How to Convert an OutputStream to an InputStream in Java?

How to Convert an OutputStream to an InputStream

In software development, it's not uncommon to encounter situations where you need to convert data from one stream type to another. One such scenario is converting an OutputStream to an InputStream.

Introduction to Piped Streams

The solution to this problem lies in using Java's PipedInputStream and PipedOutputStream classes. These classes enable communication between streams by creating a bidirectional pipe.

PipedInputStream to OutputStream (Not Vice Versa)

Lambda Expression:

PipedInputStream in = new PipedInputStream();
final PipedOutputStream out = new PipedOutputStream(in);
// in a background thread, write the given output stream to the PipedOutputStream for consumption
new Thread(() -> {
    originalOutputStream.writeTo(out);
}).start();

Note: It's essential to handle situations where the OutputStream may be prematurely closed, leading to a ClosedPipeException. To avoid this, you can invert the constructors:

PipedInputStream in = new PipedInputStream(out);
new Thread(() -> {
    originalOutputStream.writeTo(out);
}).start();

Try-With-Resources:

// take the copy of the stream and re-write it to an InputStream
PipedInputStream in = new PipedInputStream();
new Thread(new Runnable() {
    public void run() {
        // try-with-resources here
        // putting the try block outside the Thread will cause the PipedOutputStream resource to close before the Runnable finishes
        try (final PipedOutputStream out = new PipedOutputStream(in)) {
            // write the original OutputStream to the PipedOutputStream
            // note that in order for the below method to work, you need to ensure that the data has finished writing to the ByteArrayOutputStream
            originalByteArrayOutputStream.writeTo(out);
        } catch (IOException e) {
            // logging and exception handling should go here
        }
    }
}).start();

PipedOutputStream to InputStream

If you don't have a ByteArrayOutputStream, you can use the following code:

PipedInputStream in = new PipedInputStream();
final PipedOutputStream out = new PipedOutputStream(in);
new Thread(new Runnable() {
    public void run() {
        try {
            // write the original OutputStream to the PipedOutputStream
            // note that in order for the below method to work, you need to ensure that the data has finished writing to the OutputStream
            originalOutputStream.writeTo(out);
        } catch (IOException e) {
            // logging and exception handling should go here
        } finally {
            // close the PipedOutputStream here because we're done writing data
            // once this thread has completed its run
            if (out != null) {
                // close the PipedOutputStream cleanly
                out.close();
            }
        }
    }
}).start();

Using piped streams offers several benefits, including:

• Concurrency: Data transfer occurs in a separate thread, enabling parallel processing.
• Memory efficiency: Piped streams avoid creating additional buffer copies, reducing memory consumption.

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