Question

我知道java NIO有两种模式，即异步和同步模式。当我读取SocketChannel.read（）的javadoc时，我得到下面的解释：

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.

让我困惑的是对异步和同步读取的解释。是的，在异步模式下，它会立即读取现在已经存在于缓冲区中的内容并返回。但是在同步模式下，它是不一样的？如果有的话在缓冲区中，为什么不读取它并立即返回？等待什么？

Answer 1

如果缓冲区中有东西，为什么不读它并立即返回？

我认为你理解错了。正如在int read(ByteBuffer dst)

的文档中所述

要传输字节的缓冲区。

通道读取字节并将它们写入缓冲区。

但是，如果某个频道处于阻止模式，则可以保证这个方法在缓冲区中至少剩下一个字节将阻塞，直到读取至少一个字节。

at least one byte remaining in the buffer表示缓冲区仍有可用空间。函数boolean hasRemaining() and int remaining()用于检查此内容。

等什么？

等待从频道读取一些字节或到达流末尾。在阻塞通道上读取不能返回0。

SocketChannel.read（）在异步和同步模式下的区别？

1 个答案: