StringDecoder causes direct allocation of ByteBuffer

[xavier-b]

Since my switch to Spring Boot 3, I have a lot of direct allocations in my metrics.
Indeed, my app use StringDecoder via, for example, @RequestBody String body

In org.springframework.core.codec.StringDecoder, when dataBuffer is backed by an NIO direct buffer, decode method use dataBuffer.toByteBuffer() which will call java.nio.ByteBuffer.allocateDirect(int).

In java.nio.ByteBuffer javadoc we can read :

The buffers returned by this method typically have somewhat higher allocation and deallocation costs than non-direct buffers. [...]
It is therefore recommended that direct buffers be allocated primarily for large,
long-lived buffers that are subject to the underlying system's native I/O operations.
In general it is best to allocate direct buffers only when they yield a measurable
gain in program performance.

Goal of this code in StringDecoder was to decode with a charset, so maybe it's better to use dataBuffer.toString(charset) which doesn't do direct allocation.
I see the same problem in org.springframework.http.codec.FormHttpMessageReader.

[poutsma]

Merged, thanks for fixing this!