Add custom exception assertions

リンク先の AssertJ ガイドにある既存の「2.6.2 Custom Assertions」セクションから始め、リンク先の assertj-examples リポジトリを調べて、現在の assertion の例を確認してください。カスタム例外 assertion が既存のクラス階層にどのように適合するかを確認し、例外メッセージと詳細フィールドを扱う例を両方の場所に記載してください。

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?