package servlet;

import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import helper.MailHelper;


public class MailServlet extends HttpServlet {
private static final long serialVersionUID = 3303982021932755186L;
@Override
protected void doPost(HttpServletRequest request,
        HttpServletResponse response) throws ServletException, IOException {
    processRequest(request, response);
}
public void processRequest(HttpServletRequest request,
        HttpServletResponse response) throws ServletException, IOException {
    String email = request.getParameter("mail");
    String subject = request.getParameter("subject");
    String message = request.getParameter("message");
    if (isValidParameters(email, subject, message)) {
        try {
            MailHelper.sendMail(email, subject, message);
            request.getRequestDispatcher("success.jsp").forward(request,
                    response);
        } catch (Exception ex) {
            request.getRequestDispatcher("internalError.jsp").forward(
                    request, response);
        }
    } else {
        request.getRequestDispatcher("error.jsp")
                .forward(request, response);
    }
}
private boolean isValidParameters(String... parameters) {
    boolean isValid = true;
    for (String param : parameters) {
        if (param == null || param.isEmpty()) {
            isValid = false;
            break;
        }
    }
    return isValid;
}
}

package helper;

import java.util.Properties;
import javax.mail.Message;
import javax.mail.MessagingException;
import javax.mail.PasswordAuthentication;
import javax.mail.Session;
import javax.mail.Transport;
import javax.mail.internet.AddressException;
import javax.mail.internet.InternetAddress;
import javax.mail.internet.MimeMessage;

public class MailHelper {
//input here your mail credentials
static final String username = "myemail@gmail.com";
static final String password = "mypassword";

public static void sendMail(String mail, String subject, String message)
        throws AddressException, MessagingException {
    Message msg = new MimeMessage(getSession());
    msg.setRecipients(Message.RecipientType.TO,             InternetAddress.parse(mail));
    msg.setSubject(subject);
    msg.setText(message);
    Transport.send(msg);
}
private static Session getSession() {
    Session session = Session.getDefaultInstance(getProperties(),
            new javax.mail.Authenticator() {
                protected PasswordAuthentication getPasswordAuthentication()     {
                    return new PasswordAuthentication(username, password);
                }
            });
    return session;
}
private static Properties getProperties() {
    Properties properties = new Properties();
    properties.put("mail.smtp.host", "smtp.gmail.com");
    properties.put("mail.smtp.socketFactory.port", "465");
    properties.put("mail.smtp.socketFactory.class",
            "javax.net.ssl.SSLSocketFactory");
    properties.put("mail.smtp.auth", "true");
    properties.put("mail.smtp.port", "465");
    return properties;
}