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?

You mean does it make all the methods in your class thread safe? — tim_yates, Sep 24 '14 at 19:10

score 5 · Accepted Answer · edited May 23 '17 at 12:29

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:

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() {}
}
```

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

Groovy singleton thread safe

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