For me the best in this case is Apache commons-io to handle this and similar tasks.
The IOUtils type has a static method to read an InputStream and return a byte[].
InputStream is;
byte[] bytes = IOUtils.toByteArray(is);
Internally this creates a ByteArrayOutputStream and copies the bytes to the output, then calls toByteArray().
UPDATE: as long as you have the byte array, as @Peter pointed, you have to convert to ByteBuffer
ByteBuffer.wrap(bytes)
JAVA 9 UPDATE: as stated by @saka1029 if you’re using java 9+ you can use the default InputStream API which now includes InputStream::readAllBytes function, so no external libraries needed
InputStream is;
byte[] bytes = is.readAllBytes()