Thread safe ?

Question

Hi
This code is currently running on my companys server in a java class implementing the singleton pattern:
public static ShoppingAssistant getInstance() {
    if (instance == null) {
        synchronized (ShoppingAssistant.class) {
            instance = new ShoppingAssistant();
    return instance;
}1 Is there really a need for synchronizing the method?
2 If you choose to synchronize it. Why not just synchronize the whole method?
3 Is this method really thread safe? Is I see it a thread A could be preempted after checking the instance and seing that it is null. Thereafter another thread B could get to run, also see that instance is null and go on and create the instance. Thereafter B modifies some of the instance�s instance variables. After that B is preempted and A gets to run. A, thinking instance is null, goes on and create a new instance of the class and assigns it to instance, thus owerwriting the old instance and creating a faulty state.
By altering the code like this I think that problem is solved although the code will probably run slower. Is that correct?
public static ShoppingAssistant getInstance() {
    synchronized (ShoppingAssistant.class) {
        if (instance == null) {
            instance = new ShoppingAssistant();
}Regards, Mattias

Answer 1

public class Singleton {
private static final instance = newSingleton();
public static Singleton getInstance() {
return instance;
}This seems like it's so obviously the standard
solution, why do
people keep trying to give lazy solutions? Do they
expect it to be
more efficient that this?I can see one possible performance gains with a lazy solution:
If you use something else in that class (besides getInstance()), but don't use getInstance, then you don't take the performance hit of instantiating the thing.
Of course, this is dubious at best because how often do you have other static methods in your singleton class? And not use the instance? And have construction be so expensive that a single unnecessary one has a noticeably detrimental impact on your app's performance?