Question

Storing the mail session (javax.mail.Session) in a singleton is a good practice? Me and my team decided to keep a single mail session in a static variable inside a Singleton class.

So, in the private constructor we do this:

try {
        Properties props = new Properties();

        props.put("mail.transport.protocol", config.getMailTransportProtocol());
        props.put("mail.smtp.starttls.enable", config.getMailStarttlsEnable());
        props.put("mail.smtp.host", config.getMailHost());
        props.put("mail.smtp.auth", config.getMailAuth());
        props.put("mail.smtp.user", config.getMailFrom());
        props.put("mail.debug", config.getMailDebug());
        props.put("mail.smtp.port", config.getMailPort());
        props.put("mail.smtp.socketFactory.port", config.getMailPort());
        props.put("mail.smtp.socketFactory.class", config.getMailSocketFactoryClass());
        props.put("mail.smtp.socketFactory.fallback", config.getMailSocketFactoryFallback());

        props.put("mail.pop3.host", config.getMailPop3Host());
        props.put("mail.store.protocol", config.getMailStoreProtocol());

        SimpleAuth auth = new SimpleAuth(config.getMailUser(), config.getMailPass());

        MailSession.session = Session.getDefaultInstance(props, auth);
        session.setDebug(config.getMailDebug());
    } catch (Throwable ex) {
        System.err.println("Initial MailSession creation failed." + ex);
        throw new ExceptionInInitializerError(ex);
    }

But I’m worried if is best keeping it like this or opening and closing a session for each e-mail.

Answer 1

First, you’ll want to read this JavaMail FAQ entry on common mistakes.

And if you read the javadocs for the Session.getDefaultInstance method, you’ll see that JavaMail already maintains a singleton Session for you. If you never change the configuration of the Session, this might work for you, although generally I recommend against it.