Groovy singleton thread safe

Question

Does @Singleton annotation in Groovy make a singleton thread-safe?

If it's not, which is the easiest way to create a thread-safe singleton using Groovy?

Answer 1

The actual class used as the instance is not threadsafe (unless you make it). There are lots of examples around here (e.g. Are final static variables thread safe in Java?: a static final HashMap is used there, which is not threadsafe)

The creation of the singleton using groovys @Singleton annotation is thread-safe (and you should rely on that).

The docs show two versions, of the corresponding Java code generated by the transformation:

1. Here is the regular version @Singleton, which results in a static final variable, which again is threadsafe in java:

   public class T {
       public static final T instance = new T();
       private T() {}
   }
   

2. For the lazy version (@Singleton(lazy=true)) Double-checked locking is created:

   class T {
       private static volatile T instance
       private T() {}
       static T getInstance () {
           if (instance) {
               instance
           } else {
               synchronized(T) {
                   if (instance) {
                       instance
                   } else {
                       instance = new T ()
                   }
               }
           }
       }
   }
   

For reference, here is a gist with a simple class and the disassembled code