Deallocating Direct Buffer Native Memory in Java for JOGL

Solution 1:

The direct NIO buffers use unmanaged memory. It means that they are allocated on the native heap, not on the Java heap. As a consequence, they are freed only when the JVM runs out of memory on the Java heap, not on the native heap. In other terms, it's unmanaged = it's up to you to manage them. Forcing the garbage collection is discouraged and won't solve this problem most of the time.

When you know that a direct NIO buffer has become useless for you, you have to release its native memory by using its sun.misc.Cleaner (StaxMan is right) and call clean() (except with Apache Harmony), call free() (with Apache Harmony) or use a better public API to do that (maybe in Java > 12, AutoCleaning that extends AutoCloseable?).

It's not JOGL job to do that, you can use plain Java code to do it yourself. My example is under GPL v2 and this example is under a more permissive license.

Edit.: My latest example works even with Java 1.9 and supports OpenJDK, Oracle Java, Sun Java, Apache Harmony, GNU Classpath and Android. You might have to remove some syntactical sugar to make it work with Java < 1.7 (the multi catches, the diamonds and the generics).

Reference: http://www.ibm.com/developerworks/library/j-nativememory-linux/

Direct ByteBuffer objects clean up their native buffers automatically but can only do so as part of Java heap GC — so they do not automatically respond to pressure on the native heap. GC occurs only when the Java heap becomes so full it can't service a heap-allocation request or if the Java application explicitly requests it (not recommended because it causes performance problems).

Reference: http://docs.oracle.com/javase/7/docs/api/java/nio/ByteBuffer.html#direct

The contents of direct buffers may reside outside of the normal garbage-collected heap

This solution is integrated in Java 14:

try (MemorySegment segment = MemorySegment.allocateNative(100)) {
   ...
}

You can wrap a byte buffer into a memory segment by calling MemorySegment.ofByteBuffer(ByteBuffer), get its memory address and free it (this is a restricted method) in Java 16:

CLinker.getInstance().freeMemoryRestricted(MemorySegment.ofByteBuffer(myByteBuffer).address());

Note that you still need to use reflection in many non trivial cases in order to find the buffer that can be deallocated, typically when your direct NIO buffer isn't a ByteBuffer.

N.B: sun.misc.Cleaner has been moved into jdk.internal.ref.Cleaner in Java 1.9 in the module "java.base", the latter implemented java.lang.Runnable (thanks to Alan Bateman for reminding me that difference) for a short time but it's no longer the case. You have to call sun.misc.Unsafe.invokeCleaner(), it's done in JogAmp's Gluegen. I preferred using the Cleaner as a Runnable as it avoided to rely on sun.misc.Unsafe but it doesn't work now.

My last suggestion works with Java 9, 10, 11 and 12.

My very latest example requires the use of an incubated feature (requires Java >= 14) but is very very simple.

There is a good example in Lucene under a more permissive license.

Solution 2:

Direct buffers are tricky and don't have the usual garbage collection guarantees - see for more detail: http://docs.oracle.com/javase/7/docs/api/java/nio/ByteBuffer.html#direct

If you are having issues, I'd suggest allocating once and re-using the buffer rather than allocating and deallocating repeatedly.

Solution 3:

The way in which deallocation is done is awful -- a soft reference is basically inserted into a Cleaner object, which then does deallocation when owning ByteBuffer is garbage collected. But this is not really guaranteed to be called in timely manner.