Use Python standard library which instead of OS which

[dogatekin]

When pyjnius is installed in an environment that doesn't have the which command, the two functions get_jdk_home and get_jre_home might fail if they try to invoke the OS which command.

This PR fixes this issue by instead using the which implementation in the Python standard library, namely shutil.which for Python 3 and distutils.spawn.find_executable for Python 2. If the searched executable wasn't found in the original code the setup was failing with an error, so similarly I raised an explanatory exception in those cases.

If the conditional import seems icky, we could:

• Drop the find_executable import if you decide to drop Python 2 support, or
• Drop the shutil.which import since find_executable also works on Python 3, I only kept it because shutil.which seems to have some minor improvements over find_executable e.g. checking PATHEXT for Windows.

[cmacdonald]

Hi @dogatekin

This switch makes sense.
There is a related PR under review at #541 - that may solve your underlying problem?
Could this patch be rebased for that?

[cmacdonald]

Python 2 support is also being removed, AIUI

[dogatekin]

Hi @cmacdonald, thanks for checking my changes and for pointing me to the other PR!

The PR at #541 might indeed solve the problem I've been running into since it looks like the which calls would be avoided as long as one of JAVA_HOME / JDK_HOME / JRE_HOME is set. And I could definitely rebase this patch on that PR, or we could add this change to that PR as well.

However, since that PR has been open for so long and since @tshirtman doesn't currently have that much time to fully review and test it, I thought I might have a better chance fixing this problem earlier with this more specific PR that only changes this small part, while #541 has more major changes. My ideal scenario would be to merge this PR quickly, and #541 can get merged when it receives the necessary attention and review. And since that PR still retains the calls to the system which, we could rebase it on these changes easily if this PR indeed gets merged quicker.

[cmacdonald]

Sorry I hadnt realised you were on #541 as well!

[tshirtman]

Looks good to me!
I didn’t test it, but which seems to do exactly what we need, so i don’t see any reason to block this.
and TIL about it, so thanks :)