前面介绍过Java Buffer使用的内存分堆内内存(Heap)和堆外内存(No Heap)，本文将介绍DirectBuffer的实现原理，以DirectByteBuffer为例。
DirectByteBuffer
DirectByteBuffer是ByteBuffer的一个实现。如果需要实例化一个DirectByteBuffer，可以使用java.nio.ByteBuffer#allocateDirect()。
1
2
3
| public static ByteBuffer allocateDirect(int capacity) {
return new DirectByteBuffer(capacity);
}
|
内存申请
先来看一下DirectByteBuffer是如何构造，如何申请与释放内存的。下面是 DirectByteBuffer 的构造器代码:
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
| DirectByteBuffer(int cap) {
super(-1, 0, cap, cap);
boolean pa = VM.isDirectMemoryPageAligned();
int ps = Bits.pageSize();
long size = Math.max(1L, (long)cap + (pa ? ps : 0));
Bits.reserveMemory(size, cap);
long base = 0;
try {
base = unsafe.allocateMemory(size);
} catch (OutOfMemoryError x) {
Bits.unreserveMemory(size, cap);
throw x;
}
unsafe.setMemory(base, size, (byte) 0);
if (pa && (base % ps != 0)) {
address = base + ps - (base & (ps - 1));
} else {
address = base;
}
cleaner = Cleaner.create(this, new Deallocator(base, size, cap));
att = null;
}
|