question: public void assertUsable ( ) throws IOException { if ( throwable . get ( ) == null ) { if ( connection == null ) { throw new IOException ( "Connection is not open" ) ; } return ; } Throwable t = throwable . get ( ) ; if ( t instanceof IOException ) { throw ( IOException ) t ; } else { throw Throwables . propagate ( t ) ; } }
target:  Before the websocket is used, it's recommended to call this method to ensure that any exceptions caught while processing the messages received are acknowledged.