If you want to work with Groovy (Jython,JavaScript) or a Java editor (JDT)
with ImageJ API code completion you can download Bio7 which integrates JDT
and the aforementioned macro editor plugin (an advanced R editor by the way,
too):
You can compile and test ImageJ Java plugins on the fly (dynamic in process
compilation) or install an advanced
Groovy plugin (Bio7 comes with some simple default editors) which also has
access to the ImageJ API for code completion.
I tested several other Eclipse editors (JavaScript, Pydev). However until
now the Groovy editor plugin offers the complete code completion toolchain
of JDT.
Here as an example a video how to use ImageJ with a Eclipse JavaScript
editor in Bio7:
The Groovy plugin (or other Eclipse editors) work similar.
My opinion to BeanShell: BeanShell very popular in the past is a little bit
outdated and I would prefer Groovy instead which also offers Java code
compatibility.