Saturday, August 6, 2011

This is not a joke, this is Java EE.

Extracted from the Book "Real World Java EE Patterns":
public class BookFacadeBean implements BookFacade {
    private EntityManager em;
    private Book currentBook;
    public Book find(long id) { 
        this.currentBook = this.em.find(Book.class, id); 
        return this.currentBook;

    public void create(Book book){
        this.currentBook = book;
    public Book getCurrentBook() { return currentBook; }

    public void save() {
        // nothing to do here

The transaction setting of the BookFacade may appear a little esoteric at the first glance.
On the class level the TransactionAttributeType.NEVER is used. Therefore it is guaranteed, that no method of the bean will be invoked in an active transaction. The method save, however, overwrites this rule with the setting TransactionAttribute.REQUIRES_NEW at the method level, which in turn starts a new transaction every time. The method create in turn was not annotated with TransactionAttributeType.REQUIRES_NEW even though the EntityManager#persist methos is invoqued inside. This invocation will not fail with the TransactionRequiredException. It will not fail because on an EntityManager with an EXTENDED Persistence context, the perssit,remove,merge, and refresh methdos may be called regardless of whether a transaction is active or not. The effects of these operations will be committed to the database when the exteneded persistence context is enlisted in a transaction and the transaction commits. It actually happens in the empty method save.

So in order to avoid a few extra lines of intuitive standard java code in charge of transaction demarcation the author has developed a counter-intuitive code that just work when calling empty function (and just if invoqued in the correct order). What's is worse, the author looks really excited with this way of working.

I'm completly sure no one except those in charge of developing Java EE containers will think calling and empty function with metadata is the proper way to work.

Fortunatelly for the rest of us, we have Spring.

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