InvalidKeySpecException:如何从.der文件中提取私钥？

import java.io.FileInputStream;
import java.security.KeyFactory;
import java.security.PrivateKey;
import java.security.spec.PKCS8EncodedKeySpec;

public class PrivateKeyExtractor {
    public static void main(String[] args) {
        try {
            // 1. 读取.der文件
            FileInputStream fis = new FileInputStream("private_key.der");
            byte[] privateKeyBytes = new byte[fis.available()];
            fis.read(privateKeyBytes);
            fis.close();

            // 2. 解析.der文件
            KeyFactory keyFactory = KeyFactory.getInstance("RSA");
            PKCS8EncodedKeySpec privateKeySpec = new PKCS8EncodedKeySpec(privateKeyBytes);

            // 3. 构建密钥规范
            PrivateKey privateKey = keyFactory.generatePrivate(privateKeySpec);

            // 提取私钥成功
            System.out.println("提取私钥成功：" + privateKey);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}