How to post error messages to the HTML from in Struts 2?

I have a register program. When I insert a record on the database, I'll instantiate a class and invoke the method insert(). When I insert a the same record, of course there is a duplicate data error and tons of error messages. I want to capture them with try-catch. I can do that. However, I have no idea how to display the error messages to the JSP.

What I have understood, in an action class, there's validate() method, and the validation.xml is running first. The insert duplicate error happened after those methods are invoked.

import com.opensymphony.xwork2.ActionSupport;
import lotmovement.business.crud.InsertUserProfile;
import lotmovement.business.entity.UserProfile;
import org.apache.commons.lang3.StringUtils;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

public class RegisterAction extends ActionSupport {

    private static String userId;
    private static String password;
    private static String firstName;
    private static String lastName;
    private static int securityLevel;

    @Override
     public String execute() {    
         ApplicationContext context = 
                 new ClassPathXmlApplicationContext("spring.xml");
     
          InsertUserProfile iup = 
                 (InsertUserProfile)context.getBean("insertuserprofile");
         iup.Insert();       
    
      return SUCCESS;
    }

Here is my insert user profile method of

 public void Insert() {          
    ApplicationContext context = new ClassPathXmlApplicationContext("spring.xml");
    UserProfile up = (UserProfile)context.getBean("userprofile");
    RegisterAction ra = (RegisterAction)context.getBean("registeraction");
    EntityStart es = (EntityStart)context.getBean("entitystart");
    
    es.StartDbaseConnection();
            
    up.setUserId(ra.getUserId());
    up.setFirstName(ra.getFirstName());
    up.setLastName(ra.getLastName());
    up.setPassword(ra.getPassword());
    up.setSecurityLevel(ra.getSecurityLevel());
    
    es.StartPopulateTransaction(up);
    
    es.CloseDbaseConnection();        
}

This is my JSP:

<%@ taglib prefix="s" uri="/struts-tags" %>
<html>
<head>
  <meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
  <link rel="stylesheet" type="text/css" href="CSS/register.css">
  <title>Register</title>
  <s:head />
</head>
<body>
<s:form method="POST" action="register" >
  </tr>
  <tr>
    <td colspan="2">
      <s:actionerror/>
      <s:fielderror/>
    </td>
  </tr>
  <s:textfield label="UserID"     key="userId" maxLength="20"/>
  <s:password label="Password"    key="password" maxLength="20"/>
  <s:password label="retype-Password"  key="retypepassword" maxLength="20"/>
  <s:textfield label="Firstname"  key="firstName" maxLength="20"/>
  <s:textfield label="Lastname"   key="lastName" maxLength="20"/>
  <s:textfield label="SecurityLevel" key="securityLevel" maxLength="20"/>
  <s:submit   value="Register"/>
</s:form>
</body>
</html>

Solution

try this:

1.define your own exception class

public class MyException extends Exception {}

2.throw this exception, if there is an exception in your insert method

if(insertError){
    throw new MyExecption("your message");
}

3.in the action class, invoke insert method with try-catch block

try{
    insert();
} catch(MyException me){
    addActionError(me.getMessage());
    return "anything_you_want";
}

4.you can show the message in the jsp using <s:actionerror/>