虎牙二面：說說你對 Java “零拷貝”的理解？

public class MappedByteBufferTest {
    public static void main(String[] args) throws Exception {        File file = new File("D://db.txt");        long len = file.length();        byte[] ds = new byte[(int) len];        MappedByteBuffer mappedByteBuffer = new FileInputStream(file).getChannel().map(FileChannel.MapMode.READ_ONLY, 0,                len);        for (int offset = 0; offset < len; offset++) {            byte b = mappedByteBuffer.get();            ds[offset] = b;        }        Scanner scan = new Scanner(new ByteArrayInputStream(ds)).useDelimiter(" ");        while (scan.hasNext()) {            System.out.print(scan.next() + " ");        }    }}

public abstract MappedByteBuffer map(MapMode mode,  long position, long size)    throws IOException;

public MappedByteBuffer map(MapMode mode, long position, long size)    throws IOException{        ...省略...        int pagePosition = (int)(position % allocationGranularity);        long mapPosition = position - pagePosition;        long mapSize = size + pagePosition;        try {            // If no exception was thrown from map0, the address is valid            addr = map0(imode, mapPosition, mapSize);        } catch (OutOfMemoryError x) {            // An OutOfMemoryError may indicate that we've exhausted memory            // so force gc and re-attempt map            System.gc();            try {                Thread.sleep(100);            } catch (InterruptedException y) {                Thread.currentThread().interrupt();            }            try {                addr = map0(imode, mapPosition, mapSize);            } catch (OutOfMemoryError y) {                // After a second OOME, fail                throw new IOException("Map failed", y);            }        }
        // On Windows, and potentially other platforms, we need an open        // file descriptor for some mapping operations.        FileDescriptor mfd;        try {            mfd = nd.duplicateForMapping(fd);        } catch (IOException ioe) {            unmap0(addr, mapSize);            throw ioe;        }
        assert (IOStatus.checkAll(addr));        assert (addr % allocationGranularity == 0);        int isize = (int)size;        Unmapper um = new Unmapper(addr, mapSize, isize, mfd);        if ((!writable) || (imode == MAP_RO)) {            return Util.newMappedByteBufferR(isize,                                             addr + pagePosition,                                             mfd,                                             um);        } else {            return Util.newMappedByteBuffer(isize,                                            addr + pagePosition,                                            mfd,                                            um);        } }

ByteBuffer directByteBuffer = ByteBuffer.allocateDirect(100);

public class ChannelTransfer {    public static void main(String[] argv) throws Exception {        String files[]=new String[1];        files[0]="D://db.txt";        catFiles(Channels.newChannel(System.out), files);    }
    private static void catFiles(WritableByteChannel target, String[] files)            throws Exception {        for (int i = 0; i < files.length; i++) {            FileInputStream fis = new FileInputStream(files[i]);            FileChannel channel = fis.getChannel();            channel.transferTo(0, channel.size(), target);            channel.close();            fis.close();        }    }}

public abstract long transferTo(long position, long count,    WritableByteChannel target)  throws IOException;

public class CompositeChannelBuffer extends AbstractChannelBuffer {
    private final ByteOrder order;    private ChannelBuffer[] components;    private int[] indices;    private int lastAccessedComponentId;    private final boolean gathering;        public byte getByte(int index) {        int componentId = componentId(index);        return components[componentId].getByte(index - indices[componentId]);    }    ...省略...