[MCOMPILER-515] This plugin is not "incremental"

[cstamas]

Drop the "incremental" bit of this plugin, make it just dumb always redo everything. The incremental bits just introduced bugs that exist for over 10 years, and just make us look bad.

So just let "incremental" part go away.

This change renders 2 ITs broken:

[ERROR] The following builds failed:
[ERROR] *  MCOMPILER-500-package-info-incr/pom.xml
[ERROR] *  default-incremental-disable/pom.xml

That IMHO should be dropped as well, if this PR goes in.

---

https://issues.apache.org/jira/browse/MCOMPILER-515

[cstamas]

As expected, the two ITs did fail on GH CI.

[cstamas]

As expected, without the two IT it all pass OK.

[olamy]

-1
I'm happy to not have to recompile everything all the time.
If you think there is a bug when changing the configuration it will be better to fix it correctly (e.g comparing inputs between 2 runs).

[slawekjaranowski]

Also -1 from me.

Maybe it is not incremental compile - but detecting if module should be recompiled.

Detecting on change for project configuration should be fixed instead.

[slawekjaranowski]

.

[cstamas]

@olamy, good, am really happy for you. So, there is "nothing to be seen here", everything is cool, is it?

I have to admit, I was unaware of MCOMPILER project state: "Fun" starts that this issue was reported 10 years ago, but was closed due inactivity: https://issues.apache.org/jira/browse/MCOMPILER-213 But even more "fun" can be seen when one search for "MCOMPILER compiler incremental" in JIRA: https://issues.apache.org/jira/issues/?jql=project%20%3D%20MCOMPILER%20AND%20text%20~%20%22compiler%20incremental%22%20order%20by%20lastViewed%20DESC

There are a TON of issues, that are mostly neglected. And all is about "incremental...". There are (relatively) young issues, only 4 years without feedback https://issues.apache.org/jira/browse/MCOMPILER-345 but there are old issues as well, that got closed, while users commented on them "this is still an issue" https://issues.apache.org/jira/browse/MCOMPILER-187 There are REAL issues where users are baffled WHAT AND HOW this plugin is meant to work at all https://issues.apache.org/jira/browse/MCOMPILER-209 -- this one is really funny, and all this is just the tip of the iceberg. Seems nobody understands this "incremental" feature.

And now about the sad part: this "feature" is present in ONLY ONE class, that today has 2000 LOC. This PR removes cca 400 LOC from it (roughly 20%), and it "still works", no UT affected at all, and only two ITs out of 60 were affected. I hope we do understand what this means?

Maybe not aware, but incremental compilation (and incremental build for that matter) has been solved for Maven years ago. There are situations, where things go so bad, that really just a "let's redo it from scratch" can help, Everything else would just potentially worsen the already bad situation. And this plugin (should be) one of the "crux" of Maven (like m-install-p/m-deploy-p). It's maybe me, who is not satisfied with "current state of affairs" and want to change something about it? Maybe.

By vetoing this PR, you choose really the only possible bad outcome: nothing to happen, and continue this "status quo" of broken state. A bit better, but still bad outcome will be that someone will dedicate his time to "fix this", but we all know that solution is about "inputs and outputs", and this plugin and this "incremental" has no clue about that, so wasted time. I just say "let's burn it".

And one more thing: are there some measurements, some numbers, perf tests, whatever, that prooves the gain (seemingly at cost of correctness) Maven "wins" with this "incremental" feature?

[olamy]

maybe incremental is not the right name for this but yes on a 300 modules project not recompiling everything all the time makes a big difference at the end of the day

[rmannibucau]

think there are multiple points mixed there:

• Default behavior (incremental=true) is to not recompile if upstream didn't change -> we must keep it IMHO
• Toggle behavior (incremental=false) is to only recompile what changed (if you have 10 source files and modified 1 then you only recompile this last one). This mode is broken I agree for a lot of reasons in java ecosystem (annot proc being an example).

So overall I think it makes sense to keep at least the default behavior

[cstamas]

As everyone talks how this is "must to have" but nobody provided any number to support this claim, I tried to convince myself this is "must to have". So...

Subject build:
Jetty branch jetty-10.0.x (6e82e70edf6a106b9fd08ccfacd840efef8a95f9) with 172 modules from https://github.com/eclipse/jetty.project

Preparations

Built this checked out project "as usual" (by the book) using -DskipTests as README recommends to prime local repo. After local repo "primed" (to have no remote fetch happens), spotted that asciidoctor-maven-plugin always goes remote, so removed documentation module that uses it, to prevent huge variance (as it goes directly to central, not local MRM).

Then, as 2nd (w/o clean) invocation consistently failed, had to remove tests-integration module as well, to be able to test "incremental" use case.

This leaves us with 170 modules.

Finally,. changed jetty to use locally built 3.11.0-SNAPSHOT of maven-compile-plugin:

• master (5be3163)
• vs PR (7b59074)

Full diff of my local changes:
https://gist.github.com/cstamas/b637f4bce0967836cf835cf21e105ab9

Used Invocations

I used following 3 invocations to time:
1st invocation: mvn clean install -DskipTests
2nd invocation: mvn install -DskipTests
3rd invocation: mvn verify -DskipTests

Used Maven "total time" output as measure.

With m-c-p:master

1st master: 02:10 min
2nd master: 01:39 min
3rd master: 01:35 min

With m-c-p:PR

1st PR: 02:11 min
2nd PR: 01:35 min
3rd PR: 01:36 min

Conclusion

Please, somebody, anybody, repeat these (the numbers cannot be "cross" compared, only can be compared among runs on same HW/env), and as always, "this is not a proper benchmark" type of notice... but.

