有些代码初次阅读并不能看出其中的处理细节，但是当过一段时间对这个部分熟悉了之后再来阅读，就能发现并体会其中的细节。
本文说一下FSOutputSummer#write1的细节。
细节①： 当local buffer为empty并且此次write1要写的数据长度len > buf.length时，直接计算buf.length个数个字节的checksum并把这些数据发送到底层流。不用再做System.arraycopy到buf里了。返回值就是buf.length，即这次write1真实处理了多少数据字节。
/**
* Write a portion of an array, flushing to the underlying
* stream at most once if necessary.
*/
private int write1(byte b[], int off, int len) throws IOException {
// 细节①的判断条件
if(count==0 && len>=buf.length) {
// local buffer is empty and user buffer size >= local buffer size, so
// simply checksum the user buffer and send it directly to the underlying
// stream
final int length = buf.length;
writeChecksumChunks(b, off, length);
return length;
}
// copy user data to local buffer
int bytesToCopy = buf.length-count;
bytesToCopy = (len<bytesToCopy) ? len : bytesToCopy;
System.arraycopy(b, off, buf, count, bytesToCopy);
count += bytesToCopy;
if (count == buf.length) {
// local buffer is full
flushBuffer();
}
return bytesToCopy;
}
