Convert `.properties` files to UTF-8

Main.java

package com.basilcrow.jenkins;

import java.io.IOException;
import java.nio.ByteBuffer;
import java.nio.charset.CharacterCodingException;
import java.nio.charset.Charset;
import java.nio.charset.CharsetDecoder;
import java.nio.charset.CodingErrorAction;
import java.nio.charset.StandardCharsets;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.util.List;
import java.util.Objects;
import java.util.stream.Collectors;
import java.util.stream.Stream;
import org.apache.commons.text.translate.UnicodeUnescaper;

public class Main {

    public static void main(String[] args) throws Exception {
        List<Path> list;
        try (Stream<Path> stream =
                Files.walk(Paths.get(Objects.requireNonNull(System.getenv("JENKINS_CORE"))))) {
            list =
                    stream.filter(Files::isRegularFile)
                            .filter(f -> f.toString().endsWith(".properties"))
                            .sorted()
                            .collect(Collectors.toUnmodifiableList());
        }

        for (Path file : list) {
            if (!isEncoded(file, StandardCharsets.US_ASCII)) {
                boolean isUtf8 = isEncoded(file, StandardCharsets.UTF_8);
                boolean isIso88591 = isEncoded(file, StandardCharsets.ISO_8859_1);
                if (isUtf8 && isIso88591) {
                    throw new IllegalStateException(file + " is valid UTF-8 and valid ISO-8859-1. To avoid problems when auto-detecting the encoding, use the lowest common denominator of ASCII encoding and express non-ASCII characters with escape sequences using a tool like `native2ascii`.");
                }
            }
            if (!isEncoded(file, StandardCharsets.US_ASCII)
                    && !isEncoded(file, StandardCharsets.ISO_8859_1)) {
                throw new IllegalStateException(file + " is neither ASCII nor ISO-8859-1");
            }
            if (isEncoded(file, StandardCharsets.ISO_8859_1)) {
                String str = Files.readString(file, StandardCharsets.ISO_8859_1);
                Files.writeString(file, str, StandardCharsets.UTF_8);
            }
            if (!isEncoded(file, StandardCharsets.US_ASCII)
                    && !isEncoded(file, StandardCharsets.UTF_8)) {
                throw new IllegalStateException(file + " is neither ASCII nor UTF-8");
            }
            {
                String inStr = Files.readString(file, StandardCharsets.UTF_8);
                UnicodeUnescaper unicodeUnescaper = new UnicodeUnescaper();
                String outStr = unicodeUnescaper.translate(inStr);
                Files.writeString(file, outStr, StandardCharsets.UTF_8);
            }
            if (!isEncoded(file, StandardCharsets.US_ASCII)
                    && !isEncoded(file, StandardCharsets.UTF_8)) {
                throw new IllegalStateException(file + " is neither ASCII nor UTF-8");
            }
        }
    }

    private static boolean isEncoded(Path file, Charset charset) throws IOException {
        byte[] bytes = Files.readAllBytes(file);
        CharsetDecoder decoder = charset.newDecoder();
        decoder.onMalformedInput(CodingErrorAction.REPORT);
        decoder.onUnmappableCharacter(CodingErrorAction.REPORT);
        ByteBuffer buffer = ByteBuffer.wrap(bytes);

        try {
            decoder.decode(buffer);
            return true;
        } catch (CharacterCodingException e) {
            return false;
        }
    }
}