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《Java工程师面试突击（第3季）》重磅升级，由原来的70讲增至160讲，内容扩充一倍多，升级部分内容请参见文末

• 前言
• I/O概念
• 1.缓冲区
• 2.虚拟内存
• 3.mmap+write方式
• 4.sendfile方式
• Java零拷贝
• 1.MappedByteBuffer
• 2.DirectByteBuffer
• 3.Channel-to-Channel传输
• Netty零拷贝
• 其他零拷贝
• 总结

前言

I/O概念

1.缓冲区

2.虚拟内存

3.mmap+write方式

4.sendfile方式

Java零拷贝

1.MappedByteBuffer

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() + " ");
        }
    }
}

主要通过FileChannel提供的map()来实现映射，map()方法如下：

    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);
            }
     }

2.DirectByteBuffer

ByteBuffer directByteBuffer = ByteBuffer.allocateDirect(100);

如上开辟了100字节的直接内存空间；

3.Channel-to-Channel传输

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;

Netty零拷贝

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...

其他零拷贝

总结

END

《Java工程师面试突击第三季》加餐部分大纲：（注：1-66讲的大纲请扫描文末二维码，在课程详情页获取）

详细的课程内容，大家可以扫描下方二维码了解：