import com.alibaba.fastjson.JSONObject;
import com.alibaba.fastjson.TypeReference;
import lombok.extern.log4j.Log4j2;
import org.apache.commons.lang3.ArrayUtils;
import sun.misc.BASE64Decoder;

import javax.crypto.BadPaddingException;
import javax.crypto.Cipher;
import javax.crypto.IllegalBlockSizeException;
import javax.crypto.NoSuchPaddingException;
import java.security.InvalidKeyException;
import java.security.KeyFactory;
import java.security.NoSuchAlgorithmException;
import java.security.PublicKey;
import java.security.spec.X509EncodedKeySpec;
import java.util.TreeMap;


/**
 * 公钥解签
 */
@Log4j2
public class RUtil {
    /**     
     *  验证sign
     *  plainText 返回数据报文
     *  publicKey 公钥
     * @return
     */
    public static boolean checkSign(String plainText,String publicKey) throws Exception {
        TreeMap<String ,Object> map = JSONObject.parseObject(plainText, new TypeReference<TreeMap<String, Object>>(){});
        System.out.println(JSONObject.toJSONString(map));
        StringBuilder sb = new StringBuilder();
        String sign = "";
        for (String key : map.keySet()){
            if("sign".equals(key)){
                sign =(String) map.get(key);
                continue;
            }
            if (map.get(key) != null) {
                sb.append(key).append("=").append(map.get(key)).append("&");
            }  
        }
        String res = sb.substring(0,sb.lastIndexOf("&"));
        log.info("拼接字符串："+res);
        String sha256 = SHA256.getSHA256(res);
        log.info("拼接串的SHA256："+sha256);
        String decodeSha256 = decrypt(publicKey,sign);
        log.info("sign解密的SHA256："+decodeSha256);

        return sha256.equals(decodeSha256);
    }

    /**
     * 公钥解密过程
     *
     * @param key  公钥
     * @param data 密文数据
     * @return 明文
     * @throws Exception 解密过程中的异常信息
     */
    public static String decrypt(String key, String data) throws Exception {
        if (key == null) {
            throw new Exception("解密公钥为空, 请设置");
        }
        PublicKey publicKey = getPublicKey(key);
        byte[] cipherData = Base64Util.decode(data);
        Cipher cipher = null;
        try {
            // 使用默认RSA
            cipher = Cipher.getInstance("RSA");
            cipher.init(Cipher.DECRYPT_MODE, publicKey);

            byte[] buf = (byte[]) null;
            for (int i = 0; i < cipherData.length; i += 256) {
                byte[] doFinal = cipher.doFinal(ArrayUtils.subarray(cipherData, i, i + 256));
                if(buf == null) {
                    buf = doFinal;
                } else {
                    buf = addBytes(buf,doFinal);
                }
            }
            return new String(buf);
        } catch (NoSuchAlgorithmException e) {
            throw new Exception("无此解密算法");
        } catch (NoSuchPaddingException e) {
            e.printStackTrace();
            return null;
        } catch (InvalidKeyException e) {
            throw new Exception("解密公钥非法,请检查");
        } catch (IllegalBlockSizeException e) {
            throw new Exception("密文长度非法");
        } catch (BadPaddingException e) {
            throw new Exception("密文数据已损坏");
        }
    }

    /**
     * 数组拼接
     * @param data1 数组1
     * @param data2 数组2
     * @return
     */
    public static byte[] addBytes(byte[] data1, byte[] data2) {
        byte[] data3 = new byte[data1.length + data2.length];
        System.arraycopy(data1, 0, data3, 0, data1.length);
        System.arraycopy(data2, 0, data3, data1.length, data2.length);
        return data3;
    }

    /**
     * 得到公钥
     *
     * @param key 密钥字符串（经过base64编码）
     * @throws Exception
     */
    public static PublicKey getPublicKey(String key) throws Exception {
        byte[] keyBytes;
        keyBytes = new BASE64Decoder().decodeBuffer(key);
        X509EncodedKeySpec keySpec = new X509EncodedKeySpec(keyBytes);
        KeyFactory keyFactory = KeyFactory.getInstance("RSA");
        PublicKey publicKey = keyFactory.generatePublic(keySpec);
        return publicKey;
    }
}
其中用到的工具类Base64Util：

public class Base64Util {
    private static final char[] base64EncodeChars = new char[] { 'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z', 'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z', '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', '+', '/' };
    private static byte[] base64DecodeChars = new byte[] { -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, 62, -1, -1, -1, 63, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, -1, -1, -1, -1, -1, -1, -1, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, -1, -1, -1, -1, -1, -1, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, -1, -1, -1, -1, -1 };
    private Base64Util() {}

