JavaMail 1.4.2 Message-ID generator

InternetAddress.java

/* [...]
 *
 * Copyright 1997-2007 Sun Microsystems, Inc. All rights reserved.
 *
 * The contents of this file are subject to the terms of either the GNU
 * General Public License Version 2 only ("GPL") or the Common Development
 * and Distribution License("CDDL") (collectively, the "License").  [...]
 */

package javax.mail.internet;

// [...]

public class InternetAddress extends Address implements Cloneable {

// [...]

    /**
     * Return an InternetAddress object representing the current user.
     * The entire email address may be specified in the "mail.from"
     * property.  If not set, the "mail.user" and "mail.host" properties
     * are tried.  If those are not set, the "user.name" property and
     * <code>InetAddress.getLocalHost</code> method are tried.
     * Security exceptions that may occur while accessing this information
     * are ignored.  If it is not possible to determine an email address,
     * null is returned.
     *
     * @param   session         Session object used for property lookup
     * @return                  current user's email address
     */
    public static InternetAddress getLocalAddress(Session session) {
        String user=null, host=null, address=null;
        try {
            if (session == null) {
                user = System.getProperty("user.name");
                host = InetAddress.getLocalHost().getHostName();
            } else {
                address = session.getProperty("mail.from");
                if (address == null) {
                    user = session.getProperty("mail.user");
                    if (user == null || user.length() == 0)
                        user = session.getProperty("user.name");
                    if (user == null || user.length() == 0)
                        user = System.getProperty("user.name");
                    host = session.getProperty("mail.host");
                    if (host == null || host.length() == 0) {
                        InetAddress me = InetAddress.getLocalHost();
                        if (me != null)
                            host = me.getHostName();
                    }
                }
            }

            if (address == null && user != null && user.length() != 0 &&
                    host != null && host.length() != 0)
                address = user + "@" + host;

            if (address != null)
                return new InternetAddress(address);
        } catch (SecurityException sex) {       // ignore it
        } catch (AddressException ex) {         // ignore it
        } catch (UnknownHostException ex) { }   // ignore it
        return null;
    }

// [...]
}

UniqueValue.java

/* [...]
 *
 * Copyright 1997-2007 Sun Microsystems, Inc. All rights reserved.
 *
 * The contents of this file are subject to the terms of either the GNU
 * General Public License Version 2 only ("GPL") or the Common Development
 * and Distribution License("CDDL") (collectively, the "License").  [...]
 */

package javax.mail.internet;

// [...]

class UniqueValue {

// [...]

    /**
     * Get a unique value for use in a Message-ID.
     *
     * This implementation generates it by concatenating a newly
     * created object's <code>hashCode()</code>, a global ID
     * (incremented on every use), the current
     * time (in milliseconds), the string "JavaMail", and
     * this user's local address generated by 
     * <code>InternetAddress.getLocalAddress()</code>.
     * (The address defaults to "javamailuser@localhost" if
     * <code>getLocalAddress()</code> returns null.)
     *
     * @param ssn Session object used to get the local address
     * @see javax.mail.internet.InternetAddress
     */
    public static String getUniqueMessageIDValue(Session ssn) {
        String suffix = null;

        InternetAddress addr = InternetAddress.getLocalAddress(ssn);
        if (addr != null)
            suffix = addr.getAddress();
        else {
            suffix = "javamailuser@localhost"; // worst-case default
        }

        StringBuffer s = new StringBuffer();

        // Unique string is <hashcode>.<id>.<currentTime>.JavaMail.<suffix>
        s.append(s.hashCode()).append('.').append(getUniqueId()).append('.').
          append(System.currentTimeMillis()).append('.').
          append("JavaMail.").
          append(suffix);
        return s.toString();
    }

    /**
     * Ensure ID is unique by synchronizing access.
     * XXX - Could use AtomicInteger.getAndIncrement() in J2SE 5.0.
     */
    private static synchronized int getUniqueId() {
        return id++;
    }
}

Unique string is

<hashcode>.<counter>.<currentTimeMillis>.JavaMail.<user>@<host>