The variable *to-java-host-stream* is bound globally to an output stream that sends any output that is written to it to the Java host, by calling send-message-to-java-host. The where-keyword argument to send-message-to-java-host is :add, so the output is added at the end and scrolled if needed. If you do not want scrolling, you can use *to-java-host-stream-no-scroll* instead.
See send-message-to-java-host for details.
The connection to the Java host is made by init-java-interface. Until init-java-interface has been called, output to *to-java-host-stream* does nothing.
*to-java-host-stream* is not buffered and makes frequent calls to send-message-to-java-host. This should be OK for dealing with a few kilobytes for each user gesture.
LispWorks User Guide and Reference Manual - 20 Sep 2017