The difference of SocketChannel.read() in async and sync mode?

Question

I know that java NIO have two modes , the asynchronized and synchronized mode.When I am reading the javadoc of SocketChannel.read(), I get the explaination below:

Reads a sequence of bytes from this channel into the given buffer. 
An attempt is made to read up to r bytes from the channel, where r is the number of bytes remaining in the buffer, that is, dst.remaining(), at the moment this method is invoked. 

Suppose that a byte sequence of length n is read, where 0 <= n <= r. This byte sequence will be transferred into the buffer so that the first byte in the sequence is at index p and the last byte is at index p + n - 1, where p is the buffer's position at the moment this method is invoked. Upon return the buffer's position will be equal to p + n; its limit will not have changed. 

A read operation might not fill the buffer, and in fact it might not read any bytes at all. Whether or not it does so depends upon the nature and state of the channel. **A socket channel in non-blocking mode, for example, cannot read any more bytes than are immediately available from the socket's input buffer; similarly, a file channel cannot read any more bytes than remain in the file. It is guaranteed, however, that if a channel is in blocking mode and there is at least one byte remaining in the buffer then this method will block until at least one byte is read.** 

This method may be invoked at any time. If another thread has already initiated a read operation upon this channel, however, then an invocation of this method will block until the first operation is complete. 

what make me confused is the explanation of asynchronized and sychronized read .Yes , in asynchronized mode, it will immediately read what is now already in buffer and return.But in synchronized mode,it is not the same?if there is something in buffer, why is does't not read it and return immediately?wait for what?

Answer 1

if there is something in buffer, why is does't not read it and return immediately?

I think you understood it wrong. As it is said in docs for int read(ByteBuffer dst)

The buffer into which bytes are to be transferred.

The channel reads bytes and writes them into buffer.

It is guaranteed, however, that if a channel is in blocking mode and there is at least one byte remaining in the buffer then this method will block until at least one byte is read.

at least one byte remaining in the buffer means that buffer still has free space. Functions boolean hasRemaining() and int remaining() are used for checking this.

wait for what?

Wait for reading some bytes from the channel or reaching the end-of-stream. Reading on blocking channel couldn't return 0.