Intercept and assert private exception using junit

There are multiple question similar to mine with a small difference I want to catch private inner exception (if that's accurate definition), and then assert it. I come up with test below. I'm looking for similar method to one is Spock thrown(). Thanks for your help in advance. I do not have much experience with junit, if that's trivial for you just answer or link page that might help me understand.

Thanks for your help and support!

The Exception reactor.core.Exceptions$RetryExhaustedException

public abstract class Exceptions {

	static final class RetryExhaustedException extends IllegalStateException {

		RetryExhaustedException(String message) {
			super(message);
		}

		RetryExhaustedException(String message, Throwable cause) {
			super(message, cause);
		}
	}
}

My test

    @Test
    void exampleTest() {
        // given:
        stubToForce5xxResponse();

        // when:
        var ex = getExceptionThrown(() -> exampleService.exampleMethod());

        // then:
        Assertions.assertThat(ex).isNotNull();
        Assertions.assertThat(ex.getClass().getName()).isEqualTo("reactor.core.Exceptions$RetryExhaustedException");
        Assertions.assertThat(ex.getMessage()).isEqualTo("Retries exhausted: 3/3");
    }

    private Throwable getExceptionThrown(Runnable runnable) {
        try {
            runnable.run();
            return null;
        } catch (Exception ex) {
            return ex;
        }
    }

Given your example code, I assume you use AssertJ:

@Test
void exampleTest() {
	stubToForce5xxResponse();

	assertThatThrownBy(() -> exampleService.exampleMethod())
		.isInstanceOfSatisfying(IllegalStateException.class, ex -> {
			assertThat(ex.getClass().getName()).isEqualTo("reactor.core.Exceptions$RetryExhaustedException");
			assertThat(ex).hasMessage("Retries exhausted: 3/3");
		});
}

If you want to stick to JUnit Jupiter assertions:

@Test
void exampleTest() {
	stubToForce5xxResponse();

	var ex = assertThrows(IllegalStateException.class, () -> exampleService.exampleMethod());

	assertEquals("reactor.core.Exceptions$RetryExhaustedException", ex.getClass().getName());
	assertEquals("Retries exhausted: 3/3", ex.getMessage());
}

(Note that AssertJ is asserThat(actual).isEqualTo(expected), whereas JUnit Jupiter is assertEquals(expected, actual).)