如何从流中读取x个字节？

Question

I want to read exactly n bytes from a Socket at a time. How can I achieve that?

Answer 1

DataInputStream.readFully()

当然它可以阻止任意长时间......

Answer 2

You can create a helper method to completely fill a buffer. Something like this:

public int fillBufferCompletely(InputStream is, byte[] bytes) throws IOException {
    int size = bytes.length;
    int offset = 0;
    while (offset < size) {
        int read = is.read(bytes, offset, size - offset);
        if (read == -1) {
            if ( offset == 0 ) {
                return -1;
            } else {
                return offset;
            }
        } else {
            offset += read;
        }
    }

    return size;
}

Then you just need to pass in a buffer of size x .

Edit

Michael posted a link to a function which does essentially the same thing. The only difference with mine is that it does have the ability to return less than the buffer length, but only on the condition of an end-of-stream. DataInputStream.readFully would throw a runtime exception in this scenario.

So I'll leave my answer up in case an example of that behaviour is useful.

Answer 3

DataInputStream.readFully() throws an exception on EOF, as Mark Peters points out. But there are two other methods who don't: Commons IO's IOUtils.read() and Guavas ByteStreams.read() . These both try to read up to N bytes, stopping only at EOF, and return how many they actually read.

Answer 4

This is impossible. The underlying platforms cannot guarantee this, so neither can Java. You can attempt to read n bytes, but you always have to be prepared that you get less than what was requested.