具有静态内部类和双重检查锁定的单例

Question

I am reading about Singleton design pattern and evaluating different implementations. I have doubt with the below implementations:

A. Singleton Implementation with static inner class

public class SingletonWithStaticClass {

private SingletonWithStaticClass(){}

private static class SingletonInnerClass{
    public static SingletonWithStaticClass INSTANCE = new SingletonWithStaticClass();

}

public static SingletonWithStaticClass getInstance(){
    return SingletonInnerClass.INSTANCE;
}

}

B. Singleton double checked locking

public class SingletonWithDoubleCheck {
private static SingletonWithDoubleCheck INSTANCE = null;

private SingletonWithDoubleCheck(){
    if(INSTANCE != null){
        throw new RuntimeException("Accessing private constructor is prohibited. Use getInstance method instead");
    }
}

public static SingletonWithDoubleCheck getInstance(){
    if(INSTANCE == null){
        synchronized (SingletonWithDoubleCheck.class) {
            if(INSTANCE == null){
                INSTANCE = new SingletonWithDoubleCheck();
            }
        }
    }
    return INSTANCE;
}

}

Which one is better?

I feel we can access the private constructor with Reflection in first implementation where as second implementation is safe (From Reflection attack).

However, I am not going to use any of these in my production code, I will use enum instead. But out of these two, isn't the first implementation is broken when considered Reflection attack?

Please correct me if my understanding is wrong.

Answer 1

Both are overly complicated. The first was the best option before Java 5.0, however the second was never a good option. It didn't work before Java 5.0, it requires a volatile field and after this version you could use enum

I prefer using an enum to define a class with exactly one instance. It is a final class, thread safe, lazy loaded, and has a private constructor.

enum Singleon {
    INSTANCE;
}

The double locking singleton is only useful if you must provide configuration information in it's construction. In this case, I prefer to use Dependency Injection to provide/configure singletons and only have stateless Singletons which don't require configuration.

Answer 2

Neither are good - and attempting to compare them using different scales is counter-productive.

The first is unnecessarily complex and, as you say, is open to reflection hacking, but so is the other one, just slightly less so.

The second uses a synchronized which comes with a cost.