readFully（byte[] b）是自适应解决读取流中多少字节问题的（当然由参数byte个数决定），有则读满，流结尾时则自动退出阻塞，详细见JDK帮助

readFully

public void readFully(byte[] b)
throws IOException

Reads some bytes from an input stream and stores them into the buffer array

b

. The number of bytes read is equal to the length of

b

.
This method blocks until one of the following conditions occurs:

b.length

bytes of input data are available, in which case a normal return is made.
End of file is detected, in which case an

EOFException

is thrown.
An I/O error occurs, in which case an

IOException

other than

EOFException

is thrown.

If

b

is

null

, a

NullPointerException

is thrown. If

b.length

is zero, then no bytes are read. Otherwise, the first byte read is stored into element

b[0]

, the next one into

b[1]

, and so on. If an exception is thrown from this method, then it may be that some but not all bytes of

b

have been updated with data from the input stream.

Parameters:

b

- the buffer into which the data is read. Throws:

EOFException

- if this stream reaches the end before reading all the bytes.

IOException

- if an I/O error occurs.

还有一点参数off要注意：off是表示目的字节数组的起始位置的偏移量

void	, int, int)]readFully(byte[] b, int off, int len) 從串流中讀取 len 個位元組，並將其存儲到 b 中（從索引 off 開始）。