here is some example code , i use it to send google mail.
public void smtp(String receiver, String content) throws MessagingException {
if (smtpHost == null)
throw new MessagingException("smtpHost not found");
if (user == null)
throw new MessagingException("user not found");
if (password == null)
throw new MessagingException("password not found");
Properties properties = new Properties();
properties.put("mail.smtp.host", smtpHost); //set smtp server
properties.put("mail.smtp.auth", "true"); //use smtp authen properties.put("mail.transport.protocol", "smtp");
properties.put("mail.smtp.port", "25");
properties.put("mail.smtp.starttls.enable", "true");
Session session = Session.getDefaultInstance(properties,
new Authenticator() {
public PasswordAuthentication getPasswordAuthentication() {
return new PasswordAuthentication(user, password);
}
});
MimeMessage mimeMsg = new MimeMessage(session);
if (sender != null) {
mimeMsg.setFrom(new InternetAddress(sender));
}
if (receiver != null) {
mimeMsg.setRecipients(Message.RecipientType.TO, parse(receiver));
}
if (subject != null) {
mimeMsg.setSubject(subject, "GBK");
}
MimeBodyPart part = new MimeBodyPart();
part.setText(content == null ? "" : content, "GBK");

part.setContent(content.toString(), "text/html;charset=GBK");
Multipart multipart = new MimeMultipart();
multipart.addBodyPart(part);
mimeMsg.setContent(multipart);
mimeMsg.setSentDate(new Date());
Transport.send(mimeMsg);
}

that is simple ,if you use a j2ee container such as tomcat ,
here is a example
this is a ServletContextListener you should config it in web.xml so it will run auto when tomcat starts.

this is the web.xml config
<listener>
<listener-class>com.hecheng.util.ServletListenerTimmer</listener-class>
</listener>

you may need to change some field to fill your condition

public class ServletListenerTimmer implements ServletContextListener {
private java.util.Timer timer = null;
private Log mylogger = LogFactory.getLog(this.getClass());
public void contextInitialized(ServletContextEvent event) {
timer = new java.util.Timer(true);
mylogger.info("timmer is started");
timer.schedule(new SendReport(event.getServletContext()), 0, 60 * 60 * 1000);
mylogger.info("add sendmail task");
}
public void contextDestroyed(ServletContextEvent event) {
timer.cancel();
mylogger.info("timmer destoryed");
}
}

public class SendReport extends TimerTask {
private static final int C_SCHEDULE_HOUR = 22;
private static boolean isRunning = false;
private ServletContext context = null;
public SendReport(ServletContext context) {
this.context = context;
}
public void run() {
Calendar cal = Calendar.getInstance();
if (!isRunning) {
System.out.println("cal.get(Calendar.HOUR_OF_DAY) = " + cal.get(Calendar.HOUR_OF_DAY));
if (C_SCHEDULE_HOUR == cal.get(Calendar.HOUR_OF_DAY)) {
isRunning = true;
doReportTask();// you can write your sendmail task there
isRunning = false;
}
} else {
context.log("the prevenient task is still running!");
}
}
]