关于自定义通告ID的设置

抽象类Advertisemnt中有一个抽象方法

    /**
     *  Returns an ID which identifies this {@code Advertisement} as uniquely as 
     *  possible. This ID is typically used as the primary key for indexing of
     *  the Advertisement within databases. 
     *  <p/>
     *  Each advertisement sub-class must choose an appropriate implementation
     *  which returns canonical and relatively unique ID values for it's
     *  instances. Since this ID is commonly used for indexing, the IDs returned
     *  must be as unique as possible to avoid collisions. The value for the ID 
     *  returned can either be:
     *  <p/>
     *  <ul>
     *      <li>An ID which is already part of the advertisement definition
     *      and is relatively unique between advertisements instances. For
     *      example, the Peer Advertisement returns the Peer ID.</li>
     *
     *      <li>A static CodatID which is generated via some canonical process
     *      which will produce the same value each time and different values for
     *      different advertisements of the same type.</li>
     *
     *      <li>ID.nullID for advertisement types which are not readily indexed.
     *      </li>
     *  </ul>
     *  <p/>
     *  For Advertisement types which normally return non-ID.nullID values
     *  no ID should be returned when asked to generate an ID while the
     *  Advertisement is an inconsistent state (example: uninitialized index
     *  fields). Instead {@link java.lang.IllegalStateException} should be
     *  thrown.
     *
     *  @return An ID that relatively uniquely identifies this advertisement 
     *  or {@code ID.nullID} if this advertisement is of a type that is not 
     *  normally indexed.
     */
    public abstract ID getID();

 英语不好，对上面的注释不是很理解：对于不需要索引的通告类型，可以将ID设置为ID.nullID，不知道是不是这个意思！

对索引的理解是凡是由getIndexFields返回的字段都是需要索引的

    /**
     * Returns the element names on which this advertisement should be indexed.
     *
     * @return The element names on which this advertisement should be indexed.
     */
    public abstract String[] getIndexFields();

 总结起来就是说，只要你的ID字段名不需要索引（没有通过getIndexFields返回，当然此时也不能通过ID来发现通告了），就可以设置为ID.nullID。

另外注意，自定义通告必须使用以下方法进行注册才能使用：

    /**
     *  Register an instantiator for and advertisement type to allow instances
     *  of that type to be created.
     *
     *  @param rootType  the identifying value for this advertisement instance
     *   type.
     *  @param instantiator the instantiator to use in constructing objects
     *   of this rootType.
     *  @return boolean  true if the rootType type is registered. If there is
     *   already a constructor for this type then false will be returned.
     */
    public static boolean registerAdvertisementInstance(String rootType, Instantiator instantiator) {
        boolean result = factory.registerAssoc(rootType, instantiator);

        return result;
    }