[Java的写法]之MD5对字符串签名操作

对字符串签名后，1：长字符串变为32位字符：aacfbe08d042fddd8ee778b148efc923
2 ： 只要长字符串内容不变，签名后得到的32位字符不变。适合用来做ID等。
private String genKeyId(String keyStr) {
return Md5Utils.getStringMD5(keyStr);
}

Md5Utils 类如下：
`

import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import org.apache.commons.lang3.StringUtils;

public class Md5Utils {
protected static char[] hexDigits = new char[]{'0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'a', 'b', 'c', 'd', 'e', 'f'};
protected static MessageDigest messagedigest = null;

public Md5Utils() {      }
static {
    try {
        messagedigest = MessageDigest.getInstance("MD5");
    } catch (NoSuchAlgorithmException var1) {
        var1.printStackTrace();
    }

}

public static String getFileMD5String(File file) throws IOException {
    InputStream fis = new FileInputStream(file);
    byte[] buffer = new byte[1024];
    boolean var3 = false;

    int numRead;
    while((numRead = fis.read(buffer)) > 0) {
        messagedigest.update(buffer, 0, numRead);
    }

    fis.close();
    return bufferToHex(messagedigest.digest());
}

public static String getStringMD5(String str) {
    if (StringUtils.isEmpty(str)) {
        return "";
    } else {
        byte[] buffer = str.getBytes();
        messagedigest.update(buffer);
        return bufferToHex(messagedigest.digest());
    }
}

public static String bufferToHex(byte[] bytes) {
    return bufferToHex(bytes, 0, bytes.length);
}

private static String bufferToHex(byte[] bytes, int m, int n) {
    StringBuffer stringbuffer = new StringBuffer(2 * n);
    int k = m + n;

    for(int l = m; l < k; ++l) {
        appendHexPair(bytes[l], stringbuffer);
    }

    return stringbuffer.toString();
}

private static void appendHexPair(byte bt, StringBuffer stringbuffer) {
    char c0 = hexDigits[(bt & 240) >> 4];
    char c1 = hexDigits[bt & 15];
    stringbuffer.append(c0);
    stringbuffer.append(c1);
}

private static final String toHex(byte[] hash) {
    if (hash == null) {
        return null;
    } else {
        StringBuffer buf = new StringBuffer(hash.length * 2);

        for(int i = 0; i < hash.length; ++i) {
            if ((hash[i] & 255) < 16) {
                buf.append("0");
            }

            buf.append(Long.toString((long)(hash[i] & 255), 16));
        }

        return buf.toString();
    }
}

public static String hash(String s) {
    try {
        return new String(toHex(getStringMD5(s).getBytes("UTF-8")).getBytes("UTF-8"), "UTF-8");
    } catch (Exception var2) {
        return s;
    }
}

public static void main(String[] args) {
    System.out.println(getStringMD5("admin123"));
    System.out.println(hash("123456"));
}

}
`