How to put data from an OutputStream into a ByteBuffer?

Though the above-mention answers solve your problem, none of them are efficient as you expect from NIO. ByteArrayOutputStream or MyByteArrayOutputStream first write the data into a Java heap memory and then copy it to ByteBuffer which greatly affects the performance.

An efficient implementation would be writing ByteBufferOutputStream class yourself. Actually It's quite easy to do. You have to just provide a write() method. See this link for ByteBufferInputStream.

You can create a ByteArrayOutputStream and write to it, and extract the contents as a byte[] using toByteArray(). Then ByteBuffer.wrap(byte []) will create a ByteBuffer with the contents of the output byte array.

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.