NPE exception when forType used with Jupiter

[luvarqpp]

Testing Problem

When using Arbitraries.forType() method, there is NPE exception, when it is used outside of jqwik test runner (i.e. in junit 5 default, jupiter, runner).

Stacktrace:

java.lang.NullPointerException
	at net.jqwik.engine.facades.ArbitrariesFacadeImpl.allDefaultsFor(ArbitrariesFacadeImpl.java:196)
	at net.jqwik.engine.facades.ArbitrariesFacadeImpl.lambda$defaultFor$3(ArbitrariesFacadeImpl.java:164)
	at net.jqwik.engine.properties.arbitraries.LazyArbitrary.generator(LazyArbitrary.java:18)
	at net.jqwik.api.Combinators$ListCombinator$1.lambda$generator$0(Combinators.java:697)
	at java.base/java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:195)
	at java.base/java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1654)
	at java.base/java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:484)
	at java.base/java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:474)
	at java.base/java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:913)
	at java.base/java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
	at java.base/java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:578)
	at net.jqwik.api.Combinators$ListCombinator$1.generator(Combinators.java:698)
	at net.jqwik.engine.properties.arbitraries.IgnoreGenerationErrorArbitrary.generator(IgnoreGenerationErrorArbitrary.java:18)
	at net.jqwik.engine.properties.shrinking.FlatMappedShrinkable.lambda$new$0(FlatMappedShrinkable.java:17)
	at net.jqwik.engine.properties.shrinking.FlatMappedShrinkable.generateShrinkable(FlatMappedShrinkable.java:29)
	at net.jqwik.engine.properties.shrinking.FlatMappedShrinkable.<init>(FlatMappedShrinkable.java:24)
	at net.jqwik.engine.properties.shrinking.FlatMappedShrinkable.<init>(FlatMappedShrinkable.java:17)
	at net.jqwik.engine.facades.RandomGeneratorFacadeImpl.flatMap(RandomGeneratorFacadeImpl.java:21)
	at net.jqwik.api.RandomGenerator.lambda$flatMap$2(RandomGenerator.java:58)

Test code throwing exception is something like this:

    public static UserForTests aUserFromForType() {
        return Arbitraries.forType(UserForTests.class).usePublicConstructors()
                .generator(0).next(rnd).value();
    }

Problem is test engine. I have also one property based test, which is passing ok. Here it is:

    @Property
    @Report(Reporting.GENERATED)
    void justTestOfForTypeThing(@ForAll @UseType UserForTests aPerson) {
        System.out.println(aPerson);
    }
    @Provide
    Arbitrary<UserForTests> people() {
        return Arbitraries.forType(UserForTests.class).usePublicConstructors();
    }

Suggested Solution

I would check, if forType is called in right context (i.e. correct test engine executor) and throw some more informative exception.

Perhaps in DefaultTypeArbitrary constructor would be a nice place. Other imho valid place could be in DomainContextFacadeImpl. Change its static property called currentContext into "static getter" and check for valid value in thread local there.

Discussion

I am not aware if jqwik internal code hierarchy, so not much here from me. I would just suggest to document for jqwik engine in forType javadoc. (I can try to make pull request for this if you like).

[jlink]

There could be a (somewhat) useful behaviour outside a property: Just using the global domain context, i.e. use all globally registered arbitrary providers. I might implement that...

[jlink]

Fixed in 1.2.2-SNAPSHOT

[jlink]

It now works outside jqwik properties using default global domain, i.e. all globally registered arbtirary providers are considered.