I know I can simply iterate from start to end and clear those cells but I was wondering if it was possible in any faster way (perhaps using JNI-ed System.arrayCopy)?
If I got it right, you need to nullify an array, or a sub-range of an array containing references to objects to make them eligible for GC. And you have a regular Java array, which stores data on-heap.
Answering your question, System.arrayCopy is the fastest way to null a sub-range of an array. It is worse memory-wise than Arrays.fill though, since you would have to allocate twice as much memory to hold references at worst case for an array of nulls you can copy from. Though if you need to fully null an array, even faster would be just to create a new empty array (e.g. new Object[desiredLength]) and replace the one you want to nullify with it.
Unsafe, DirectByteBuffer, DirectLongBuffer implementations doesn't provide any performance gain in a naive straight-forward implementation (i.e. if you just replace the Array with DirectByteBuffer or Unsafe). They are slower then bulk System.arrayCopy as well. Since those implementations have nothing to do with Java Array, they're out of scope of your question anyway.
Here's my JMH benchmark (full benchmark code available via gist) snippet for those including unsafe.setMemory case as per @apangin comment; and including ByteBuffer.put(long[] src, int srcOffset, int longCount) as per @jan-chaefer; and an equivalent of Arrays.fill loop as per @scott-carey to check if Arrays.fill could be an intrinsic in JDK 8.
@Benchmark
@BenchmarkMode(Mode.SampleTime)
@OutputTimeUnit(TimeUnit.NANOSECONDS)
public void arrayFill() {
Arrays.fill(objectHolderForFill, null);
}
@Benchmark
@BenchmarkMode(Mode.SampleTime)
@OutputTimeUnit(TimeUnit.NANOSECONDS)
public void arrayFillManualLoop() {
for (int i = 0, len = objectHolderForFill.length; i < len; i++) {
objectHolderForLoop[i] = null;
}
}
@Benchmark
@BenchmarkMode(Mode.SampleTime)
@OutputTimeUnit(TimeUnit.NANOSECONDS)
public void arrayCopy() {
System.arraycopy(nullsArray, 0, objectHolderForArrayCopy, 0,
objectHolderForArrayCopy.length);
}
@Benchmark
@BenchmarkMode(Mode.SampleTime)
@OutputTimeUnit(TimeUnit.NANOSECONDS)
public void directByteBufferManualLoop() {
while (referenceHolderByteBuffer.hasRemaining()) {
referenceHolderByteBuffer.putLong(0);
}
}
@Benchmark
@BenchmarkMode(Mode.SampleTime)
@OutputTimeUnit(TimeUnit.NANOSECONDS)
public void directByteBufferBatch() {
referenceHolderByteBuffer.put(nullBytes, 0, nullBytes.length);
}
@Benchmark
@BenchmarkMode(Mode.SampleTime)
@OutputTimeUnit(TimeUnit.NANOSECONDS)
public void directLongBufferManualLoop() {
while (referenceHolderLongBuffer.hasRemaining()) {
referenceHolderLongBuffer.put(0L);
}
}
@Benchmark
@BenchmarkMode(Mode.SampleTime)
@OutputTimeUnit(TimeUnit.NANOSECONDS)
public void directLongBufferBatch() {
referenceHolderLongBuffer.put(nullLongs, 0, nullLongs.length);
}
@Benchmark
@BenchmarkMode(Mode.SampleTime)
@OutputTimeUnit(TimeUnit.NANOSECONDS)
public void unsafeArrayManualLoop() {
long addr = referenceHolderUnsafe;
long pos = 0;
for (int i = 0; i < size; i++) {
unsafe.putLong(addr + pos, 0L);
pos += 1 << 3;
}
}
@Benchmark
@BenchmarkMode(Mode.SampleTime)
@OutputTimeUnit(TimeUnit.NANOSECONDS)
public void unsafeArraySetMemory() {
unsafe.setMemory(referenceHolderUnsafe, size*8, (byte) 0);
}
Here's what I got (Java 1.8, JMH 1.13, Core i3-6100U 2.30 GHz, Win10):
100 elements
Benchmark Mode Cnt Score Error Units
ArrayNullFillBench.arrayCopy sample 5234029 39,518 ± 0,991 ns/op
ArrayNullFillBench.directByteBufferBatch sample 6271334 43,646 ± 1,523 ns/op
ArrayNullFillBench.directLongBufferBatch sample 4615974 45,252 ± 2,352 ns/op
ArrayNullFillBench.arrayFill sample 4745406 76,997 ± 3,547 ns/op
ArrayNullFillBench.arrayFillManualLoop sample 5549216 78,677 ± 13,013 ns/op
ArrayNullFillBench.unsafeArrayManualLoop sample 5980381 78,811 ± 2,870 ns/op
ArrayNullFillBench.unsafeArraySetMemory sample 5985884 85,062 ± 2,096 ns/op
ArrayNullFillBench.directLongBufferManualLoop sample 4697023 116,242 ± 2,579 ns/op <-- wow
ArrayNullFillBench.directByteBufferManualLoop sample 7504629 208,440 ± 10,651 ns/op <-- wow
I skipped all** the loop implementations from further tests
** - except arrayFill and arrayFillManualLoop for scale
1000 elements
Benchmark Mode Cnt Score Error Units
ArrayNullFillBench.arrayCopy sample 6780681 184,516 ± 14,036 ns/op
ArrayNullFillBench.directLongBufferBatch sample 4018778 293,325 ± 4,074 ns/op
ArrayNullFillBench.directByteBufferBatch sample 4063969 313,171 ± 4,861 ns/op
ArrayNullFillBench.arrayFillManualLoop sample 6270397 543,801 ± 20,325 ns/op
ArrayNullFillBench.arrayFill sample 6590416 548,250 ± 13,475 ns/op
10000 elements
Benchmark Mode Cnt Score Error Units
ArrayNullFillBench.arrayCopy sample 2551851 2024,543 ± 12,533 ns/op
ArrayNullFillBench.directLongBufferBatch sample 2958517 4469,210 ± 10,376 ns/op
ArrayNullFillBench.directByteBufferBatch sample 2892258 4526,945 ± 33,443 ns/op
ArrayNullFillBench.arrayFill sample 2578580 5532,063 ± 20,705 ns/op
ArrayNullFillBench.arrayFillManualLoop sample 2562569 5550,195 ± 40,666 ns/op
P.S.
Speaking of ByteBuffer and Unsafe - their main benefits in your case is that they store data off-heap, and you can implement your own memory deallocation alghorithm which would siut your data-structure better than regular GC. So you won't need to nullify them, and could compact memory as you please. Most likely the efforts won't worth much, since it would be much easier to get a less performant and more error-prone code then you have now.