Make java.nio.ByteBuffer a sealed class

[Paul Sandoz]

I don’t see much advantage to this. Most of the (byte) buffer hierarchy is package private. Further, the hierarchy is set up for convenience: DirectByteBuffer extends MappedByteBuffer [*], even though not all DirectByteBuffer instances are mapped.

Internally we can assume if a ByteBuffer instance is also an instance sun.nio.ch.DirectBuffer (which in turn implies that ByteBuffer.isDirect() == true) then it’s a direct buffer. Otherwise, its a heap buffer and if hasArray() == true then the underlying array is accessible.

In the referenced code there is a precondition that the byte buffer is not read-only. Therefore, we can assume for a heap buffer the src array is accessible (use an assert if you wish).

My hope is this kind of code can improve when we can use MemorySegment.

Paul.

[*]
public abstract class MappedByteBuffer
    extends ByteBuffer
{

    // This is a little bit backwards: By rights MappedByteBuffer should be a
    // subclass of DirectByteBuffer, but to keep the spec clear and simple, and
    // for optimization purposes, it's easier to do it the other way around.
    // This works because DirectByteBuffer is a package-private class.

> On Jul 23, 2021, at 3:59 AM, Sebastian Stenzel <sebastian.stenzel at gmail.com> wrote:
> 
> Any thoughts on this?
> 
>> On 8. Jul 2021, at 16:54, Sebastian Stenzel <sebastian.stenzel at gmail.com> wrote:
>> 
>> A while back, when working on JDK-8264110, we had a discussion about the nature of ByteBuffers, where I asked whether we could eventually make this a sealed class. [1]
>> 
>> The idea is, that basically there are only two kinds of ByteBuffers:
>> - HeapByteBuffers
>> - MappedByteBuffer → DirectByteBuffer
>> 
>> While the former is backed by an array, the latter isn't, which leads to branching such as `if (buffer.hasArray())`, which could be simplified, if we knew for sure to deal with either of the two kinds.
>> 
>> Now, that sealed classes are stable with JDK 17, I'd like to ask for your approval to make ByteBuffer a sealed class, so we can rely on an exhaustive list of ByteBuffer "flavours" at compile time.
>> 
>> This is a requirement for follow-up tasks, I'd like to work on, such as JDK-8264341.
>> 
>> 
>> [1] https://github.com/openjdk/jdk/pull/3217#discussion_r602528233
>