Groovy single threaded stream safe

The actual class used as an instance is not thread safe (unless you do this). There are many examples here (e.g. Are final static variables thread safe in Java?: It uses static final HashMap

, which is not thread safe)

creating a singleton using groovys @Singleton

annotation is thread safe (and you have to rely on it).

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

• Here is the regular version @Singleton
  
  that results in a variable static final
  
  which is again thread safe in java:

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

• For version lazy
  
  ( @Singleton(lazy=true)
  
  ) Double lock 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: gist with simple class and disassembled code