heroku app runs OK remotely, but fails locally with "Failed to get driver instance for jdbcUrl=jdbc:postgresql://username@localhost"

after an insane day of troubleshooting, i got it to work. as near as i can tell, the commands gradlew build and heroku local web are only happy if the database user has a password.

by default when you install postgres on ubuntu you end up with a postgres account with the same name as your linux account, and with no password.

in my original attempts described in my OP above, i had some problems with the formatting of the values for environment variables DATABASE_URL and JDBC_DATABASE_URL. once i fixed that, i started getting the error The server requested password-based authentication, but no password was provided, which i fixed by setting a password.

here are the steps that i followed:

1) set a password in postgres:

$psql#alter user username with password 'password';#\q

2) set the environment variables:

$export DATABASE_URL=postgres://username:password@localhost:5432/username$export JDBC_DATABASE_URL=jdbc:postgresql://localhost:5432/username?user=username\&password=password

(i am specifying a database with the same name as my user name).(the second export command contains the & character, which i must escape with a backslash).

3) build the project:

$./gradlew clean build

(you probably don't usually need the clean part, but at least once today i found the clean part necessary to reset some state).

4) start heroku local:

$heroku local web

5) browse to the endpoint that triggers a call to the database:

http://localhost:5000/db

this now displays the hoped-for output:

Database OutputRead from DB: 2019-04-27 19:48:38.764037Read from DB: 2019-04-27 19:58:30.007374

whew!