Failing Hamcrest Matcher does not print expected value

[bastian-schlueter]

Currently using json-unit-core:3.4.1, facing incomplete output on test failure when using a Hamcrest Matcher as a custom Matcher.

Problem

Given the following assertion:

net.javacrumbs.jsonunit.assertj.JsonAssertions.assertThatJson(
        """
    {
      "someText": "abc123"
    }
    """)
    .withMatcher("exampleMatcher", org.hamcrest.Matchers.equalTo("def456"))
    .isEqualTo(
        """
    {
      "someText": "${json-unit.matches:exampleMatcher}"
    }
    """);

I expected the resulting message to contain the expected value def456.
Instead the output looks like this:

JSON documents are different:
Matcher "exampleMatcher" does not match value "abc123" in node "someText". was "abc123"

Notice the . was "abc123" at the end, which just repeats the actual value again.
This can be highly confusing as there is no information in the message, what the expected value was.

Cause

This happens due to a missing call between these two lines:

JsonUnit/json-unit-core/src/main/java/net/javacrumbs/jsonunit/core/internal/HamcrestHandler.java

Lines 36 to 37 in 16eb5c2

	Description description = new StringDescription();
	matcher.describeMismatch(value, description);

Didn't look too deep into the Matcher interface, but looking at existing Hamcrest Matchers it seems like matcher.describeMismatch(value, description) only populates the description with the actual value.

Possible solution

Calling matcher.describeTo(description) between lines 36 and 37, mentioned above, should allow the Hamcrest Matcher to populate the description with the expected value, although I must admit, the formatting looks a little funky due to a missing space character:

JSON documents are different:
Matcher "exampleMatcher" does not match value "abc123" in node "someText". "def456"was "abc123"

[lukas-krecan]

Hi, thanks for report. You are right, this should fix it