^(?![0-9]+$)(?![a-zA-Z]+$)[0-9A-Za-z]{8,16}$

public class Test {
  public static void main(String[] args) throws Exception {
    String regex = "^(?![0-9]+$)(?![a-zA-Z]+$)[0-9A-Za-z]{8,16}$";    
    String value = "aaa"; // 长度不够
    System.out.println(value.matches(regex));
    value = "1111aaaa1111aaaaa"; // 太长
    System.out.println(value.matches(regex));
    value = "111111111"; // 纯数字
    System.out.println(value.matches(regex));
    value = "aaaaaaaaa"; // 纯字母
    System.out.println(value.matches(regex));
    value = "####@@@@#"; // 特殊字符
    System.out.println(value.matches(regex));
    value = "1111aaaa"; // 数字字母组合
    System.out.println(value.matches(regex));
    value = "aaaa1111"; // 数字字母组合
    System.out.println(value.matches(regex));
    value = "aa1111aa";  // 数字字母组合
    System.out.println(value.matches(regex));
    value = "11aaaa11";  // 数字字母组合
    System.out.println(value.matches(regex));
    value = "aa11aa11"; // 数字字母组合
    System.out.println(value.matches(regex));
  }
}