jooby-test: NullPointerException when using MockRouter::tryError with custom error handler

[zoltan-kollar]

Please see the code snippet below illustrating an issue I encountered when testing an ErrorHandler with MockRouter's tryError method.

The snippet does the following:

1. Configures a Jooby application with a custom ErrorHandler that catches CustomException.
   • The handler creates a HttpProblem and delegates to the underlying ErrorHandler, following the Error Handler - Problem Details - Custom Problems section of the documentation.
2. Asserts the custom ErrorHandler by creating a MockRouter and invoking its tryError method.

Expectation

• The assertion in ApplicationTest::test runs and passes.

Observed behaviour

• The MockRouter::tryError call in ApplicationTest::test throws a NullPointerException. See the exception details below.

Example code snippet

package joobysandbox;

import io.jooby.Jooby;
import io.jooby.StatusCode;
import io.jooby.problem.HttpProblem;
import io.jooby.test.MockContext;
import io.jooby.test.MockRouter;
import org.junit.jupiter.api.Test;

import static joobysandbox.Application.CustomException;
import static org.junit.jupiter.api.Assertions.assertEquals;

class ApplicationTest {

    private final MockRouter router = new MockRouter(new Application()).setFullExecution(true);

    @Test
    void test() {
        var ctx = new MockContext();
        router.tryError(new CustomException(), ctx); // throws java.lang.NullPointerException
        assertEquals(StatusCode.BAD_REQUEST, ctx.getResponseCode());
    }
}

class Application extends Jooby {
    {
        get("/throw", _ -> {
            throw new CustomException();
        });

        error(CustomException.class, (ctx, _, statusCode) -> {
            var problem = HttpProblem.badRequest("A Client Error — Obviously");
            ctx.getRouter().getErrorHandler().apply(ctx, problem, statusCode);
        });
    }

    public static void main(String[] args) {
        runApp(args, Application::new);
    }

    static class CustomException extends RuntimeException {}
}

Expectation details

java.lang.NullPointerException: Cannot invoke "io.jooby.Router.getErrorHandler()" because the return value of "io.jooby.Context.getRouter()" is null

	at joobysandbox.Application.lambda$new$0(Application.java:34)
	at io.jooby.Router.lambda$error$0(Router.java:747)
	at io.jooby.test.MockRouter.tryError(MockRouter.java:476)
	at joobysandbox.ApplicationTest.test(Application.java:20)

Environment

jooby and jooby-test v4.0.7
junit-jupiter v6.0.0
JDK 24

[kliushnichenko]

Here is the test proving that the approach generally works

https://github.com/jooby-project/jooby/blob/main/tests/src/test/java/io/jooby/problem/ProblemDetailsHandlerTest.java#L378
https://github.com/jooby-project/jooby/blob/main/tests/src/test/java/io/jooby/problem/data/App.java#L177

The issue here is that the MockRouter doesn't set the router to the context in tryError, hence, the NPE arises. We can fix it easily.
But it still won't work with ProblemDetails as expected, cause MockRouter doesn't run the initial start logic of RouterImpl where ProblemDetailsHandler is instantiated.

The WA that can be applied is manually adding error(new ProblemDetailsHandler()); to your Application. Otherwise, MockRouter probably requires some more re-work, not sure it was intended to support this, maybe this is the case where it's better to switch to @JoobyTest. @jknack, what is your vision?

[jknack]

Thank you, @kliushnichenko .

I didn't look yet BUT you are right about the start setup and logic. I would like to fix this NPE (not sure how hard is it) but yea we can't call start. Would you like any of you give a try?

[kliushnichenko]

let me check it