`SingletonSupplier` is not singleton if `null` is returned on first supplier call

[mhalbritter]

When null is returned from the supplier passed to SingletonSupplier and some non-null value on subsequent calls, it breaks the singleton nature.

It returns on first call of singletonSupplier.get() null and then afterwards Hello.

private static int call = 0;

public static void main(String[] args) {
    SingletonSupplier<String> singletonSupplier = SingletonSupplier.ofNullable(() -> supply());
    System.out.println(singletonSupplier.get());
    System.out.println(singletonSupplier.get());
    System.out.println(singletonSupplier.get());
    System.out.println(singletonSupplier.get());
}

private static String supply() {
    call++;
    if (call % 2 == 1) {
        return null;
    }
    return "Hello";
}

prints

null
Hello
Hello
Hello

In Framework 7 it has this signature:

public static <T> @Nullable SingletonSupplier<T> ofNullable(@Nullable Supplier<@Nullable T> supplier) {

suggesting that using a Supplier<@Nullable String> is fine.

[sbrannen]

Indeed, the following line stores null in the singleInstance field in such use cases.

spring-framework/spring-core/src/main/java/org/springframework/util/function/SingletonSupplier.java

Line 109 in 8f41079

this.singletonInstance = instance;

And for a subsequent get(), a null value indicates that another attempt should be made to retrieve the value from the Supplier.

In other words, SingletonSupplier does not track that null was actually returned from instanceSupplier or defaultSupplier.

Thanks for raising the issue and providing the reproducer.

We'll look into it.