In PostgreSQL releases 9.2 and later, PL/Java can be installed entirely
without disturbing the postgresql.conf file or reloading/restarting the
server: the configuration variables can be set interactively in a session
until PL/Java loads sucessfully, then saved with a simple
ALTER DATABASE dbname SET var FROM CURRENT for each setting
that had to be changed.
Releases earlier than 9.2 are slightly less convenient. It is still possible
to work out the right settings in an interactive session, but once found,
the settings must be added to postgresql.conf to be persistent, and the
postmaster signalled (with pg_ctl reload) to pick up the new settings.
Releases before 9.2 also require setting custom_variable_classes in
postgresql.conf to include the prefix pljava, and that assignment must
be earlier in the file than any settings of pljava.* variables.
It is still possible to do an exploratory session to find the variable settings
that work before touching postgresql.conf at all, but
the details are slightly different.
In later PostgreSQL versions, you would typically use some SET commands
followed by a LOAD (followed, perhaps, by more SET commands unless you
always get things right on the first try).
Before release 9.2, however, the order has to be LOAD first, which typically
will lead to an incompletely-started warning because the configuration settings
have not been made yet. Then, because the module has been loaded,
pljava.* variables will be recognized and can be set and changed until
PL/Java successfully loads, just as in the newer versions of PostgreSQL.
Once working settings are found, edit postgresql.conf, make sure that
custom_variable_classes includes pljava, copy in the variable settings
that worked, and use pg_ctl reload to make the new settings effective.
The procedure above relies on the way loading stops when the settings are not
right, giving you a chance to adjust them interactively. That turns out to be
a problem if there are previously-saved settings, or the original defaults,
that happen to work even if they are not the settings you want. In that case,
the LOAD command starts PL/Java right up, leaving you no chance in the
interactive session to change anything.
To escape that behavior, there is one more very simple configuration variable,
pljava.enable. If it is off, LOADing PL/Java will always stop early and
allow you to set other variables before setting pljava.enable to on.
To answer the hen-and-egg question of how to set pljava.enable to off
before loading the module, it defaults to off on PostgreSQL releases
earlier than 9.2, so you will always have the chance to test your settings
interactively (and you will always have to set it explicitly on when
you are ready).
If it is already on because of an earlier configuration saved in
postgresql.conf, it will be recognized in your interactive session and you
can set it off as needed.
On later PostgreSQL releases with no such complications, it defaults to on
and can be ignored.