<html:errors> struts tutorial or example

Here's one: //struts.apache.org/1.3.5/struts-taglib/apidocs/org/apache/struts/taglib/html/package-summary.html#package_description

Here I'm assuming Struts 1. I don't know if it has changed for Struts 2.

You can put an errors.header and errors.footer into your message resources file:

errors.header=<h3><font color="red">Errors:</font></h3><ul>
errors.footer=</ul>

The header and footer are displayed only if the ActionErrors object has any errors in it.

In your Action class, do this:

ActionErrors errors = new ActionErrors();
if (badInput) {
  errors.add(ActionErrors.GLOBAL_ERROR,
    new ActionError("error.bad.input", badString);    // key in messages resource file
                                    // badString will replace {0} in message
}

Then before returning:

saveErrors(request, errors);

In your messages resource file:

error.bad.input=<li>Bad input:  '{0}' is invalid.</li>

Now when the <html:errors/> tag is processed, it will turn into:

<h3><font color="red">Errors:</font></h3><ul>
<li>Bad input: 'xxyyzzz' is invalid.<li>
</ul>

Here's a quick summary. You have an ActionForm class, say MyForm:

<form-bean name="myForm" type="myapp.forms.MyForm"/>

You have an Action class, say MyAction:

<action path="/insert" type="myapp.actions.MyAction" name="myForm"
   input="/insert.jsp" validate="true" />
  <forward name="success" path="/insertDone.jsp"/>
</action>

"name" in the action refers to "name" in the form-bean. Because you have validate="true" your ActionForm class MyForm must define validate() method which will automatically be called:

public ActionErrors validate(ActionMapping mapping, HttpServletRequest request) {
  ActionErrors errors = new ActionErrors();
  if ((username==null) || (username.length() < 1)) 
      errors.add("username", new ActionError("error.username.required"));
  return errors;
}

If it returns an empty ActionErrors object, Struts goes on to call your MyAction.execute(). Otherwise, Struts displays /insert.jsp (because that's the input= parm you gave) and expands the html.errors tag to display your errors from ActionErrors.