gistfile1.java

import com.fasterxml.jackson.databind.ObjectMapper;
import org.bouncycastle.jce.provider.BouncyCastleProvider;
import org.bouncycastle.util.io.pem.PemObject;
import org.bouncycastle.util.io.pem.PemReader;

import java.io.File;
import java.io.FileReader;
import java.io.Reader;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.security.KeyFactory;
import java.security.PrivateKey;
import java.security.Security;
import java.security.spec.PKCS8EncodedKeySpec;
import java.util.Arrays;

public class Jwt {
    public static class KeyInfo {
        public String id;
        public String service_account_id;
        public String private_key;
    }

    public static void main(String[] args) throws Exception {
        Security.addProvider(new BouncyCastleProvider());

        String content = new String(Files.readAllBytes(Paths.get("/home/codecraftman/IdeaProjects/YandexTranslator/keys.json")));
        JavaJwt.KeyInfo keyInfo = (new ObjectMapper()).readValue(content, JavaJwt.KeyInfo.class);

        // преобразовать строку в файл - start
        Path filePath = Paths.get("/home/codecraftman/IdeaProjects/YandexTranslator/file.txt");
        //Files.writeString(filePath, privateKeyString, StandardCharsets.UTF_8);
        File file = new File(filePath.toString());
        Reader reader0 = new FileReader(file);
        // end

        PemReader reader1 = new PemReader(reader0);
        PemObject privateKeyPem = reader1.readPemObject();
        byte[] keyBytes = privateKeyPem.getContent();
        System.out.println(Arrays.toString(keyBytes));
        PKCS8EncodedKeySpec keySpec = new PKCS8EncodedKeySpec(keyBytes);
        KeyFactory keyFactory = KeyFactory.getInstance("RSA");
        PrivateKey privateKey = keyFactory.generatePrivate(keySpec);

        System.out.println("Private Key: " + privateKey);