Get an input stream from an output stream

问题

I have a component that's given me data in an output stream (ByteArrayOutputStream) and I need to write this into a blob field of a SQL database without creating temp buffers hence the need to get an input stream.

Based on answers here and here I came up the following method to get an input stream from an output stream:

private PipedInputStream getInputStream(ByteArrayOutputStream outputStream) throws InterruptedException
{
    PipedInputStream pipedInStream = new PipedInputStream();
    Thread copyThread = new Thread(new CopyStreamHelper(outputStream, pipedInStream));
    copyThread.start();
    // Wait for copy to complete
    copyThread.join();
    return pipedInStream;
}

class CopyStreamHelper implements Runnable
{
    private ByteArrayOutputStream outStream;
    private PipedInputStream pipedInStream;

    public CopyStreamHelper (ByteArrayOutputStream _outStream, PipedInputStream _pipedInStream)
    {
        outStream = _outStream;
        pipedInStream = _pipedInStream;
    }

    public void run()
    {
        PipedOutputStream pipedOutStream = null;
        try
        {
            // write the original OutputStream to the PipedOutputStream
            pipedOutStream = new PipedOutputStream(pipedInStream);
            outStream.writeTo(pipedOutStream);
        }
        catch (IOException e) 
        {
            // logging and exception handling should go here
        }
        finally
        {
            IOUtils.closeQuietly(pipedOutStream);
        }
    }
}

Please note that the output stream already contains the written data and it can run up to 1-2 MB. However regardless of trying to do this in two separate threads or the same thread I am finding that always PipedInputStream hangs at the following:

Object.wait(long) line: not available [native method]   
PipedInputStream.awaitSpace() line: not available

回答1:

You are overcomplicating the solution

ByteArrayOutputStream baos = ...;
byte[] data = baos.toByteArray();
return new ByteArrayInputStream(data);

回答2:

I have worked out a very simple demo for use of PipedInput/OutputStream. It may or may not fit your usecase.

A producing class writing into PipedOutputStream:

public class Producer implements Runnable {

    private final PipedOutputStream pipedOutputStream;
    private final PipedInputStream pipedInputStream;

    public Producer() throws IOException {
        this.pipedOutputStream = new PipedOutputStream();
        this.pipedInputStream = new PipedInputStream(pipedOutputStream);
    }

    public PipedInputStream getPipedInputStream() {
        return pipedInputStream;
    }

    @Override
    public void run() {

        try(InputStream inputStream = ByteStreams.limit(new RandomInputStream(), 100000)) {
            // guava copy function
            ByteStreams.copy(inputStream, pipedOutputStream);
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            try {
                pipedOutputStream.close();
            } catch (IOException e) {
                // no-op
            }
        }
    }

    public static void main(String[] args) throws IOException {

        try {
            Producer producer = new Producer();
            Consumer consumer = new Consumer(producer);

            Thread thread1 = new Thread(producer);
            Thread thread2 = new Thread(consumer);

            thread1.start();
            thread2.start();

            thread1.join();
            thread2.join();

        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

A consumer just counting the bytes:

public class Consumer implements Runnable {

    private final Producer producer;

    public Consumer(Producer producer) {
        this.producer = producer;
    }

    @Override
    public void run() {

        try (PipedInputStream pipedInputStream = producer.getPipedInputStream()) {

            int counter = 0;
            while (pipedInputStream.read() != -1) {
                counter++;
            }

            System.out.println(counter);

        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

回答3:

At some level there has to be a buffer. See Connecting an input stream to an outputstream.

My favorite answer from there is from Dean Hiller:

void feedInputToOutput(InputStream in, OutputStream out) {
   IOUtils.copy(in, out);
}

See the api for details

来源：https://stackoverflow.com/questions/29286599/get-an-input-stream-from-an-output-stream

标签

java

inputstream

outputstream