api contract on java public int read(byte[] buffer[], int off, int len):
If len is zero, then no bytes are read and 0 is returned; otherwise,
there is an attempt to read at least one byte. If no byte is available
because the stream is at end of file, the value -1 is returned;
otherwise, at least one byte is read and stored into b.
DFSInputStream in hadoop 1 and 2 returns 0 in if len is 0 as long as the
position is not currently at the end of a stream but if at the end of a
stream and len is 0, read returns -1 instead of 0 because pos =
getFileLength()
/**
* Read the entire buffer.
*/
@Override
public synchronized int read(byte buf[], int off, int len) throws
IOException {
checkOpen();
if (closed) {
throw new IOException("Stream closed");
}
failures = 0;
if (pos < getFileLength()) {
int retries = 2;
while (retries > 0) {
try {
if (pos > blockEnd) {
currentNode = blockSeekTo(pos);
}
int realLen = Math.min(len, (int) (blockEnd - pos + 1));
int result = readBuffer(buf, off, realLen);
if (result >= 0) {
pos += result;
} else {
// got a EOS from reader though we expect more data on it.
throw new IOException("Unexpected EOS from the reader");
}
if (stats != null && result != -1) {
stats.incrementBytesRead(result);
}
return result;
} catch (ChecksumException ce) {
throw ce;
} catch (IOException e) {
if (retries == 1) {
LOG.warn("DFS Read: " + StringUtils.stringifyException(e));
}
blockEnd = -1;
if (currentNode != null) { addToDeadNodes(currentNode); }
if (--retries == 0) {
throw e;
}
}
}
}
return -1;
}
