Servlet Parameter Validator Utility Class

ExampleUsage.java

public class ExampleUsage extends HttpServlet {

  private static final long serialVersionUID = 1L;
  
  private static final ParameterValidator validator = new ParameterValidator()
		  .regex("userName", "^[a-z0-9.]{1,20}$")
		  .regex("fullName", "^[A-Za-z0-9.]{1,40}$", false)
		  .regex("street", "^[A-Za-z0-9 ]{1,30}$")
		  .regex("city", "^[A-Za-z ]{1,30}$")
		  .regex("zip", "^[A-Za-z0-9 ]{1,20}$")
		  .regex("ccn", "^[0-9]{1,30}$");
		  
  public void doPost(HttpServletRequest request, HttpServletResponse response)
      throws IOException, ServletException {
    response.setContentType("text/html");

    // Validate input
    if(!validator.isValid(request, response)) return;

ParameterValidator.java

import java.io.IOException;
import java.util.LinkedHashMap;
import java.util.Map;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import lombok.NoArgsConstructor;
import lombok.Value;

@NoArgsConstructor
public class ParameterValidator {

	@Value(staticConstructor="of")
	private static class RegexTest {
		String param;
		String regex;
		boolean required;
		public boolean isValid(String value) {
			if (value == null) {
				return this.required ? false : true;
			} else {
				return value.matches(regex);
			}
		}
	}

	private final Map<String, RegexTest> template = new LinkedHashMap<String, RegexTest>();

	public ParameterValidator regex(String param, String regex) {
		return this.regex(param, regex, true);
	}

	public ParameterValidator regex(String param, String regex, boolean required) {
		template.put(param, RegexTest.of(param, regex, required));
		return this;
	}

	public boolean isValid(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		  for(String key: template.keySet()) {
			  RegexTest test = template.get(key);
			  if(!test.isValid(request.getParameter(test.getParam()))) {
				  request.setAttribute("message", "Invalid value for '"+test.getParam()+"'");
				  request.getRequestDispatcher("ErrorHandler").forward(request, response);
				  return false;
			  }
		  }
		  return true;
	}
}