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Hello Dali!

An introduction to the Eclipse Dali Java Persistence API tools project

On June 26, 2006 the Eclipse Foundation announced the availability of new releases of 10 Open Source projects. This simultaneous release event, named Callisto, garnered a lot of attention for the 10 projects involved. But, meanwhile, on the same day and without much fanfare, not even a press release, the Dali JPA Tools project shipped its first formal release numbered 0.5. With the release of Dali 0.5, developers now have a solid set of tools for developing applications for the new Java Persistence API (JPA) in Eclipse.

The Java Persistence API
The Java Persistence API is part of the new Java EE 5 EJB 3.0 specification and defines a vendor-neutral standard for object-relational mapping. But don't be fooled by the term "EJB." The JPA specification was certainly developed under the umbrella EJB 3.0 specification, but that doesn't mean it's just for Java EE. JPA is designed to work in Java SE as well as EE, and will likely be split off into its own specification in the future.

JPA defines a way to map plain old Java objects (POJOs), not Entity Beans, to relational databases. This means you can use JPA to store the Java objects you write without having to subclass a JPA-provided class or implement any JPA interfaces. One of the driving goals of the JPA specification was ease of use and it shows.

JPA in Eclipse
One of the most striking features of JPA is the use of Java 5 annotations to define object-relational mappings. By adding annotations to your classes you can make instances persistent. JPA uses the term "Entity" for persistent objects and uses the @Entity annotation to identify them. This means that you can use a simple text editor or the Eclipse Java editor to work with JPA. (see Figure 1)

Unfortunately the Java editor doesn't understand what the annotations mean. As far as it's concerned annotations are just metadata markup. It can validate the syntax but not the semantics. For example, in Figure 2 the Phone Entity's number field is mapped to a column named "NUM." That column may or may not exist in the database but without JPA-aware validation you won't find out until runtime - a very bad time to find out. This is essentially what Dali provides: JPA-aware tooling and validation to ensure that what developers build at design time will run at deployment time.

Dali Overview
Dali provides tools to develop JPA applications targeted at either Java SE or Java EE and supports top-down, bottom-up, and meet-in-the-middle development approaches. Regardless of whether you want to persist an existing Java object model, manipulate data in an existing database, or connect your existing Java classes with an existing database, Dali can improve your productivity and help ensure that you don't spend your time in an endless edit, deploy, run, debug cycle.

For example, Figure 3 shows the same Phone Entity as Figure 2. But when using Dali, a problem is found in the JPA mapping for the number field. Dali has validated the column name specified in the @Column annotation against the Phone table and found that there's no such column.

JPA Defaults
One of the most useful features of the JPA is its defaulting rules. For example, if an Entity is not explicitly mapped to a table then the table name defaults to that of the Entity. Defaulting rules let developers "program by exception." That is, they only need to add annotations for things that don't match the defaults. In the case of our Phone example, Dali has confirmed that a table exists in the database with the name "Phone" - the same name as the Entity. Since there's no problem, no errors are displayed.

Dali Views
Dali contributes two views to the Eclipse user interface along with a perspective that defines a layout suitable for performing object-relational mapping. Those two views are the Persistence Outline and Persistence Properties.

Persistence Outline
The Persistence Outline view is similar to the Eclipse Java Outline but offers a JPA view of your object. In Figure 4 the Persistence Outline shows the Phone Entity and its mapped attributes. In JPA you can either put your mapping annotations on a Class's fields or properties (JavaBean style getters). The Persistence Outline displays the mappings the same way regardless of which of the two approaches you choose. Using the outline you can get a quick thumbnail sketch of the mappings for an Entity, even if those mappings are spread throughout the Java source file. For Phone you can see the id holds the primary key of the Entity and is a Basic mapping - mapped directly to the database column. The number attribute is also a Basic mapping while the custs attribute is a collection of objects mapped as a ManyToMany.

By default, the Persistence Outline selection is linked with the Java editor so you can navigate quickly around a Class to individual mappings. The linking is reciprocal - selection of attributes in the Java editor will also update the selection in the Persistence Outline. This quick navigation to mappings is useful if you want to jump to them in the Java source editor, but is more useful when paired with the Persistence Properties view.


More Stories By Shaun Smith

Shaun Smith is a Principal Product Manager for Oracle TopLink and an active member of the Eclipse community. He's Ecosystem Development Lead for the Eclipse Persistence Services Project (EclipseLink) and a committer on the Eclipse EMF Teneo and Dali Java Persistence Tools projects. He’s currently involved with the development of JPA persistence for OSGi and Oracle TopLink Grid, which integrates Oracle Coherence with Oracle TopLink to provide JPA on the grid.

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