Easiest would be to have placeholders for the validation error messages in your JSP.
The JSP /WEB-INF/foo.jsp:
<form action="${pageContext.request.contextPath}/foo" method="post">
<label for="foo">Foo</label>
<input id="foo" name="foo" value="${fn:escapeXml(param.foo)}">
<span class="error">${messages.foo}</span>
<br />
<label for="bar">Bar</label>
<input id="bar" name="bar" value="${fn:escapeXml(param.bar)}">
<span class="error">${messages.bar}</span>
<br />
...
<input type="submit">
<span class="success">${messages.success}</span>
</form>
In the servlet where you submit the form to, you can use a Map<String, String> to get hold of the messages which are to be displayed in JSP.
The Servlet @WebServlet("foo"):
@Override
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
request.getRequestDispatcher("/WEB-INF/foo.jsp").forward(request, response);
}
@Override
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
Map<String, String> messages = new HashMap<String, String>();
request.setAttribute("messages", messages); // Now it's available by ${messages}
String foo = request.getParameter("foo");
if (foo == null || foo.trim().isEmpty()) {
messages.put("foo", "Please enter foo");
} else if (!foo.matches("\\p{Alnum}+")) {
messages.put("foo", "Please enter alphanumeric characters only");
}
String bar = request.getParameter("bar");
if (bar == null || bar.trim().isEmpty()) {
messages.put("bar", "Please enter bar");
} else if (!bar.matches("\\d+")) {
messages.put("bar", "Please enter digits only");
}
// ...
if (messages.isEmpty()) {
messages.put("success", "Form successfully submitted!");
}
request.getRequestDispatcher("/WEB-INF/foo.jsp").forward(request, response);
}
In case you create more JSP pages and servlets doing less or more the same, and start to notice yourself that this is after all a lot of repeated boilerplate code, then consider using a MVC framework instead.