如何克隆输入流但仍可重复使用原始流

Question

I am trying to copy the InputStream from a URLConnection which is returning a stream of type HttpInputStream (inner class of HttpUrlConnection )

In other cases, I can copy the original stream to a ByteArrayOutputStream and then use mark/reset on the original, but HttpInputStream does not support mark/reset.

Is there a way I can still copy the stream and reset the original or keep it from being consumed? The original stream inside URLConnection has to be readable because it is passed into another library. I only need to copy the stream so I can read the first two lines of data. Here is what I have for streams that support mark/reset:

InputStream input = null;
    ByteArrayOutputStream baos = new ByteArrayOutputStream();
    try {
      input = connection.getInputStream();
      byte[] buffer = new byte[200];
      input.mark(200);
      int len = input.read(buffer);
      input.reset();
      baos.write(buffer, 0, len);
      baos.flush();

      String content = baos.toString("UTF-8");
      //I set flags based on the value of content, but omitting here for the sake of simplicity.

    } catch (IOException ex) {
      //I do stuff here, but omitting for sake of simplicity in this
    }

Answer 1

ImputStreams are not generally cloneable, and neither do all streams support mark/reset. There are some possible workarounds within the standard JRE.

Wrap the InputStream into a BufferedInputStream. That one supports mark/reset within the limits of its buffer size. That enables you to read a limited amount of data from the beginning, then reset the stream.

Another alternative is PushBackInputStream, which allows you to "unread" data previously read. You need to buffer the data to be pushed back yourself though, so it may be a bit inconvinient to handle.

If the whole stream isn't terribly big, you could also read the entire stream first, then construct as many ByteArrayInputStreams as needed from the pre-read data. Only feasible if the data fits in the heap (eg less than approximately 2GB max).

Answer 2

Apache commons library has a really nice TeeInput stream. https://commons.apache.org/proper/commons-io/javadocs/api-1.4/org/apache/commons/io/input/TeeInputStream.html