在UTF-8流中间打开InputStreamReader [英] Opening InputStreamReader in the middle of UTF-8 stream

问题描述

我正在使用一个可寻求的InputStream，它将特定位置的流返回给我。流中的底层数据用UTF-8编码。我想使用inputStreamReader打开此流，并一次读取一个字符。

I am using a seekable InputStream which returns the stream to me at a specific position. The underlying data in the stream is encoded with UTF-8. I want to open this stream using inputStreamReader and read one character at a time.

这是我的代码片段

inputStream.seek(position-1);
InputStreamReader reader = new InputStreamReader(inputStream, "UTF-8");

问题是如果position-1可能指向多字节UTF- 8序列。如何检测，确保从新的UTF-8编码序列开始？感谢提前。

The problem is that if position-1 could be pointing to the middle of a multi-byte UTF-8 sequence. How can I detect that make sure it starts from a new UTF-8 encoded sequence? Thanks in advance.

推荐答案

假设您可以随时重新定位流，您可以简单地读取字节，而前两位是10。所以这样的东西：

Assuming you can reposition the stream whenever you want, you can simply read bytes while the top two bits are "10". So something like:

// InputStream doesn't actually have a seek method, but I'll assume you're using
// a subclass which does...
inputStream.seek(position);
while (true) {
    int nextByte = inputStream.read();
    if (nextByte == -1 || (nextByte & 0xc0) != 0xc0) {
       break;
    }
    position++;
}
// Undo the last read, effectively
inputStream.seek(position);
InputStreamReader reader = new InputStreamReader(inputStream, StandardCharsets.UTF_8);

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