import org.apache.hadoop.fs.ByteBufferUtil; //导入依赖的package包/类
@Override
public ByteBuffer read(ByteBufferPool bufferPool, int maxLength,
    EnumSet<ReadOption> opts) 
        throws IOException, UnsupportedOperationException {
  try {
    return ((HasEnhancedByteBufferAccess)in).read(bufferPool,
        maxLength, opts);
  }
  catch (ClassCastException e) {
    ByteBuffer buffer = ByteBufferUtil.
        fallbackRead(this, bufferPool, maxLength);
    if (buffer != null) {
      extendedReadBuffers.put(buffer, bufferPool);
    }
    return buffer;
  }
}
 

import org.apache.hadoop.fs.ByteBufferUtil; //导入依赖的package包/类
@Override
public synchronized ByteBuffer read(ByteBufferPool bufferPool,
    int maxLength, EnumSet<ReadOption> opts) 
        throws IOException, UnsupportedOperationException {
  if (maxLength == 0) {
    return EMPTY_BUFFER;
  } else if (maxLength < 0) {
    throw new IllegalArgumentException("can't read a negative " +
        "number of bytes.");
  }
  if ((blockReader == null) || (blockEnd == -1)) {
    if (pos >= getFileLength()) {
      return null;
    }
    /*
     * If we don't have a blockReader, or the one we have has no more bytes
     * left to read, we call seekToBlockSource to get a new blockReader and
     * recalculate blockEnd.  Note that we assume we're not at EOF here
     * (we check this above).
     */
    if ((!seekToBlockSource(pos)) || (blockReader == null)) {
      throw new IOException("failed to allocate new BlockReader " +
          "at position " + pos);
    }
  }
  ByteBuffer buffer = null;
  if (dfsClient.getConf().shortCircuitMmapEnabled) {
    buffer = tryReadZeroCopy(maxLength, opts);
  }
  if (buffer != null) {
    return buffer;
  }
  buffer = ByteBufferUtil.fallbackRead(this, bufferPool, maxLength);
  if (buffer != null) {
    getExtendedReadBuffers().put(buffer, bufferPool);
  }
  return buffer;
}
 

import org.apache.hadoop.fs.ByteBufferUtil; //导入依赖的package包/类
@Override
public synchronized ByteBuffer read(ByteBufferPool bufferPool,
    int maxLength, EnumSet<ReadOption> opts)
        throws IOException, UnsupportedOperationException {
  if (maxLength == 0) {
    return EMPTY_BUFFER;
  } else if (maxLength < 0) {
    throw new IllegalArgumentException("can't read a negative " +
        "number of bytes.");
  }
  if ((blockReader == null) || (blockEnd == -1)) {
    if (pos >= getFileLength()) {
      return null;
    }
    /*
     * If we don't have a blockReader, or the one we have has no more bytes
     * left to read, we call seekToBlockSource to get a new blockReader and
     * recalculate blockEnd.  Note that we assume we're not at EOF here
     * (we check this above).
     */
    if ((!seekToBlockSource(pos)) || (blockReader == null)) {
      throw new IOException("failed to allocate new BlockReader " +
          "at position " + pos);
    }
  }
  ByteBuffer buffer = null;
  if (dfsClient.getConf().getShortCircuitConf().isShortCircuitMmapEnabled()) {
    buffer = tryReadZeroCopy(maxLength, opts);
  }
  if (buffer != null) {
    return buffer;
  }
  buffer = ByteBufferUtil.fallbackRead(this, bufferPool, maxLength);
  if (buffer != null) {
    getExtendedReadBuffers().put(buffer, bufferPool);
  }
  return buffer;
}
 

import org.apache.hadoop.fs.ByteBufferUtil; //导入依赖的package包/类
@Override
public synchronized ByteBuffer read(ByteBufferPool bufferPool,
    int maxLength, EnumSet<ReadOption> opts) 
        throws IOException, UnsupportedOperationException {
  if (maxLength == 0) {
    return EMPTY_BUFFER;
  } else if (maxLength < 0) {
    throw new IllegalArgumentException("can't read a negative " +
        "number of bytes.");
  }
  if ((blockReader == null) || (blockEnd == -1)) {
    if (pos >= getFileLength()) {
      return null;
    }
    /*
     * If we don't have a blockReader, or the one we have has no more bytes
     * left to read, we call seekToBlockSource to get a new blockReader and
     * recalculate blockEnd.  Note that we assume we're not at EOF here
     * (we check this above).
     */
    if ((!seekToBlockSource(pos)) || (blockReader == null)) {
      throw new IOException("failed to allocate new BlockReader " +
          "at position " + pos);
    }
  }
  ByteBuffer buffer = null;
  if (dfsClient.getConf().shortCircuitMmapEnabled) {
    buffer = tryReadZeroCopy(maxLength, opts);
  }
  if (buffer != null) {
    return buffer;
  }
  buffer = ByteBufferUtil.fallbackRead(this, bufferPool, maxLength);
  if (buffer != null) {
    extendedReadBuffers.put(buffer, bufferPool);
  }
  return buffer;
}