Question

This page: http://blog.ostermiller.org/convert-java-outputstream-inputstream
describes how to create an InputStream from OutputStream:

new ByteArrayInputStream(out.toByteArray())

Other alternatives are to use PipedStreams and new threads which is cumbersome.

I do not like the idea of copying many megabytes to new in memory byte array.
Is there a library that does this more efficiently?

EDIT:

By advice from Laurence Gonsalves, i tried PipedStreams and it turned out they are not that hard to deal with.
Here’s the sample code in clojure:

(defn #^PipedInputStream create-pdf-stream [pdf-info]
  (let [in-stream (new PipedInputStream)
        out-stream (PipedOutputStream. in-stream)]
    (.start (Thread. #(;Here you write into out-stream)))
    in-stream))

Answer 1

If you don’t want to copy all of the data into an in-memory buffer all at once then you’re going to have to have your code that uses the OutputStream (the producer) and the code that uses the InputStream (the consumer) either alternate in the same thread, or operate concurrently in two separate threads. Having them operate in the same thread is probably much more complicated that using two separate threads, is much more error prone (you’ll need to make sure that the consumer never blocks waiting for input, or you’ll effectively deadlock) and would necessitate having the producer and consumer running in the same loop which seems way too tightly coupled.

So use a second thread. It really isn’t that complicated. The page you linked to had reasonable example. Here’s a somewhat modernized version, which also closes the streams:

try (PipedInputStream in = new PipedInputStream()) {
    new Thread(() -> {
        try (PipedOutputStream out = new PipedOutputStream(in)) {
            writeDataToOutputStream(out);
        } catch (IOException iox) {
            // handle IOExceptions
        }
    }).start();
    processDataFromInputStream(in);
}