java: clean up the EvmcVm class a bit

[axic]

1. Make the native methods explicitly private (they are not supposed to be available due to requiring a VM pointer).
2. Properly return exception on loading failures.

Some references:

• https://developer.android.com/training/articles/perf-jni#java
• https://www.protechtraining.com/content/java_fundamentals_tutorial-_java_native_interface_jni
• https://rkennke.wordpress.com/2007/07/30/efficient-jni-programming-iv-wrapping-native-data-objects/

[atoulme]

Why is this static?

[axic]

Because it only exists once, it is the native library which can only be loaded once.

[atoulme]

OK, if it's purely functional, that may work.

[atoulme]

Consider making it extend RuntimeException - this way it's an unchecked exception.

[axic]

I'd argue this must be actually checked because it is an error which should not be ignored or bubbled up.

[axic]

But perhaps I don't fully understand the distinction between Exception and RuntimeException -- could you explain please?

[atoulme]

Sure thing. In Java, you must catch checked exceptions, or the code doesn't compile. You can let unchecked exceptions "unchecked", meaning you don't need to catch them explicitly. They still happen and bubble up.
The net difference is that sometimes your Java code ends up looking like a bunch of try/catch statements or throws.
This is a subject of hot debate and frankly, I'm fine either way. I just prefer RuntimeExceptions myself.
https://stackoverflow.com/questions/6115896/understanding-checked-vs-unchecked-exceptions-in-java

[axic]

Thanks! My argument for checked exception here is that this EVMC-specific loading error should be handled at the place this is integrated in, likely it would be translated into some local exception in the project?

Though I don't have a very strong opinion, fine either way.

[atoulme]

OK, let’s do it.

[atoulme]

Please don't make the methods static - this would only allow one EVM in a JVM at a time. Having multiple EVMs is a good thing.

[axic]

Please don't make the methods static - this would only allow one EVM in a JVM at a time. Having multiple EVMs is a good thing.

The JNI binding (libevmc-java.so) should be loaded once. Multiple EVMC VMs can be loaded and that is distinguished by the nativeVM object which the JNI side looks up and dispatches.

[axic]

@atoulme okay to merge this?

[axic]

@atoulme merging this now to progress with the refactoring but we can revert certain changes if it turns out to be bad.

[atoulme]

All good. Sorry for the slow response.