One common question that you may encounter one day when using object-relational-mapping in your application is how to slim down data that you’re retrieving from the persistence layer down to a specific subset for your use-case in an efficient manner and without using complex additional mapping frameworks. In some situations you might declare lazy loaded fields but another approach that I’d like to share with you here are JPA2 constructor expressions.

Constructor expressions allow us to create plain old java objects from the result of an JPA query. The advantage is that we may use different projections for different scenarios and without being managed, the POJOs might help us save some resources here.

In the following tutorial, we’re going to persist 100 book entities with multiple properties to an embedded database and we’re using constructor expressions afterwards to create smaller POJOs using a subset of the information available from each persisted book.

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Java EE 6 is out and it indeed offers an interesting stack of technologies. So in today’s tutorial we are going to build a small sample web application that builds on this stack using Enterprise JavaBeans, Java Persistence API, Bean Validation, CDI and finally Java Server Faces and PrimeFaces.

The application we’re going to develop is a simple blog app that allows us to create new articles, list them and – finally delete them. We’re also covering some additional topics like JSF navigation, i18n, Ajax-enabled components and the deployment on the GlassFish application server.
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Today we’re going to take a look at the world of object-relational Mapping and how it is done using the Java Persistence API by creating some basic examples, mapping some relations and querying objects using JPQL or the Criteria API..

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