I.T. is full of complex things that should (and sometimes could) be simple. Getting the JQPL/SQL String representation for a JPA 2.0 CriteriaQuery is one of them.
By now you all know the JPA 2.0 Criteria API : a type safe way to write a JQPL query. This API is clever in the way that you don’t use Strings to build your query, but is quite verbose… and sometimes you get lost in dozens of lines of Java code, just to write a simple query. You get lost in your CriteriaQuery, you don’t know why your query doesn’t work, and you would love to debug it. But how do you debug it ? Well, one way would be by just displaying the JPQL and/or SQL representation. Simple, isn’t it ? Yes, but JPA 2.0 javax.persistence.Query doesn’t have an API to do this. You then need to rely on the implementation… meaning, the code is different if you use EclipseLink, Hibernate or OpenJPA.
The CriteriaQuery we want to debug
Let’s say you have a simple Book entity and you want to retrieve all the books sorted by their id. Something like SELECT b FROM Book b ORDER BY b.id DESC. How would you write this with the CriteriaQuery ? Well, something like these 5 lines of Java code :
[sourcecode language=”java” highlight=”5″]
CriteriaBuilder cb = em.getCriteriaBuilder();
CriteriaQuery<Book> q = cb.createQuery(Book.class);
Root<Book> b = q.from(Book.class);
TypedQuery<Book> findAllBooks = em.createQuery(q);
So imagine when you have more complex ones. Sometimes, you just get lost, it gets buggy and you would appreciate to have the JPQL and/or SQL String representation to find out what’s happening. You could then even unit test it.
Getting the JPQL/SQL String Representations for a Criteria Query
So let’s use an API to get the JPQL/SQL String representations of a CriteriaQuery (to be more precise, the TypedQuery created from a CriteriaQuery). The bad news is that there is no standard JPA 2.0 API to do this. You need to use the implementation API hoping the implementation allows it (thank god that’s (nearly) the case for the 3 main JPA ORM frameworks). The good news is that the Query interface (and therefore TypedQuery) has an unwrap method. This method returns the provider’s query API implementation. Let’s see how you can use it with EclipseLink, Hibernate and OpenJPA.
EclipseLink‘s Query representation is the org.eclipse.persistence.jpa.JpaQuery interface and the org.eclipse.persistence.internal.jpa.EJBQueryImpl implementation. This interface gives you the wrapped native query (org.eclipse.persistence.queries.DatabaseQuery) with two very handy methods : getJPQLString() and getSQLString(). Unfortunatelly the getJPQLString() method will not translate a CriteriaQuery into JPQL, it only works for queries originally written in JPQL (dynamic or named query). The getSQLString() method relies on the query being “prepared”, meaning you have to run the query once before getting the SQL String representation.
findAllBooks.unwrap(JpaQuery.class).getDatabaseQuery().getJPQLString(); // doesn’t work for CriteriaQuery
Hibernate‘s Query representation is org.hibernate.Query. This interface has several implementations and the very useful method that returns the SQL query string : getQueryString(). I couldn’t find a method that returns the JPQL representation, if I’ve missed something, please let me know.
OpenJPA‘s Query representation is org.apache.openjpa.persistence.QueryImpl and also has a getQueryString() method that returns the SQL (not the JPQL). It delegates the call to the internal org.apache.openjpa.kernel.Query interface. I couldn’t find a method that returns the JPQL representation, if I’ve missed something, please let me know.
Once you get your SQL String, why not unit test it ? Hey, but I don’t want to test my ORM, why would I do that ? Well, it happens that I’ve discovered a but in the new releases of OpenJPA by unit testing a query… so, there is a use case for that. Anyway, this is how you could do it :
assertEquals("SELECT b FROM Book b ORDER BY b.id DESC", findAllBooksCriteriaQuery.unwrap(org.apache.openjpa.persistence.QueryImpl.class).getQueryString());
As you can see, it’s not that simple to get a String representation for a TypedQuery. Here is a digest of the three main ORMs :
|ORM Framework||Query implementation||How to get the JPQL String||How to get the SPQL String|
(*) Only possible on a dynamic or named query. Not possible on a CriteriaQuery
(**) You need to execute the query first, if not, the value is null
And what about having an API in JPA 2.1 ?
For a developers’ point of view it would be great to have two methods in the javax.persistence.Query (and therefore javax.persistence.TypedQuery) interface that would be able to easily return the JPQL and SQL String representations, e.g : Query.getJPQLString() and Query.getSQLString(). Hey, that would be the perfect time to have it in JPA 2.1 that will be shipped in less than a year. Now, as an implementer, this might be tricky to do, I would love to ear your point of view on this.
Anyway, I’m going to post an email to the JPA 2.1 Expert Group… just in case we can have this in the next version of JPA ;o)