src/main/java/org/codehaus/plexus/archiver/war/WarArchiver.java
protected void initZipOutputStream( ZipArchiveOutputStream zOut )
        throws ArchiverException, IOException
    {
        // If no webxml file is specified, it's an error.
        if ( expectWebXml && deploymentDescriptor == null && !isInUpdateMode() )
        {
            throw new ArchiverException( "webxml attribute is required (or pre-existing WEB-INF/web.xml if executing in update mode)" );
        }
        super.initZipOutputStream( zOut );
    }
protected void initZipOutputStream( ZipArchiveOutputStream zOut )
        throws ArchiverException, IOException
    {
        // If no webxml file is specified, it's an error.
        if ( expectWebXml && deploymentDescriptor == null && !isInUpdateMode() )
        {
            throw new ArchiverException( "webxml attribute is required (or pre-existing WEB-INF/web.xml if executing in update mode)" );
        }
        super.initZipOutputStream( zOut );
    }
override of  parent; validates configuration
before initializing the output stream.