Sunday, September 4, 2022

Returning local references from Java JNI

Wrote some tests on Java JNI. Lesson learned: it is ok to return a local reference from JNI. Just don't explicitly call DeleteLocalRef on it otherwise Java side will get a null value.


Regions highlighted on the graph:

(1) Running the JNI calls normally, freeing local references as it goes. Everything works as expected.

(2) Running the JNI calls without explicitly freeing local references. It is still fine. There were reclaimable objects left in the memory. But triggering the GC (from VisualVM) will free up the space.

(3) Running the JNI calls incorrectly by freeing the local references before returning from JNI. Memory usage normal. But the program printout should indicate 0 test case passed. This is because null is returned instead of the expected HashMap.

(4) !! This will cause OutOfMemoryError !!. The JNI calls explicitly create global references without freeing them, causing out-of-memory issue.