Yes, JTcl is based on Jacl. One of the main goals of JTcl is to continue the modernization of Jacl that began with the Google Summer of Code (GSOC) 2009 Jacl Modernization Project.
JTcl is solely focused on Jacl, and does not include the TclBlend extension. Users of TclBlend should continue to use the extension from the TclJava project.
Tcl 8.4 compatibility was the goal of the GSOC 2009 project, and represents a usable Tcl implementation for a wide variety of code. Tcl 8.5 is the result of several years of development by a much larger team, and contains a large number of additions and changes from Tcl 8.4. The JTcl Project plans to implement some Tcl 8.5 commands to enable more modern Tcl code (e.g., apply, chan, dict, lassign, etc.)
JTcl cannot load C-based Tcl extensions. C-based extensions call functions of the C-based Tcl intepreter, which are not available in JTcl. JTcl can utilize extensions coded in Java.
