Provide a method to run functions on JVM thread

[jonmcclung]

This is an attempt to provide a way for C++-created threads to call into Java without risking a memory leak, as requested in #176 . The basic idea is this:

• you pass in a lambda
• the current thread is attached to the JVM if it's not already
• the lambda is executed and the result, if any, stored
• the current thread is detached only if it was this function that attached it
• the result, if it was stored, is returned.

There are two functions because I'm not skilled enough with lambdas to make one function that will accept any kind of lambda.

[mjmacleod]

I've tested this and confirm it works my side.

However I don't personally like the extra code clutter to use this, nor the overhead of constantly attaching/detaching.
I've implemented the code suggestions in #372 as an alternative pull request #405 that could possibly be used instead of this though I guess there is no reason they could not both co-exist also.

[shahzadlone]

Please make the if one line if's consistent with the rest, just have a {} for if's scope like the majority.

-    if (attached) g_cachedJVM->DetachCurrentThread();
+    if (attached) {
+        g_cachedJVM->DetachCurrentThread();
+    }

[shahzadlone]

Please make the if one line if's consistent with the rest, just have a {} for if's scope like the majority.

-    if (attached) g_cachedJVM->DetachCurrentThread();
+    if (attached) {
+        g_cachedJVM->DetachCurrentThread();
+    }

[mjmacleod]

Just a note that #405 has been merged now, I'm not sure if there is a good reason to have more than one way to do this or not.

[shahzadlone]

Just a note that #405 has been merged now, I'm not sure if there is a good reason to have more than one way to do this or not.

That's fair enough. I just pointed out because I have seen some pretty tedious bugs introduced because of the missing braces after an if.

[artwyman]

#405 provided an alternative option to this. This repo isn't going to be adding any new features so I'm putting this PR on hold.