springframework Order 该怎么理解？

Question

22.2.2. 使用 JavaMailSender 和 MimeMessagePreparator
下面是OrderManager的另一种实现, 使用了MimeMessagePreparator回调接口。 请注意在这个用例中，mailSender属性是JavaMailSender类型， 所以我们可以使用JavaMail的MimeMessage类：import javax.mail.Message;
import javax.mail.MessagingException;
import javax.mail.internet.InternetAddress;
import javax.mail.internet.MimeMessage;import javax.mail.internet.MimeMessage;
import org.springframework.mail.MailException;
import org.springframework.mail.javamail.JavaMailSender;
import org.springframework.mail.javamail.MimeMessagePreparator;public class SimpleOrderManager implements OrderManager {    private JavaMailSender mailSender;
    
    public void setMailSender(JavaMailSender mailSender) {
        this.mailSender = mailSender;
    }    public void placeOrder(final Order order) {        /////我要问的问题
        // Do the business calculations...        // Call the collaborators to persist the order...
        
        MimeMessagePreparator preparator = new MimeMessagePreparator() {
        
            public void prepare(MimeMessage mimeMessage) throws Exception {
        
                mimeMessage.setRecipient(Message.RecipientType.TO, 
                        new InternetAddress(order.getCustomer().getEmailAddress()));
                mimeMessage.setFrom(new InternetAddress("[email protected]"));
                mimeMessage.setText(
                    "Dear " + order.getCustomer().getFirstName() + " "
                        + order.getCustomer().getLastName()
                        + ", thank you for placing order. Your order number is "
                        + order.getOrderNumber());
            }
        };
        try {
            this.mailSender.send(preparator);
        }
        catch (MailException ex) {
            // simply log it and go on...
            System.err.println(ex.getMessage());            
        }
    }
}