That's not exactly what Netty does. It does allocate large direct buffers and slices up pieces, but the pieces are not pooled because they are never returned to Netty. For this reason they also don't need to implement malloc in Java. All the slices reference the "parent" buffer, and once they are all collected, the parent can be collected as well. When there is no more room in the buffer, another one is allocated. (I just read that code last week because I wanted to know what was going on precisely because there was no way of returning a sliced buffer to the pool).

It is very easy (and efficient) to implement java.io input and output streams backed by a buffer (as long as the buffer doesn't have to grow).