multithreading - Java - is there any reason to check if a singleton is null twice?

Question

I have come across some code, where the developer is constantly checking if the singleton is null twice with a nested if - like in the code below:

private static processManager singleton = null;

...

public synchronized static processManager getInsatnce() throws Exception {

    if(singleton == null) {
      if(singleton == null){
             singleton = new processManager();
       }
     }
     return singleton
}

I cannot see any reason why this might be, but there are numerous instances in the code, so thought there might be a reason?

Answer 1

Your code doesn't demonstrate the case properly. This stems from the double-checked idiom, where it does make sense:

// Double-check idiom for lazy initialization of instance fields.
private volatile FieldType field;
FieldType getField() {
    FieldType result = field;
    if (result == null) { // First check (no locking)
        synchronized(this) {
            result = field;
            if (result == null) // Second check (with locking)
                field = result = computeFieldValue();
        }
    }
    return result;
}

Read about it over here.

Be careful to note that this idiom is a good choice only for instance fields. In your question you have a static field, for which case a much simpler idiom is the primary choice: the lazy initialion holder class idiom:

// Lazy initialization holder class idiom for static fields
private static class FieldHolder {
    static final FieldType field = computeFieldValue();
}
static FieldType getField() { return FieldHolder.field; }