废话不多说，直接上代码

public static String camelToUnderline(String str) {
    if (str == null || "".equals(str.trim())) {
        return "";
    }
    int len = str.length();
    StringBuilder sb = new StringBuilder(len);
    for (int i = 0; i < len; i++) {
        char c = str.charAt(i);
        if (Character.isUpperCase(c)) {
            sb.append("_").append(Character.toLowerCase(c));
        } else {
            sb.append(c);
        }
    }
    return sb.toString();
}