JNI-谁释放Java方法返回的ByteBuffer？

Question

I understand when a call is made from Java to C, there is no need to release memory obtained by GetDirectBufferAddress method. The garbage collector knows when to reclaim the memory. In my case, I am calling a Java method from C. Here is a code snippet for my Java method:

ByteBuffer processTask() {
   ...
   ByteBuffer retVal = ByteBuffer.allocateDirect(len);
   return retVal;
}

Here is a code snippet from my C function:

jobject retVal = invokeProcessTask();
uint8_t* buf = (uint8_t*) env->GetDirectBufferAddress(retVal);

In this case, I am thinking the garbage collector has no idea when to collect the ByteBuffer instance. I am thinking it will be my C code's responsibility to release this memory. Is this right? If so, what function do I need to call to release it? Regards.

Answer 1

The JNI docs do not seem to make this clear. They do say that all Java objects returned by JNI functions are local references, and I see no reason to doubt that that applies whether or not the JNI function is called inside the scope of a broader native method invocation. The VM is responsible for keeping track of objects passed to native code, so as not to GC them while still in use, and I think it's necessary to interpret "passed" as including "returned to".

I am therefore inclined to agree with the interpretation that when a Java VM is embedded in a C program, and the program obtains a reference to a Java object by calling a JNI function outside the scope of a Java -> C native method invocation, that the referred-to object and all those reachable from it remain ineligible for GC until the C program explicitly releases its reference (at least). The program can release local references via the DeleteLocalRef() function.

Note, too, JNI provides for only a limited number of local references to live in the same JNI environment at any given time. That number can be managed via EnsureLocalCapacity() , PushLocalFrame() , and PopLocalFrame() , but you must take care to avoid exceeding it, which may be more of a risk in the embedding scenario you describe than it is in a short-running, narrow-scoped native method invocation.

Answer 2

您可以调用DeleteLocalRef() ，但是除非C方法真正长时间运行，否则可能不需要：GC可以在JNI方法返回后的任何时间收集它。