[ Groovy Doc ] AST : Singleton transformation

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Whether the singleton is pattern or an anti-pattern, there are still some cases where we need to create singletons. We're used to create a private constructor, a getInstance() method for a static field or even an initialized public static final field. So instead of writing code like this in Java: 

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1. public class T {  
2.     public static final T instance = new T();  
3.     private T() {}  
4. }  

You just need to annotate your type with the @Singleton annotation: 

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1. @Singleton class T {}  

The singleton instance can then simply be accessed with T.instance (direct public field access). You can also have the lazy loading approach with an additional annotation parameter: 

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1. @Singleton(lazy = true) class T {}  

Would become more or less equivalent to this Groovy class: 

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1. class T {  
2.     private static volatile T instance  
3.     private T() {}  
4.     static T getInstance () {  
5.         if (instance) {  
6.             instance  
7.         } else {  
8.             synchronized(T) {  
9.                 if (instance) {  
10.                     instance  
11.                 } else {  
12.                     instance = new T ()  
13.                 }  
14.             }  
15.         }  
16.     }  
17. }  

Lazy or not, once again, to access the instance, simply do T.instance (property access, shorcut for T.getInstance()). 

Below is the singleton usage example: 

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1. @Singleton(strict=false,lazy=true)  
2. class T {  
3.     static int i=0  
4.     public T(){i++}  
5. }  
6.   
7. printf("T.i=%d\n", T.i)  // lazy=false:1, lazy=true:0  
8. printf("T.instance.i=%d\n", T.instance.i)  
9. printf("T.instance.i=%d\n", T.instance.i)  
10. T t = new T()  // Not access T object through T.instance  
11. printf("T.instance.i=%d\n", T.instance.i)  

Execution Result: 

T.i=0
T.instance.i=1
T.instance.i=1
T.instance.i=2

Supplement 
* Compile-time Metaprogramming - AST Transformations