MapStruct EnumMappingStrategy to deal with enums generated by the OpenAPI Tools

OpenApiToolEnumMappingStrategy.java

package nl.trifork.mapstruct;

import org.mapstruct.ap.spi.DefaultEnumMappingStrategy;

import javax.lang.model.element.AnnotationMirror;
import javax.lang.model.element.AnnotationValue;
import javax.lang.model.element.ExecutableElement;
import javax.lang.model.element.Name;
import javax.lang.model.element.TypeElement;
import java.io.IOException;
import java.util.List;
import java.util.Map;
import java.util.regex.Pattern;

/**
 * Recognizes enums that are generated by the OpenAPI tools and uses
 * the constants' constructor argument rather than their name for mapping
 * if that String argument represents a valid Java identifier.
 * We do this because the tools do things like stripping common prefixes
 * and inserting underscores for digits, which we don't want in the
 * enum constants for our corresponding domain enums.
 * When that constructor orgument cannot be determined, simply falls back
 * to the given {@code enumConstant}.
 */
public class OpenApiToolEnumMappingStrategy extends DefaultEnumMappingStrategy {

    private static final Pattern VALID_IDENTIFIER = Pattern.compile("^[a-zA-Z_]\\w*$");

    private static boolean isGenerated(TypeElement enumType) {
        TypeElement toplevelClass = enumType;
        while (toplevelClass.getEnclosingElement() instanceof TypeElement owningType) {
            toplevelClass = owningType;
        }
        for (AnnotationMirror annotation : toplevelClass.getAnnotationMirrors()) {
            Name annotationName = annotation.getAnnotationType().asElement().getSimpleName();
            if (annotationName.contentEquals("Generated")) {
                for (var entry : annotation.getElementValues().entrySet()) {
                    if (hasOpenApiCodeGenValue(entry)) return true;
                }
            }
        }
        return false;
    }

    private static boolean hasOpenApiCodeGenValue(Map.Entry<? extends ExecutableElement, ? extends AnnotationValue> entry) {
        return entry.getKey().getSimpleName().contentEquals("value") &&
               entry.getValue().getValue() instanceof List values &&
               !values.isEmpty() && values.getFirst() instanceof AnnotationValue annValue &&
               annValue.getValue() instanceof String attrValue &&
               attrValue.startsWith("org.openapitools.codegen");
    }

    @Override
    public String getEnumConstant(TypeElement enumType, String enumConstant) {
        if (isGenerated(enumType)) {
            return constantForGeneratedEnum(enumType, enumConstant);
        }
        return super.getEnumConstant(enumType, enumConstant);
    }

    private String constantForGeneratedEnum(TypeElement enumType, String enumConstant) {
        // we can't obtain the string arg of the enum instances programmatically,
        // so we're going to parse the source file to obtain those as we know the format
        try {
            var content = elementUtils.getFileObjectOf(enumType).getCharContent(true).toString();
            String prefix = enumConstant + "(\"";
            int i = content.indexOf(prefix);
            if (i != -1) {
                int start = i + prefix.length();
                int end = content.indexOf('"', start);
                String openApiEnum = content.substring(start, end);
                if (VALID_IDENTIFIER.matcher(openApiEnum).matches()) return openApiEnum;
            }
        } catch (IOException e) {
            System.out.println("Couldn't read source file for " + enumType + ", mapping '" + enumConstant + "' as-is");
        }
        return enumConstant;
    }
}