MessageQueue.java

// copyright (c) 1997,1998 stephen f. white
//
// This program is free software; you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation; either version 2, or (at your option)
// any later version.
//
// This program is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.
//
// You should have received a copy of the GNU General Public License
// along with this program; see the file COPYING.  If not, write to
// the Free Software Foundation, 675 Mass Ave, Cambridge, MA 02139, USA.
package sai.eai;

class MessageQueue {
    private Message     head;
    private Message     tail;

    public              MessageQueue() {
        head = tail = null;
    }

    public synchronized void            enqueue(Message msg) {
        msg.next = head;
        msg.prev = null;
        if (head == null) {
            tail = msg;
        } else {
            head.prev = msg;
        }
        head = msg;
    }

    public synchronized void            enqueueUnique(Message msg) {
        for (Message m = head; m != null; m = m.next) {
            if (m.id == msg.id && m.field == msg.field) {
                m.value = msg.value;
                return;
            }
        }
        enqueue(msg);
    }

    public synchronized Message         dequeue() {
        if (tail == null) return null;
        Message msg = tail;
        tail = tail.prev;
        if (tail == null) {
            head = null;
        } else {
            tail.next = null;
        }
        msg.prev = msg.next = null;
        return msg;
    }

    public boolean isEmpty() {
        return head == null;
    }
}