Java 异步和同步模式下SocketChannel.read（）的区别是什么？

我知道java NIO有两种模式，异步模式和同步模式。当我阅读SocketChannel.read（）的javadoc时，我得到以下解释：

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. 

从该通道将字节序列读入给定的缓冲区。
尝试从通道读取最多r个字节，其中r是调用此方法时缓冲区中剩余的字节数，即dst.remaining（）。
假设读取长度为n的字节序列，其中0
若缓冲区中有东西，为什么不读取它并立即返回

我想你理解错了。正如文件中所说的

要将字节传输到其中的缓冲区

通道读取字节并将其写入缓冲区

但是，可以保证，如果通道处于阻塞模式
缓冲区中至少剩余一个字节，然后使用此方法
将阻塞，直到至少读取一个字节

缓冲区中至少剩余一个字节
表示缓冲区仍有可用空间。函数用于检查这一点

等什么

等待从通道读取一些字节或到达流的末尾。
在阻塞通道上读取无法返回0。
我认为第一个混淆的来源是您使用的术语。
SocketChannel.read（）
的Javadoc从未提到“同步”或“异步”。相反，这是关于阻塞和非阻塞I/O的。您应该重新考虑您的问题，并在适当的情况下用非阻塞替换“异步”，用“阻塞”替换“同步”。这将使你和阅读问题的用户都更加清楚。