如何用java代码实现 openssl dgst -sha256 -sign

使用Java代码实现相当于使用Java的加密库进行签名，可以使用Java Security API中的Signature类。下面是一个示例代码：

import java.io.*;
import java.security.*;
import java.security.spec.PKCS8EncodedKeySpec;

public class Signer {
    public static void main(String[] args) throws Exception {
        String data = "Hello World";
        String privateKeyPath = "/path/to/private.key";

        byte[] privateKeyBytes = readAllBytes(new FileInputStream(privateKeyPath));
        PKCS8EncodedKeySpec keySpec = new PKCS8EncodedKeySpec(privateKeyBytes);
        KeyFactory keyFactory = KeyFactory.getInstance("RSA");
        PrivateKey privateKey = keyFactory.generatePrivate(keySpec);

        Signature signature = Signature.getInstance("SHA256withRSA");
        signature.initSign(privateKey);
        signature.update(data.getBytes());
        byte[] signatureBytes = signature.sign();

        System.out.println("Signature: " + bytesToHex(signatureBytes));
    }

    private static byte[] readAllBytes(InputStream input) throws IOException {
        ByteArrayOutputStream output = new ByteArrayOutputStream();
        byte[] buffer = new byte[8192];
        int n;
        while ((n = input.read(buffer)) != -1) {
            output.write(buffer, 0, n);
        }
        return output.toByteArray();
    }

    private static final char[] HEX_ARRAY = "0123456789ABCDEF".toCharArray();

    private static String bytesToHex(byte[] bytes) {
        char[] hexChars = new char[bytes.length * 2];
        for (int i = 0; i < bytes.length; i++) {
            int v = bytes[i] & 0xFF;
            hexChars[i * 2] = HEX_ARRAY[v >>> 4];
            hexChars[i * 2 + 1] = HEX_ARRAY[v & 0x0F];
        }
        return new String(hexChars);
    }
}

这个示例代码使用RSA算法进行签名，并使用SHA256算法对数据进行哈希。其中，data是待签名的数据，privateKeyPath是私钥文件的路径。签名结果会以十六进制字符串形式输出。