Convert OutputStream to ByteArrayOutputStream

You could use the writeTo method of ByteArrayOutputStream.

ByteArrayOutputStream bos = new ByteArrayOutputStream();
byte[] bytes = new byte[8];
bos.write(bytes);
bos.writeTo(oos);

You can create an instance of ByteArrayOutputStream. You then need to write the data to this ByteOutputStream instance and then using the writeTo method, which accepts an OutputStream, you can enable the ByteArrayOutputStream to write the output, to the instance of OutputStream which you passed as the argument.

Hope it works!

There are multiple possible scenarios:

a) You have a ByteArrayOutputStream, but it was declared as OutputStream. Then you can do a cast like this:

void doSomething(OutputStream os)
{
    // fails with ClassCastException if it is not a BOS
    ByteArrayOutputStream bos = (ByteArrayOutputStream)os;
...

b) if you have any other type of output stream, it does not really make sense to convert it to a BOS. (You typically want to cast it, because you want to access the result array). So in this case you simple set up a new stream and use it.

void doSomething(OutputStream os)
{
    ByteArrayOutputStream bos = new ByteArrayOutputStream();
    bos.write(something);
    bos.close();
    byte[] arr = bos.toByteArray();
    // what do you want to do?
    os.write(arr); // or: bos.writeTo(os);
...

c) If you have written something to any kind of OutputStream (which you do not know what it is, for example because you get it from a servlet), there is no way to get that information back. You must not write something you need later. A solution is the answer b) where you write it in your own stream, and then you can use the array for your own purpose as well as writing it to the actual output stream.

Keep in mind ByteArrayOutputStreams keep all Data in Memory.