-one relationship between them, and all the attributes of the target object can be retrieved from the same data source representation as the source object. This means that if the source object exists, then the target object must also exist, and if the source object is destroyed, then the target object is also destroyed.
In this case, the descriptors for the source and target objects must be designated to reflect this relationship.
In EJB 3.0, an aggregate is known as an embeddable. In the EJB 3.0 specification, an embeddable may not contain another embeddable (that is, the EJB 3.0 specification does not support nested aggregates).
For more information, see Aggregate and Composite Descriptors and Inheritance.
You can customize a descriptor at run time by specifying a descriptor customizer—a Java class that implements the org.eclipse.persistence.config.DescriptorCustomizer interface and provides a default (zero-argument) constructor.
You use a descriptor customizer to customize a descriptor at run time through code API similar to how you use an amendment method to customize a descriptor. See Amendment and After-Load Methods.
You can associate a static Java method that is called when a descriptor is loaded at run time. This method can amend the run-time descriptor instance through the descriptor Java code API.
You can only modify descriptors before the session has been connected; you should not modify descriptors after the session has been connected.
In relational projects, EclipseLink raises various instances of DescriptorEvent during the persistence life cycle. Each descriptor owns an instance of DescriptorEventManager that is responsible for receiving these events and dispatching them to the descriptor event handlers registered with it.
Using a descriptor event handler, you can execute your own application specific logic whenever descriptor events occur, allowing you to take customized action at various points in the persistence life-cycle. For example, using a descriptor event handler, you can do the following:
Synchronize persistent objects with other systems, services, and frameworks
Maintain nonpersistent attributes of which EclipseLink is not aware
Notify other objects in the application when the persistent state of an object changes
Implement complex mappings or optimizations not directly supported by EclipseLink mappings
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