ErrorCollector for JUnit 5

ErrorCollector.java

import org.hamcrest.Matcher;
import org.hamcrest.MatcherAssert;
import org.junit.jupiter.api.extension.AfterTestExecutionCallback;
import org.junit.jupiter.api.extension.ExtensionContext;
import org.opentest4j.MultipleFailuresError;

import java.lang.reflect.Field;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;

public class ErrorCollector {

    public static class Extension implements AfterTestExecutionCallback {

        @Override
        public void afterTestExecution(ExtensionContext context) throws Exception {
            if (context.getExecutionException().isPresent()) return;

            ErrorCollector errorCollector = getOnlyField(context.getRequiredTestInstance(), ErrorCollector.class);
            if (!errorCollector.failures.isEmpty()) {
                throw new MultipleFailuresError(null, errorCollector.failures);
            }
        }

        private <T> T getOnlyField(Object object, Class<T> fieldType) throws IllegalAccessException {
            Class<?> cl = object.getClass();
            List<Field> fields = Arrays.stream(cl.getFields())
                                       .filter(f -> f.getType().equals(fieldType))
                                       .collect(Collectors.toList());
            if (fields.size() != 1) {
                throw new AssertionError("there must be exactly one field of type " + fieldType.getName() + ": " + fields);
            }

            Field field = fields.get(0);
            Object value = field.get(object);
            return fieldType.cast(value);
        }

    }

    private final List<Throwable> failures = new ArrayList<>();

    public <T> void checkThat(String reason, T value, Matcher<T> matcher) {
        try {
            MatcherAssert.assertThat(reason, value, matcher);
        } catch (Throwable e) {
            failures.add(e);
        }
    }

}