FileChannelA channel for reading, writing, mapping, and manipulating a file.
一个用来写、读、映射和操作文件的通道。
FileChannel的read、write和map通过其实现类FileChannelImpl实现。
- read实现
public int read(ByteBuffer dst) throws IOException { ensureOpen(); if (!readable) throw new NonReadableChannelException(); synchronized (positionLock) { int n = 0; int ti = -1; try { begin(); ti = threads.add(); if (!isOpen()) return 0; do { n = IOUtil.read(fd, dst, -1, nd); } while ((n == IOStatus.INTERRUPTED) && isOpen()); return IOStatus.normalize(n); } finally { threads.remove(ti); end(n > 0); assert IOStatus.check(n); } }}FileChannelImpl的read方法通过IOUtil的read实现：
static int read(FileDescriptor fd, ByteBuffer dst, long position, NativeDispatcher nd) IOException { if (dst.isReadOnly()) throw new IllegalArgumentException("Read-only buffer"); if (dst instanceof DirectBuffer) return readIntoNativeBuffer(fd, dst, position, nd); // Substitute a native buffer ByteBuffer bb = Util.getTemporaryDirectBuffer(dst.remaining()); try { int n = readIntoNativeBuffer(fd, bb, position, nd); bb.flip(); if (n > 0) dst.put(bb); return n; } finally { Util.offerFirstTemporaryDirectBuffer(bb); }}通过上述实现可以看出，基于channel的文件数据读取步骤如下：
1、申请一块和缓存同大小的DirectByteBuffer bb。
2、读取数据到缓存bb，底层由NativeDispatcher的read实现。
3、把bb的数据读取到dst（用户定义的缓存，在jvm中分配内存）。
read方法导致数据复制了两次。
- write实现
public int write(ByteBuffer src) throws IOException { ensureOpen(); if (!writable) throw new NonWritableChannelException(); synchronized (positionLock) { int n = 0; int ti = -1; try { begin(); ti = threads.add(); if (!isOpen()) return 0; do { n = IOUtil.write(fd, src, -1, nd); } while ((n == IOStatus.INTERRUPTED) && isOpen()); return IOStatus.normalize(n); } finally { threads.remove(ti); end(n > 0); assert IOStatus.check(n); } }}和read实现一样，FileChannelImpl的write方法通过IOUtil的write实现：
static int write(FileDescriptor fd, ByteBuffer src, long position, NativeDispatcher nd) throws IOException { if (src instanceof DirectBuffer) return writeFromNativeBuffer(fd, src, position, nd); // Substitute a native buffer int pos = src.position(); int lim = src.limit(); assert (pos <= lim); int rem = (pos <= lim ? lim - pos : 0); ByteBuffer bb = Util.getTemporaryDirectBuffer(rem); try { bb.put(src); bb.flip(); // Do not update src until we see how many bytes were written src.position(pos); int n = writeFromNativeBuffer(fd, bb, position, nd); if (n > 0) { // now update src src.position(pos + n); } return n; } finally { Util.offerFirstTemporaryDirectBuffer(bb); }}通过上述实现可以看出，基于channel的文件数据写入步骤如下：
1、申请一块DirectByteBuffer，bb大小为byteBuffer中的limit - position。
2、复制byteBuffer中的数据到bb中。
3、把数据从bb中写入到文件，底层由NativeDispatcher的write实现，具体如下：
private static int writeFromNativeBuffer(FileDescriptor fd, ByteBuffer bb, long position, NativeDispatcher nd) throws IOException { int pos = bb.position(); int lim = bb.limit(); assert (pos <= lim); int rem = (pos <= lim ? lim - pos : 0); int written = 0; if (rem == 0) return 0; if (position != -1) { written = nd.pwrite(fd, ((DirectBuffer)bb).address() + pos, rem, position); } else { written = nd.write(fd, ((DirectBuffer)bb).address() + pos, rem); } if (written > 0) bb.position(pos + written); return written;}write方法也导致了数据复制了两次
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