data matrix barcode generator java Object-relational mapping in Java

Generating DataMatrix in Java Object-relational mapping

Object-relational mapping
Data Matrix 2d Barcode Generation In Java
Using Barcode drawer for Java Control to generate, create ECC200 image in Java applications.
www.OnBarcode.com
Scanning Data Matrix In Java
Using Barcode decoder for Java Control to read, scan read, scan image in Java applications.
www.OnBarcode.com
8.4 Mapping inheritance
UPC Symbol Drawer In Java
Using Barcode printer for Java Control to generate, create GS1 - 12 image in Java applications.
www.OnBarcode.com
USS Code 39 Printer In Java
Using Barcode generation for Java Control to generate, create Code-39 image in Java applications.
www.OnBarcode.com
In section 8.1.1 we mentioned the difficulties in mapping OO inheritance into relational databases. We also alluded to the three strategies used to solve this problem: putting all classes in the OO hierarchy in the same table, using joined tables for the super- and subclasses, or using completely separate tables for each class. We ll explore how each strategy is actually implemented in this section. Recall from section 8.2.7 that we could easily utilize different strategies for generating sequences more or less by changing configuration parameters for the @GeneratedValue annotation. The @Inheritance annotation used to map OO inheritance tries to follow the same philosophy of isolating strategy-specific settings into the configuration. We ll explore inheritance mapping using the three strategies offered through the @Inheritance annotation by implementing a familiar example. As we mentioned earlier, the ActionBazaar system has several user types, including sellers and bidders. We have also introduced the idea of creating a User superclass common to all user types. In this scheme of things, the User entity would hold data and behavior common to all users, while subclasses like Bidder and Seller would hold data and behavior specific to user types. Figure 8.8 shows a simplified class diagram for this OO hierarchy. In this section you ll learn how all the entities in the hierarchy in figure 8.8 can be mapped to database tables using different types of inheritance mapping strategies supported by JPA:
Make GTIN - 13 In Java
Using Barcode printer for Java Control to generate, create GTIN - 13 image in Java applications.
www.OnBarcode.com
Print QR-Code In Java
Using Barcode printer for Java Control to generate, create QR Code JIS X 0510 image in Java applications.
www.OnBarcode.com
Single table Joined tables Table per class
Encoding Data Matrix ECC200 In Java
Using Barcode printer for Java Control to generate, create Data Matrix ECC200 image in Java applications.
www.OnBarcode.com
Painting Code 9/3 In Java
Using Barcode printer for Java Control to generate, create Code 93 Full ASCII image in Java applications.
www.OnBarcode.com
You ll also learn about polymorphic relationships.
Paint Data Matrix In None
Using Barcode encoder for Microsoft Excel Control to generate, create Data Matrix image in Excel applications.
www.OnBarcode.com
Data Matrix Encoder In Objective-C
Using Barcode drawer for iPhone Control to generate, create Data Matrix image in iPhone applications.
www.OnBarcode.com
Figure 8.8 The ActionBazaar user hierarchy. Each user type like Bidder and Seller inherit from the common User superclass. The empty arrow signifies there may be some other subclasses of the User class.
ECC200 Drawer In .NET
Using Barcode generator for Reporting Service Control to generate, create ECC200 image in Reporting Service applications.
www.OnBarcode.com
Barcode Generator In Visual C#.NET
Using Barcode drawer for .NET framework Control to generate, create Barcode image in Visual Studio .NET applications.
www.OnBarcode.com
Mapping inheritance
QR-Code Drawer In Java
Using Barcode drawer for Eclipse BIRT Control to generate, create Quick Response Code image in BIRT reports applications.
www.OnBarcode.com
Make UPC Symbol In None
Using Barcode creator for Office Excel Control to generate, create UPC Symbol image in Excel applications.
www.OnBarcode.com
8.4.1 Single-table strategy
GS1 - 13 Scanner In Visual Basic .NET
Using Barcode reader for Visual Studio .NET Control to read, scan read, scan image in VS .NET applications.
www.OnBarcode.com
Barcode Scanner In Java
Using Barcode decoder for Java Control to read, scan read, scan image in Java applications.
www.OnBarcode.com
In the single-table strategy, all classes in the inheritance hierarchy are mapped to a single table. This means that the single table will contain a superset of all data stored in the class hierarchy. Different objects in the OO hierarchy are identified using a special column called a discriminator column. In effect, the discriminator column contains a value unique to the object type in a given row. The best way to understand this scheme is to see it implemented. For the ActionBazaar schema, assume that all user types, including Bidders and Sellers, are mapped into the USERS table. Figure 8.9 shows how the table might look.
