Summary
-------

Improve the `sjavac` tool so that it can be used by default in the JDK
build, and generalize it so that it can be used to build large projects
other than the JDK.


Goals
-----

Due to various issues relating to stability and portability, `sjavac` is
not used by default in the JDK build scripts. The first goal of this JEP
is to resolve these issues. This involves making sure the tool produces
reliable results on all software/hardware configurations at all times.

The overarching goal is to improve the quality of `sjavac` to the point
where it can serve as a general purpose `javac` wrapper able to compile
large arbitrary Java projects.

A follow-on project will explore how `sjavac` is to be exposed in the JDK
tool chain, if at all; this might be a separate supported standalone
tool, an unsupported standalone tool, integration with `javac`, or
something else.


Non-Goals
---------

It is not a goal of this JEP to introduce new features that are not in
place today, not required by our own build scripts, and do not contribute
towards a general purpose `javac` wrapper.  The intent is not to do a
rewrite of the wrapper. Current functionality that works well will be
kept as-is.

The incorporation of `sjavac` into the product, if desired, will be the
subject of a separate JEP.


Motivation
----------

The current implementation has proved to be useful and does indeed
improve build speed and allow for incremental builds. The quality of the
code and stability of the tool as a whole, however, is not satisfactory
and it is certainly not ready for public release.


Description
-----------

This work builds upon the previously-delivered implementation described
in [JEP 139](http://openjdk.java.net/jeps/139). The work described in
this JEP will be performed gradually by writing, testing, and committing
incremental patches to the existing implementation.

Initially the work focus on resolving issues related to building the
JDK. This includes addressing:

  - [JDK-8014510](https://bugs.openjdk.java.net/browse/JDK-8014510)
    Fix `sjavac` on all platforms in JPRT
  - [JDK-8019239](https://bugs.openjdk.java.net/browse/JDK-8019239)
    Properly clean up `sjavac` heuristics

Other issues that will be dealt with include:

  - Refactor the protocol, possibly by leveraging RMI
  - Make the option names and syntax more consistent with those of
    `javac`
  - Replace `System.{out,err}` logging with something more suitable
  - Increase parallelism by sharing more resources in `javac`, where
    possible

Issues to be addressed further down the road include, but are not limited
to:

  - Internationalization
  - Support for Jigsaw

Certain adaptations of `javac` will be required. Most notably, `javac`
will have to be refactored to allow for multiple concurrent compilations
to share common data structures to enable better speed up and higher CPU
utilization on multicore machines.


Testing
-------

Beyond the usual mandatory unit tests, `sjavac` will be thoroughly tested
on all platforms since parts of the tool involve platform-specific
aspects such as file paths and the spawning of background processes.

Furthermore, `sjavac` will be stress-tested with concurrency in mind,
since the server-client architecture is inherently parallel.


Dependences
-----------

The work proposed by this JEP builds upon the result of
[JEP 139](http://openjdk.java.net/jeps/139): Enhance `javac` to Improve
Build Speed.