Find singletons in a Java application

There might be situations where a constraint needs to verify that a certain type is implemented as singleton. In the world of containers (e.g. EJB, CDI) these can be easily identified by the presence of specific annotations:

• javax.inject.Singleton
• javax.ejb.Singleton
• javax.enterprise.context.ApplicationScoped (provides singleton-like semantics)

The situation is a bit more difficult if no container is available and we have to go back to the „classical“ singleton pattern. In Java this usually means an implementation of a class like the following one:

public class Singleton {
  private static final INSTANCE = new Singleton();

  private Singleton() {
  }

  public static Singleton getInstance() {
    return INSTANCE;
  }
}

Our singleton implementation has the following properties:

• It declares a static field with the class itself as type
• All constructors are private
• It declares a static method which reads the static field and returns an instance of the class

It’s easy to translate these conditions into Cypher:

match
   (singleton:Class)-[:DECLARES]->(constructor:Constructor)
with
   singleton, collect(constructor) as constructors
where
   all(constructor in constructors where constructor.visibility='private')
with
   singleton
match
   (singleton)-[:DECLARES]->(instance:Field)-[:OF_TYPE]->(singleton),
   (singleton)-[:DECLARES]->(getInstance:Method)-[:RETURNS]->(singleton),
   (getInstance)-[:READS]->instance
where
   instance.static and getInstance.static
set
  singleton:Singleton
return
   singleton.fqn, instance.name, getInstance.name

The statement is „weak“ enough to be agnostic against eager or lazy initialization thus it can be used as a template for detecting singletons which are present in an application.

Reference:
http://en.wikipedia.org/wiki/Singleton_pattern