Add deprecation notice for Java<11.

[yuchenshi]

Description

This PR checks the Java version before starting emulators that require Java, and warns if Java version <= 11 or cannot be determined.

As a side effect, this also catches situations where the java command is not actually Java, and fixes #4116.

Scenarios Tested

Manually tested emulators:start and emulators:exec with Java 11 and Java 8. Only Java 8 leads to warnings. Tried to empty PATH and got the Java not installed error as expected.

Could use some help testing on OS X WITHOUT Java.

Sample Commands

$ firebase emulators:start
⚠  emulators: Support for Java version <= 10 will be dropped soon in firebase-tools@11.Please upgrade to Java version 11 or higher to continue using the emulators.
<snip>

┌─────────────────────────────────────────────────────────────┐
│ ✔  All emulators ready! It is now safe to connect your app. │
│ i  View Emulator UI at http://localhost:4000                │
└─────────────────────────────────────────────────────────────┘

┌────────────────┬────────────────┬─────────────────────────────────┐
│ Emulator       │ Host:Port      │ View in Emulator UI             │
├────────────────┼────────────────┼─────────────────────────────────┤
│ Authentication │ localhost:9099 │ http://localhost:4000/auth      │
├────────────────┼────────────────┼─────────────────────────────────┤
│ Functions      │ localhost:5001 │ http://localhost:4000/functions │
├────────────────┼────────────────┼─────────────────────────────────┤
│ Firestore      │ 0.0.0.0:8080   │ http://localhost:4000/firestore │
├────────────────┼────────────────┼─────────────────────────────────┤
│ Database       │ localhost:9999 │ http://localhost:4000/database  │
├────────────────┼────────────────┼─────────────────────────────────┤
│ Hosting        │ localhost:5000 │ n/a                             │
├────────────────┼────────────────┼─────────────────────────────────┤
│ Storage        │ localhost:9199 │ http://localhost:4000/storage   │
└────────────────┴────────────────┴─────────────────────────────────┘
  Emulator Hub running at localhost:4400
  Other reserved ports: 4500

Issues? Report them at https://github.com/firebase/firebase-tools/issues and attach the *-debug.log files.
 
⚠  emulators: Support for Java version <= 10 will be dropped soon in firebase-tools@11.Please upgrade to Java version 11 or higher to continue using the emulators.

To increase visibility, the deprecation notice is printed again after start (or at the very end for emulators:exec).

Internal note: Crashlytics is not affected by the deprecation (i.e. keeping Java 8 support), so we're not adding a check or warnings there. (Googlers: See go/crashlytics-android-cli for more info)

[joehan]

LGTM - I can help test this on Windows 11 shortly

[joehan]

Tested this on Windows 11, and it seems to be working nicely:
Error: Could not spawn 'java --version'. Please make sure Java is installed and on your system path

[michaeldebetaz]

Thanks for the warning! I followed the link from the Firebase documentation and installed openjdk-8-jre (so, just the runtime) on my WSL2 with Ubuntu and it works fine. It's however possible that I should install something else to have a more recent version of Java. and the doc is not very helpful with that.

Any tip for an easy install on Ubuntu ? My openjdk-8-jre package is already up to date.