data matrix barcode generator java Inheritance mapping using the table-per-class strategy in Java

Creator Data Matrix ECC200 in Java Inheritance mapping using the table-per-class strategy

Listing 8.14 Inheritance mapping using the table-per-class strategy
Printing ECC200 In Java
Using Barcode creator for Java Control to generate, create Data Matrix ECC200 image in Java applications.
www.OnBarcode.com
Scan Data Matrix In Java
Using Barcode reader for Java Control to read, scan read, scan image in Java applications.
www.OnBarcode.com
@Entity @Table(name="USERS") @Inheritance(strategy=InheritanceType.TABLE_PER_CLASS) public class User { Inheritance ... strategy @Entity @Table(name="SELLERS") public class Seller extends User { ... @Entity @Table(name="BIDDERS") public class Bidder extends User {
Barcode Maker In Java
Using Barcode drawer for Java Control to generate, create Barcode image in Java applications.
www.OnBarcode.com
Generating Data Matrix 2d Barcode In Java
Using Barcode encoder for Java Control to generate, create Data Matrix ECC200 image in Java applications.
www.OnBarcode.com
Table mappings
Matrix Maker In Java
Using Barcode creation for Java Control to generate, create 2D Barcode image in Java applications.
www.OnBarcode.com
Creating USS Code 128 In Java
Using Barcode drawer for Java Control to generate, create Code 128A image in Java applications.
www.OnBarcode.com
As you can see, the inheritance strategy C is specified in the superclass entity, User. However, all of the concrete entities in the OO hierarchy are mapped to separate tables and each have a @Table specification B. The greatest disadvantage of using this mapping type is that it does not have good support for polymorphic relations or queries because each subclass is mapped to its own table.
Generating Code39 In Java
Using Barcode generator for Java Control to generate, create USS Code 39 image in Java applications.
www.OnBarcode.com
Paint EAN8 In Java
Using Barcode creation for Java Control to generate, create UPC - 8 image in Java applications.
www.OnBarcode.com
Object-relational mapping
Data Matrix Reader In C#.NET
Using Barcode reader for VS .NET Control to read, scan read, scan image in VS .NET applications.
www.OnBarcode.com
Reading Data Matrix ECC200 In VB.NET
Using Barcode decoder for .NET Control to read, scan read, scan image in VS .NET applications.
www.OnBarcode.com
The limitation is that when you want to retrieve entities over the persistence provider, you must use SQL UNION or retrieve each entity with separate SQL for each subclass in the hierarchy. Besides being awkward, this strategy is the hardest for an EJB 3 provider to implement reliably. As a result, implementing this strategy has been made optional for the provider by the EJB 3 specification. We recommend that you avoid this strategy altogether. This completes our analysis of the three strategies for mapping OO inheritance. Choosing the right strategy is not as straightforward as you might think. Table 8.2 compares each strategy. The single-table strategy is relatively simple and is fairly performance friendly since it avoids joins under the hood. Even inserts and updates in the single-table strategy perform better when compared to the joined-tables strategy. This is because in the joined-tables strategy, both the parent and child tables need to be modified for a given entity subclass. However, as we mentioned, the single-table
Making EAN13 In Visual Basic .NET
Using Barcode maker for Visual Studio .NET Control to generate, create EAN-13 image in .NET applications.
www.OnBarcode.com
Barcode Generator In None
Using Barcode generator for Microsoft Excel Control to generate, create Barcode image in Excel applications.
www.OnBarcode.com
Table 8.2 The EJB 3 JPA supports three different inheritance strategies. The table-per-class choice is optional and the worst of these strategies. Feature Table support Single Table One table for all classes in the entity hierarchy:
Encode Barcode In Objective-C
Using Barcode generator for iPhone Control to generate, create Barcode image in iPhone applications.
www.OnBarcode.com
Making Linear 1D Barcode In Visual Studio .NET
Using Barcode generator for .NET Control to generate, create Linear image in VS .NET applications.
www.OnBarcode.com
Joined Tables One for the parent class, and each subclass has a separate table to store polymorphic properties Mapped tables are normalized. Yes
Draw QR Code In VB.NET
Using Barcode generation for .NET framework Control to generate, create Denso QR Bar Code image in .NET framework applications.
www.OnBarcode.com
Generating Barcode In Visual Studio .NET
Using Barcode generator for VS .NET Control to generate, create Barcode image in .NET applications.
www.OnBarcode.com
Table per Class One table for each concrete class in the entity hierarchy
Generating UPC Symbol In Java
Using Barcode maker for Android Control to generate, create UPCA image in Android applications.
www.OnBarcode.com
Barcode Drawer In None
Using Barcode creation for Font Control to generate, create Barcode image in Font applications.
www.OnBarcode.com
Mandatory columns may be nullable. The table grows when more subclasses are added.
Scanning Barcode In Java
Using Barcode Control SDK for Java Control to generate, create, read, scan barcode image in Java applications.
www.OnBarcode.com
Decode Barcode In .NET Framework
Using Barcode recognizer for .NET Control to read, scan read, scan image in .NET framework applications.
www.OnBarcode.com
Uses discriminator column SQL generated for retrieval of entity hierarchy SQL for insert and update
Simple SELECT
SELECT clause joining multiple tables
One SELECT for each subclass or UNION of SELECT One INSERT or UPDATE for every subclass
Single INSERT or UPDATE for all entities in the hierarchy
Multiple INSERT, UPDATE: one for the root class and one for each involved subclass Good Good Required
Polymorphic relationship Polymorphic queries Support in EJB 3 JPA
Good Good Required
Poor Poor Optional
Mapping inheritance
strategy results in a large number of NULL -valued columns. Moreover, adding a new subclass essentially means updating the unified table each time. In the joined-tables strategy, adding a subclass means adding a new child table as opposed to altering the parent table (which may or may not be easier to do). By and large, we recommend the joined-tables strategy since it best utilizes the strengths of the relational database, including the ability to use normalization techniques to avoid redundancy. In our opinion, the performance cost associated with joins is relatively insignificant, especially with surrogate keys and proper database indexing. The table-per-class strategy is probably the worst choice of the three. It is relatively counterintuitive, uses almost no relational database features, and practically magnifies the object relational impedance instead of attempting to bridge it. The most important reason to avoid this strategy, however, is that EJB 3 makes it optional for a provider to implement it. As a result, choosing this strategy might make your solution nonportable across implementations. Beside these inheritance strategies, the EJB 3 JPA allows an entity to inherit from a nonentity class. Such a class is annotated with @MappedSuperClass. Like an embeddable object, a mapped superclass does not have an associated table.
Eclipse ORM tool
As we mentioned earlier, EJB 3 makes O/R mapping a lot easier, but not quite painless. This is largely because of the inherent complexity of ORM and the large number of possible combinations to handle. The good news is that a project to create an Eclipse-based EJB 3 mapping tool code-named Dali is under way (www.eclipse.org/ dali/). The project is led by Oracle and supported by JBoss and BEA. It will be a part of Eclipse web tool project (WTP) and will support creating and editing EJB 3 O/R mappings using either annotations or XML. Also, two commercial products, Oracle JDeveloper and BEA Workshop, support development of EJB 3 applications.
Copyright © OnBarcode.com . All rights reserved.