A Curious Incidence of a jOOQ API Design Flaw

jOOQ is an internal domain-specific language (DSL), modelling the SQL language (external DSL) in Java (the host language). The main mechanism of the jOOQ API is described in this popular article:

The Java Fluent API Designer Crash Course.

Anyone can implement an internal DSL in Java (or in most other host languages) according to the rules from that article.

An example SQL language feature: BOOLEANs

One of the nice things about the SQL language, however, is the BOOLEAN type, which has been introduced late into the language as of SQL:1999. Sure, without booleans, you can just model TRUE and FALSE values via 1 and 0, and transform the predicates into the value using CASE

CASE WHEN A = B THEN 1 ELSE 0 END

But with true BOOLEAN support, you can do awesome queries like the following PostgreSQL query that is run against the Sakila database:

SELECT
  f.title, 
  string_agg(a.first_name, ', ') AS actors
FROM film AS f
JOIN film_actor AS fa USING (film_id)
JOIN actor AS a USING (actor_id)
GROUP BY film_id
HAVING every(a.first_name LIKE '%A%')

The above yields:

TITLE                    ACTORS
-----------------------------------------------------
AMISTAD MIDSUMMER        CARY, DARYL, SCARLETT, SALMA
ANNIE IDENTITY           CATE, ADAM, GRETA
ANTHEM LUKE              MILLA, OPRAH
ARSENIC INDEPENDENCE     RITA, CUBA, OPRAH
BIRD INDEPENDENCE        FAY, JAYNE
...

In other words, we’re looking for all the films where all the actors who played in the film contain the letter “A” in their first names. This is done via an aggregation on the boolean expression / predicate first_name LIKE '%A%':

HAVING every(a.first_name LIKE '%A%')

Now, in the terms of the jOOQ API, this means we’ll have to provide overloads of the having() method that take different argument types, such as:

// These accept "classic" predicates
having(Condition... conditions);
having(Collection<? extends Condition> conditions);

// These accept a BOOLEAN type
having(Field<Boolean> condition);

Of course, these overloads are available for any API method that accepts predicates / boolean values, not just for the HAVING clause.

As mentioned before, since SQL:1999, jOOQ’s Condition and Field<Boolean> are really the same thing. jOOQ allows for converting between the two via explicit API:

Condition condition1 = FIRST_NAME.like("%A%");
Field<Boolean> field = field(condition1);
Condition condition2 = condition(field);

… and the overloads make conversion more conveniently implicit.

So, what’s the problem?

The problem is that we thought it might be a good idea to add yet another convenient overload, the having(Boolean) method, where constant, nullable BOOLEAN values could be introduced into the query, for convenience, which can be useful when building dynamic SQL, or commenting out some predicates:

DSL.using(configuration)
   .select()
   .from(TABLE)
   .where(true)
// .and(predicate1)
   .and(predicate2)
// .and(predicate3)
   .fetch();

The idea is that the WHERE keyword will never be commented out, regardless what predicate you want to temporarily remove.

Unfortunately, adding this overload introduced a nuisance to developers using IDE auto-completion. Consider the following two method calls:

// Using jOOQ API
Condition condition1 = FIRST_NAME.eq   ("ADAM");
Condition condition2 = FIRST_NAME.equal("ADAM");

// Using Object.equals (accident)
boolean = FIRST_NAME.equals("ADAM");

By (accidentally) adding a letter “s” to the equal() method – mostly because of IDE autocompletion – the whole predicate expression changes semantics drastically, from a jOOQ expression tree element that can be used to generate SQL to an “ordinary” boolean value (which always yields false, obviously).

Prior to having added the last overload, this wasn’t a problem. The equals() method usage wouldn’t compile, as there was no applicable overload taking a Java boolean type.

// These accept "classic" predicates
having(Condition condition);
having(Condition... conditions);
having(Collection<? extends Condition> conditions);

// These accept a BOOLEAN type
having(Field<Boolean> condition);

// This method didn't exist prior to jOOQ 3.7
// having(Boolean condition);

After jOOQ 3.7, this accident started to go unnoticed in user code as the compiler no longer complained, leading to wrong SQL.

Conclusion: Be careful when designing an internal DSL. You inherit the host language’s “flaws”

Java is “flawed” in that every type is guaranteed to inherit from java.lang.Object and with it, its methods: getClass(), clone(), finalize() equals(), hashCode(), toString(), notify(), notifyAll(), and wait().

In most APIs, this isn’t really that much of a problem. You don’t really need to re-use any of the above method names (please, don’t).

But when designing an internal DSL, these Object method names (just like the language keywords) limit you in your design space. This is particularly obvious in the case of equal(s).

We’ve learned, and we’ve deprecated and will remove the having(Boolean) overload, and all the similar overloads again.

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