How to put data from an OutputStream into a ByteBuffer?

Question

In Java I need to put content from an OutputStream (I fill data to that stream myself) into a ByteBuffer. How to do it in a simple way?

Answer 1

There is a more efficient variant of @DJClayworth's answer.

As @seh correctly noticed, ByteArrayOutputStream.toByteArray() returns a copy of the backing byte[] object, which may be inefficient. However, the backing byte[] object as well as the count of the bytes are both protected members of the ByteArrayOutputStream class. Hence, you can create your own variant of the ByteArrayOutputStream exposing them directly:

public class MyByteArrayOutputStream extends ByteArrayOutputStream {
  public MyByteArrayOutputStream() {
  }

  public MyByteArrayOutputStream(int size) {
    super(size);
  }

  public int getCount() {
    return count;
  }

  public byte[] getBuf() {
    return buf;
  }
}

Using this class is easy:

MyByteArrayOutputStream out = new MyByteArrayOutputStream();
fillTheOutputStream(out);
return new ByteArrayInputStream(out.getBuf(), 0, out.getCount());

As a result, once all the output is written the same buffer is used as the basis of an input stream.