    /**
     * 将字节数组编码为字符串
     *
     * @param data
     */
    public static String encode(byte[] data) {
        StringBuffer sb = new StringBuffer();
        int len = data.length;
        int i = 0;
        int b1, b2, b3;
        while (i < len) {
            b1 = data[i++] & 0xff;
            if (i == len) {
                sb.append(base64EncodeChars[b1 >>> 2]);
                sb.append(base64EncodeChars[(b1 & 0x3) << 4]);
                sb.append("==");
                break;
            }
            b2 = data[i++] & 0xff;
            if (i == len) {
                sb.append(base64EncodeChars[b1 >>> 2]);
                sb.append(base64EncodeChars[((b1 & 0x03) << 4) | ((b2 & 0xf0) >>> 4)]);
                sb.append(base64EncodeChars[(b2 & 0x0f) << 2]);
                sb.append("=");
                break;
            }
            b3 = data[i++] & 0xff;
            sb.append(base64EncodeChars[b1 >>> 2]);
            sb.append(base64EncodeChars[((b1 & 0x03) << 4) | ((b2 & 0xf0) >>> 4)]);
            sb.append(base64EncodeChars[((b2 & 0x0f) << 2) | ((b3 & 0xc0) >>> 6)]);
            sb.append(base64EncodeChars[b3 & 0x3f]);
        }
        return sb.toString();
    }


    /**
     * 将base64字符串解码为字节数组
     *
     * @param str
     */
    public static byte[] decode(String str) throws Exception{
        byte[] data = str.getBytes("GBK");
        int len = data.length;
        ByteArrayOutputStream buf = new ByteArrayOutputStream(len);
        int i = 0;
        int b1, b2, b3, b4;
        while (i < len) {


            /* b1 */
            do {
                b1 = base64DecodeChars[data[i++]];
            } while (i < len && b1 == -1);
            if (b1 == -1) {
                break;
            }


            /* b2 */
            do {
                b2 = base64DecodeChars[data[i++]];
            } while (i < len && b2 == -1);
            if (b2 == -1) {
                break;
            }
            buf.write((int) ((b1 << 2) | ((b2 & 0x30) >>> 4)));


            /* b3 */
            do {
                b3 = data[i++];
                if (b3 == 61) {
                    return buf.toByteArray();
                }
                b3 = base64DecodeChars[b3];
            } while (i < len && b3 == -1);
            if (b3 == -1) {
                break;
            }
            buf.write((int) (((b2 & 0x0f) << 4) | ((b3 & 0x3c) >>> 2)));


            /* b4 */
            do {
                b4 = data[i++];
                if (b4 == 61) {
                    return buf.toByteArray();
                }
                b4 = base64DecodeChars[b4];
            } while (i < len && b4 == -1);
            if (b4 == -1) {
                break;
            }
            buf.write((int) (((b3 & 0x03) << 6) | b4));
        }
        return buf.toByteArray();
    }
}
SHA256.java

import java.io.UnsupportedEncodingException;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;

public class SHA256 {
	
	/**
	 * 利用java原生的类实现SHA256加密
	 * 
	 * @param str 加密后的报文
	 * @return
	 */
	public static String getSHA256(String str) {
		MessageDigest messageDigest;
		String encodestr = "";
		try {
			messageDigest = MessageDigest.getInstance("SHA-256");
			messageDigest.update(str.getBytes("UTF-8"));
			encodestr = byte2Hex(messageDigest.digest());
		} catch (NoSuchAlgorithmException e) {
			e.printStackTrace();
		} catch (UnsupportedEncodingException e) {
			e.printStackTrace();
		}
		return encodestr;
	}
 
	/**
	 * 将byte转为16进制
	 * 
	 * @param bytes
	 * @return
	 */
	private static String byte2Hex(byte[] bytes) {
		StringBuffer stringBuffer = new StringBuffer();
		String temp = null;
		for (int i = 0; i < bytes.length; i++) {
			temp = Integer.toHexString(bytes[i] & 0xFF);
			if (temp.length() == 1) {
				stringBuffer.append("0");
			}
			stringBuffer.append(temp);
		}
		return stringBuffer.toString();
	}
}