refactor(env/java): improve tests and implementation (#1246)

[sakibguy]

This basically fixes up the changes from apache#1220.

• test(env/java): replace test that duplicates implementation
• test(env/java): stub _ensure to focus on unit under test
• test(env/java): add test for invalid output
• refactor(env/java): keep try block small
• refactor(env/java): shorten excessive comment

Platforms affected

Motivation and Context

Description

Testing

Checklist

• I've run the tests to see all new and existing tests pass
• I added automated test coverage as appropriate for this change
• Commit is prefixed with (platform) if this change only applies to one platform (e.g. (android))
• If this Pull Request resolves an issue, I linked to the issue in the text above (and used the correct keyword to close issues using keywords)
• I've updated the documentation if necessary