Data Matrix 2d Barcode Maker In Visual Basic .NET
Using Barcode generation for .NET framework Control to generate, create ECC200 image in Visual Studio .NET applications.
www.OnBarcode.com
Denso QR Bar Code Generator In None
Using Barcode generator for Font Control to generate, create QR image in Font applications.
www.OnBarcode.com
Storing all ActionBazaar user types using a single table
ANSI/AIM Code 39 Generation In VS .NET
Using Barcode drawer for ASP.NET Control to generate, create Code-39 image in ASP.NET applications.
www.OnBarcode.com
Print GS1 - 12 In None
Using Barcode printer for Software Control to generate, create GTIN - 12 image in Software applications.
www.OnBarcode.com
As figure 8.9 depicts, the USERS table contains data common to all users (such as USER_ID and USERNAME), Bidder-specific data (such as BID_FREQUENCY) and Sellerspecific data (such as CREDIT_WORTH). Records 1 and 2 contain Bidder records while record 3 contains a Seller record. This is indicated by the B and S values in the USER_TYPE column. As you can imagine, the USER_TYPE discriminator column can contain values corresponding to other user types, such as A for admin or C for CSR. The persistence provider maps each user type to the table by storing persistent data into relevant mapped columns, setting the USER_TYPE value correctly and leaving the rest of the values NULL. The next step to understanding the single-table strategy is to analyze the actual mapping implementation. Listing 8.12 shows the mapping for the User, Bidder, and Seller entities.
Listing 8.12 Inheritance mapping using a single table
@Entity @Table(name="USERS") @Inheritance(strategy=InheritanceType.SINGLE_TABLE) @DiscriminatorColumn(name="USER_TYPE", discriminatorType=DiscriminatorType.STRING, length=1) public abstract class User ...
Inheritance strategy
Discriminator column
Object-relational mapping
@Entity @DiscriminatorValue(value="S") public class Seller extends User ... @Entity @DiscriminatorValue(value="B") public class Bidder extends User
Seller discriminator
Bidder discriminator
The inheritance strategy and discriminator column has to be specified on the root entity of the OO hierarchy. In listing 8.12, we specify the strategy to be InheritanceType.SINGLE_TABLE in the @Inheritance annotation on the User entity b. The @Table annotation on the User entity specifies the name of the single table used for inheritance mapping, USERS. The @DiscriminatorColumn annotation C specifies the details of the discriminator column. The name element specifies the name of the discriminator, USER_TYPE. The discriminatorType element specifies the data type of the discriminator column, which is String, and the length element specifies the size of the column, 1. Both subclasses of User, Bidder, and Seller specify a discriminator value using the @DiscriminatorValue annotation. The Seller class specifies its discriminator value to be S D. This means that when the persistence provider saves a Seller object into the USERS table, it will set the value of the USER_TYPE column to S. Similarly, Seller entities would only be reconstituted from rows with a discriminator value of S. Likewise, the Bidder subclass specifies its discriminator value to be B E. Every subclass of User is expected to specify an appropriate discriminator value that doesn t conflict with other subclasses. If you don t specify a discriminator value for a subclass, the value is assumed to be the name of the subclass (such as Seller for the Seller entity). Single table is the default inheritance strategy for EJB 3. Although this strategy is simple to use, it has one great disadvantage that might be apparent from figure 8.5. It does not fully utilize the power of the relational database in terms of using primary/foreign keys and results in a large number of NULL column values. To understand why, consider the Seller record (number 3) in figure 8.9. The BID_FREQUENCY value is set to NULL for this record since it is not a Bidder record and the Seller entity does not map this column. Conversely, none of the Bidder records ever populate the CREDIT_WORTH column. It is not that hard to imagine the quantity of redundant NULL -valued columns in the USERS table if there are a significant numbers of users (such as a few thousand).
Copyright © OnBarcode.com . All rights reserved.