如何用java代码实现 openssl dgst -sha256 -sign pkcs8_private_test.pem -out test.sign test.txt

你可以使用Java的`java.security`包和Bouncy Castle库来实现这个功能。下面是一个示例代码：

import java.io.*;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.security.*;
import java.security.spec.PKCS8EncodedKeySpec;
import org.bouncycastle.jce.provider.BouncyCastleProvider;

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

        // 读取私钥
        byte[] privateKeyBytes = Files.readAllBytes(Paths.get("pkcs8_private_test.pem"));
        PKCS8EncodedKeySpec privateKeySpec = new PKCS8EncodedKeySpec(privateKeyBytes);
        KeyFactory keyFactory = KeyFactory.getInstance("RSA", "BC");
        PrivateKey privateKey = keyFactory.generatePrivate(privateKeySpec);

        // 计算摘要
        MessageDigest digest = MessageDigest.getInstance("SHA-256", "BC");
        byte[] data = Files.readAllBytes(Paths.get("test.txt"));
        byte[] hash = digest.digest(data);

        // 签名
        Signature signer = Signature.getInstance("SHA256withRSA", "BC");
        signer.initSign(privateKey);
        signer.update(hash);
        byte[] signature = signer.sign();

        // 写入签名文件
        OutputStream out = new FileOutputStream("test.sign");
        out.write(signature);
        out.close();
    }
}

这个代码读取了PEM格式的私钥文件，并使用Bouncy Castle实现了SHA-256摘要和RSA签名。最后将签名写入到文件中。