RFR: JDK-8306819: Consider disabling the compiler's default active annotation processing

[Rob Bygrave]

On Mon, 12 Jun 2023 23:49:51 GMT, Joe Darcy <darcy at openjdk.org> wrote:

> Change annotation processing to be opt-in rather than opt-out.

> Based on my observations, nowadays annotation processing is hardly used anymore and if it's used it mostly occurs after the actual compilation process in a build tool

Annotation processing use is fundamental to newer frameworks like Quarkus, Micronaut and Spring framework is also adopting more use of annotation processing. This is because it enables frameworks and libraries to move some work from runtime to build time for reduced memory consumption and faster startup (replacing reflection, dynamic proxies etc).

For myself I have annotation processors that:
- Perform Dependency Injection 
- Generate JSON adapters (think Jackson without any runtime reflection, all the fastest JSON libs use annotation processing)
- Generate Rest Http Clients (think JAX-RS Jersey client implemented as source code generation)
- Generate Http Server controllers (think JAX-RS Jersey server implementation as source code generation)
- Generate Validation adapters (think Jakarta Validation implemented as source code generation)
- Generate "Builders" for record types

Its also common to use annotation processing for mapping/adapting between types (e.g. MapStruct)

I'd suggest the use of annotation processing is actually taking off in the last say 5 years as we especially look to deploy applications into cloud hosted environments where we pay money for the amount of CPU and Memory used.  Annotation processing means that a **_LOT_** of libraries can move some or even most of their functionality to build time reducing the CPU required to initialise the library and the total memory consumption.


> Based on my observations, nowadays annotation processing is hardly used anymore

To me I see the exact opposite happening and it's driven by cloud adoption and the desire to move work from runtime to build time.

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PR Comment: https://git.openjdk.org/jdk/pull/14432#issuecomment-1712674323