private JAXBContext createContext( final Schemas schemas, final String model, final URI publicId )
        throws ModelException
    {
        if ( model != null && publicId != null )
        {
            throw new IllegalArgumentException( "model=" + model + ", publicId=" + publicId.toASCIIString() );
        }

        try
        {
            StringBuilder packageNames = null;

            if ( schemas != null )
            {
                packageNames = new StringBuilder( schemas.getSchema().size() * 25 );

                for ( Schema schema : schemas.getSchema() )
                {
                    if ( schema.getContextId() != null )
                    {
                        packageNames.append( ':' ).append( schema.getContextId() );
                    }
                }
            }

            if ( packageNames == null || packageNames.length() == 0 )
            {
                if ( model != null )
                {
                    throw new ModelException( getMessage( "missingSchemasForModel", model ) );
                }
                if ( publicId != null )
                {
                    throw new ModelException( getMessage( "missingSchemasForPublicId", publicId ) );
                }
            }