Adding OutputStream.write(ByteBuffer) could improve the following cases:

a) OpenJDK's implementations of InputStream.transferTo(OutputStream) have to copy the buffers, as passing byte arrays to untrusted OutputStream implementations can create security issues: https://github.com/openjdk/jdk/pull/16879 , https://github.com/openjdk/jdk/pull/16893

b) Channels.newChannel(outputStream) has to copy the data, however, it could pass the incoming ByteBuffer without copies: https://github.com/openjdk/jdk/blob/d23f4f12adf1ea26b8c340efe2c3854e50b68301/src/java.base/share/classes/java/nio/channels/Channels.java#L390

c) Passing naked byte[] to untrusted OutputStream implementations is not secure, so moving to ByteBuffer API + read-only byte buffers should improve the security.

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ByteBuffer has .asReadOnlyBuffer(), and it should be reasonably safe to pass read-only byte buffers without copying them, so if OutputStream.write(ByteBuffer) was there, then implementation of ByteArrayInputStream.transferTo could be improved as out.write(ByteBuffer.wrap(buf, ...));

It would avoid copying the data, and it might enable more efficient implementation when sending the data to a file or network.

The default implementation of OutputStream.write(ByteBuffer buffer) could be something like if (buffer.hasArray()) { write(buffer.array(), ...); } else { write(copy the contents of the buffer to a temp array); }

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A broader idea might be "OutputStream extends WritableByteChannel", however, Channel has isOpen() method, and I do not see what a default implementation in OutputStream could look like.

Alan Bateman (see https://github.com/openjdk/jdk/pull/16879#discussion_r1413859006): "Interesting but I don't think that is workable as WBC specifies that writes executing serially and also specifies the exception when attempting to write to a closed channel, both of which are not compatible with OutputStream."