Saturday, April 27, 2013

JPA - Should I become a laziness extremist?

When you spoke with the Developers about mapping objects to relational databases, they very often complain about poor JPA performance, unpredictable behavior of JPA Providers, etc. Usually at some point of the conversation you will hear: "Let's drop this technology at all, we've seen something much better on the conference last month. We will use it in our projects instead of JPA and develop them happily ever after." - Sounds familiar? :)

It's nothing wrong in learning new technologies, in fact you should do it constantly, to improve your skills and knowledge, but when you have problems with one of them, will you choose an easy path to another technology, or ask yourself: "Am I using it in a right way?"

Let's look at the JPA usage example. Suppose that we have simple database, mapped to the entities:


and we have to display all employee names, regardless their employer (and department). Nothing easier ;) - simple JPQL query will do that:

select employee from Employee employee order by employee.name

Many developers finish at this point, and celebrate with Friends another successful JPQL query in their life ;), but some of us have this strange feeling, that something creepy is lurking beneath the shiny surface. SQL queries produced by the JPA provider (ex. Hibernate) will reveal the truth: 
select [...]  from EMPLOYEE employee0_ order by employee0_.EMPLOYEE_NAME

Nothing special, so far :), but here comes the naked truth:
select [...] from DEPARTMENT department0_ left outer join EMPLOYER employer1_ on department0_.EMPLOYER_ID=employer1_.EMPLOYER_ID where department0_.DEPARTMENT_ID=?
select [...] from EMPLOYER employer0_ where employer0_.EMPLOYER_ID=?
select [...] from DEPARTMENT department0_ left outer join EMPLOYER employer1_ on department0_.EMPLOYER_ID=employer1_.EMPLOYER_ID where department0_.DEPARTMENT_ID=?
select [...] from DEPARTMENT department0_ left outer join EMPLOYER employer1_ on department0_.EMPLOYER_ID=employer1_.EMPLOYER_ID where department0_.DEPARTMENT_ID=?
select [...] from DEPARTMENT department0_ left outer join EMPLOYER employer1_ on department0_.EMPLOYER_ID=employer1_.EMPLOYER_ID where department0_.DEPARTMENT_ID=?
What the heck?! What are these queries for?! - Well the reason lies in default fetch attribute values for @ManyToOne annotations, which is EAGER. My database holds 2 Employers, one of them has 4 Departments, while second one hasn't any. When the Employee is loaded, JPA provider loads by default all EAGER associations (in our case both Department, and Employer), thus we have the additional queries. As you see above the JPA provider is clever enough to load both Employer and Department at once, when it is possible.

You've just found magical JPQL query fetching all the database content at once :). Does this situation remind you something in the past? ;)

What can we do about it? - My Friend, all you need is a laziness :) - Don't use EAGER unless it is REALLY needed (and remember that @ManyToOne and @OneToOne annotations use it by default).

You may call me a lunatic, or laziness extremist at this point :) and ask: Have you ever encountered LazyInitializationException, Bro!? Have you heard of all the mess with lazy loading problems!? Performance degradation, etc. ... Of course I did :), but don't you think that if we are getting in such troubles with JPA, maybe we use it in a wrong way?!

What we do usually in Web Applications is presenting or editing some data on UI, and usually it is only small subset of specific entities' properties. Doing it requires fetching the entities tree from the database - without batting an eye, we ask Entity Manager: give me all Employees, sorted by name, with all related entities, and then complain on degraded performance!

We don't care what we fetch from the database, because Entity Manager will do the donkey work for us. We get LazyInitializationException, so what! We will use Open Entity Manager in View pattern, and silence this stupid exception!

Give a me a break! Don't you think it's a dead end? :) - It's about time to change something :) There are sophisticated methods which you can use in your projects, like CQRS for example, along with possibilities already existing in JPA, which can help you change the bad manners described by me in this post.

To be continued ...

Few links for the dessert:




Follow-ups:


This article has been republished on Java Code Geeks (05/01/2013), and on Dzone's Javalobby (05/01/2013).

Saturday, April 6, 2013

JPA - Hibernate - Type mapping on package level

When we are finally mature enough to use some custom types mapping in JPA, we usually stuck with some provider specific solution, because JPA itself doesn't define any mechanism for doing it. Let me show you an example of custom type mapping definition for one of the JPA providers - Hibernate.

Suppose that we use Joda Money in our project, and have an entity with property having type Money. There are already pretty nice type mapping implementations for Money, provided by Jadira - User Types project. All we have to do is just let Hibernate know that we want to use specific type mapping.

When you look at the Hibernate Docs, Section 5.1.4.1.1: Type, you'll see few possibilities, starting from the simplest - using @Type annotation on each property having Money type. This choice can be good if you have only one, or very few, properties of this type in your domain mapping. It is very probable that sooner or later, when your project will grow enough, there will be more and more of them, and you end up with many similar lines defining the same type mapping.

If you aren't a big fan of repeating yourself, or you don't trust in refactorings made by your apprentices ;), you should consider another way, using @TypeDefs and @TypeDef annotations.

As you may read in Hibernate documentation: "These annotations can be placed at the class or package level." - Let's focus on the second option - package level.

We will place these annotations in package-info.java for our domain entities holding package (see: Java Language Specification - 7.4.1. Named Packages). It will look like this:

Now, when you map the property using Money type, you can do it without additional type mapping specification, just like this:

One technical note, before you become happy Money mapping user ;) - Because PersistentMoneyAmount uses single column (holding amount) for Money mapping, it requires defining of currency which will be used along with the amount. The default currency can be defined as Persistence Unit property: jadira.usertype.currencyCode

PS. Don't treat the above Money example as the guideline of Joda Money mapping :), there are probably better ways of doing it, see Jadira User Types blog.

Few links for the dessert:





Follow-ups:


This article has been republished on Java Code Geeks (04/09/2013), and on Dzone's Javalobby (04/15/2013).