如果我猜对了,您需要使一个数组或包含对象引用的数组的子范围无效,以使它们符合 GC 的条件。您有一个常规的 Java 数组,它在堆上存储数据。
回答你的问题,System.arrayCopy
是将数组的子范围置空的最快方法。它的记忆力比Arrays.fill
不过,因为在最坏的情况下,您必须分配两倍的内存来保存可以复制的空数组的引用。不过,如果您需要完全清空一个数组,那么更快的方法就是创建一个新的空数组(例如new Object[desiredLength]
) 并用它替换您想要取消的那个。
Unsafe
, DirectByteBuffer
, DirectLongBuffer
实现不会在天真的直接实现中提供任何性能增益(即,如果您只是替换Array
with DirectByteBuffer
or Unsafe
)。它们比散装慢System.arrayCopy
以及。由于这些实现与 Java 无关Array
,无论如何,它们超出了您的问题范围。
这是我的 JMH 基准测试(完整的基准测试代码可用via gist https://gist.github.com/anonymous/aeba97bdfd666b801d1c83a986f7774b) 片段包括unsafe.setMemory
根据 @apangin 评论的情况;并包括ByteBuffer.put(long[] src, int srcOffset, int longCount)
根据@jan-chaefer;和等价于Arrays.fill
按照 @scott-carey 循环检查是否Arrays.fill
可能是 JDK 8 中的内在函数。
@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);
}
这是我得到的(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
附:
说起ByteBuffer
and Unsafe
- 在您的情况下,它们的主要好处是它们在堆外存储数据,并且您可以实现自己的内存释放算法,这将比常规 GC 更好地调整您的数据结构。所以你不需要取消它们,并且可以随意压缩内存。这些努力很可能没有多大价值,因为与现在相比,获得性能较差且更容易出错的代码会容易得多。