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Another technique to create a database-wide constraint uses custom triggers that run on insertion or update of rows in particular tables. This is a procedural approach that has the already-mentioned disadvantages but is inherently flexible. By far the most common rules that span several tables are referential integrity rules. They re widely known as foreign keys, which are a combination of two things: a key value copy from a related row and a constraint that guarantees that the referenced value exists. Hibernate creates foreign key constraints automatically for all foreign key columns in association mappings. If you check the DDL produced by Hibernate, you may notice that these constraints also have automatically generated database identifiers names that aren t easy to read and that make debugging more difficult:

15.1 What are web services and what is SOAP 356 15.2 Creating a SOAP client application with Ant 357 15.3 Creating a SOAP service with Axis and Ant 363 15.4 Adding web services to an existing web application 367 15.5 Writing a client for our SOAP service 371 15.6 What is interoperability, and why is it a problem 376 15.7 Building a C# client 376 15.8 The rigorous way to build a web service 381 15.9 Reviewing web service development 382 15.10 Calling Ant via SOAP 383 15.11 Summary 384

alter table ITEM add constraint FK1FF7F1F09FA3CB90 foreign key (SELLER_ID) references USERS;

Web services are an emerging target of software development. Put simply, a web service is a web or enterprise application that provides a way for other programs to call it by using XML as the means of exchanging data. If you can build and deploy a web application, you can build and deploy a web service. If it s all so easy, why do we have a whole chapter on web services Because they add new problems to the process: integration and interoperability. Client applications need to be able to call your web service, including applications that are written in different languages or that use different web service toolkits. We need to extend our existing development process to integrate client-side and interoperability tests. In this chapter, we extend the web application we wrote in chapter 12, adding a SOAP interface to it. We use the Apache Axis library to provide our SOAP interface, rather than the Sun version, because it comes from a sister project to Ant and because we like it. After adding SOAP to our application, we build tests for it, first with a Java client, and then with a C# client running on the .NET platform. As we said, integration and interoperability are the new challenges of a web service development process. 355

This DDL declares the foreign key constraint for the SELLER_ID column in the ITEM table. It references the primary key column of the USERS table. You can customize the name of the constraint in the <many-to-one> mapping of the Item class with the foreign-key attribute:

With annotations, use a Hibernate extension:

We do not delve deeply into the details of SOAP and web services; we encourage you to read books on the subject (for example, Wesley 2002, and Graham 2001), as well as the SOAP specifications hosted in the web services working group at the W3C (http://www.w3.org/2002/ws/). We do explain the basic concepts behind web services, however, and show how you can use Ant to build, test, and call a web service.

@ManyToOne @JoinColumn(name = "SELLER_ID") @org.hibernate.annotations.ForeignKey(name = "FK_SELLER_ID") private User seller;