Add custom exception assertions

[aepedraza]

Currently, the documentation shows how to extend an assertion in 2.6.2 Custom Assertions but there is no documentation for custom exception asserts, which seems to use a slightly different mechanism with some other class hierarchy.

It would be nice to include this in the guide and in the examples repository

The goal for this section in the guide will be to present, given a custom exception, a way to assert on the specific fields in that class. For instance, given a custom exception:

package com.example.exception;

public class DetailException extends RuntimeException {

    private String detail;

    public DetailException(String message, String detail) {
        super(message);
        this.detail = detail;
    }

    public String getDetail() {
        return detail;
    }
}

and a business logic throwing this exception

package com.example.thrower;

import com.example.exception.DetailException;

public class DetailExceptionThrower {

    public void throwException() {
        throw new DetailException("message", "a detailed exception explanation");
    }
}

write an extension to write a test like:

package com.example.thrower;

import org.junit.jupiter.api.Test;

import static org.assertj.extension.DetailExceptionAssertions.assertThatDetailException;

class DetailExceptionThrowerCustomAssertionTest {

    private final DetailExceptionThrower underTest = new DetailExceptionThrower();

    @Test
    void test_customAssertion() {
        assertThatDetailException()
                .isThrownBy(underTest::throwException)
                .withMessage("message")
                .withDetail("a detailed exception explanation");
    }
}

is it even possible to write an extension like this?

[aepedraza]

Added a small POC in this repository, in my personal account