So far, it seems Jetty project (170) modules is not "large" enough to produce noticeable difference?

[olamy]

as you are using a project which need install this doesn't work....
because some artifacts are part of the reactor and rebuild (e.g re package) so they are detected as new dependency and so re compiled....
just look the output and it will say everything is recompiled...
I have some work locally which handle this case but never pushed this.

[cstamas]

Nope, I removed bits needing install, 3rd invocation is mvn verify, please reread.

[rmannibucau]

Here is a simple project showing the usage:

rmannibucau@rmannibucau-yupiik:/opt/rmannibucau/dev/demo-sunny $ mvn clean
[INFO] Scanning for projects...
[INFO] 
[INFO] -----------------------< org.example:demo-sunny >-----------------------
[INFO] Building demo-sunny 1.0-SNAPSHOT
[INFO] --------------------------------[ jar ]---------------------------------
[INFO] 
[INFO] --- maven-clean-plugin:2.5:clean (default-clean) @ demo-sunny ---
[INFO] Deleting /opt/rmannibucau/dev/demo-sunny/target
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  0.158 s
[INFO] Finished at: 2022-12-13T12:21:00+01:00
[INFO] ------------------------------------------------------------------------
rmannibucau@rmannibucau-yupiik:/opt/rmannibucau/dev/demo-sunny $ mvn compile
[INFO] Scanning for projects...
[INFO] 
[INFO] -----------------------< org.example:demo-sunny >-----------------------
[INFO] Building demo-sunny 1.0-SNAPSHOT
[INFO] --------------------------------[ jar ]---------------------------------
[INFO] 
[INFO] --- maven-resources-plugin:2.6:resources (default-resources) @ demo-sunny ---
[INFO] Using 'UTF-8' encoding to copy filtered resources.
[INFO] Copying 0 resource
[INFO] 
[INFO] --- maven-compiler-plugin:3.10.1:compile (default-compile) @ demo-sunny ---
[INFO] Changes detected - recompiling the module!
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  1.391 s
[INFO] Finished at: 2022-12-13T12:21:04+01:00
[INFO] ------------------------------------------------------------------------
rmannibucau@rmannibucau-yupiik:/opt/rmannibucau/dev/demo-sunny $ mvn compile
[INFO] Scanning for projects...
[INFO] 
[INFO] -----------------------< org.example:demo-sunny >-----------------------
[INFO] Building demo-sunny 1.0-SNAPSHOT
[INFO] --------------------------------[ jar ]---------------------------------
[INFO] 
[INFO] --- maven-resources-plugin:2.6:resources (default-resources) @ demo-sunny ---
[INFO] Using 'UTF-8' encoding to copy filtered resources.
[INFO] Copying 0 resource
[INFO] 
[INFO] --- maven-compiler-plugin:3.10.1:compile (default-compile) @ demo-sunny ---
[INFO] Nothing to compile - all classes are up to date
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  0.572 s
[INFO] Finished at: 2022-12-13T12:21:07+01:00
[INFO] ------------------------------------------------------------------------

as you can see gain is quite huge

[cstamas]

@rmannibucau i hope you understand your example does not make sense? Can you show me a real life example, a full build? What total gain are we talking about here?

[olamy]

Nope, I removed bits needing install, 3rd invocation is mvn verify, please reread.

read my comment especially on the reactor and rebuild of artifacts.

what do you see in the output
Changes detected - recompiling the module!
or
Nothing to compile - all classes are up to date

because for every build (except the module jetty-slf4j-impl everything is recompiled)
if not please post the output.

mvn clean install -DskipTests

for jetty-10.0.x branch you have 02:10 min?? really?? you have a very very beefy machine so recompiling or not doesn't make any difference for you....
I know this build and 2:10m for clean install means you have a very beefy machine.

[cstamas]

Full logs of mvn verify -DskipTests are here (they are huge) https://drive.google.com/drive/folders/1SIaAbOLegBxCLu87jB3CFtfe3cHmid2P?usp=sharing

(fixed link, sorry)

[slawekjaranowski]

My test

• Classes in target prod and unit: 4883
• Maven modules: 326
• JDK 1.8
• Os: MacOs 12.6.1 - 2,6 GHz 6-Core Intel Core i7, 32 GB 2667 MHz DDR4
• m-c-p 3.10.1

Scenario 1

mvn test-compile
mvn test-compile | grep -e "Nothing to compile\|Changes detected\|Total time" > 1.txt

Scenario 2

mvn clean
mvn test-compile | grep -e "Nothing to compile\|Changes detected\|Total time" > 2.txt

Result 1

grep "Changes detected" 1.txt | wc -l
       1

 grep "Nothing to compile" 1.txt | wc -l
     313

[INFO] Total time:  01:02 min

Result 2

grep "Changes detected" 2.txt | wc -l
     314

grep "Nothing to compile" 2.txt | wc -l
       0

[INFO] Total time:  05:59 min

[cstamas]

@rmannibucau a bit of simple math:
your "full" compile: 1391ms
your "incremental" compile: 572ms
difference is: 819ms
On "big" build (a 300 module build was mentioned): 300 x 819ms = 245700ms = 245,7s = 4,095min

So, the diff you showed would produce on 300 module build total time difference of 4 minutes? Am interested in how much percentage these 4 minutes are of the full build time....30%? 10%? 1%? 0.05%? Let's find out!

[cstamas]

@slawekjaranowski please compare m-c-p master (or any released) vs this PRl 😄 As your test really compares m-c-p against m-c-p, not m-c-p master/released against PR.

[rmannibucau]

@cstamas as Olivier mentionned, your build does not use this feature so you compare twice the same things. Also my example shows the real gain. I tested on a work project - sorry, there I can't share the full outputs but it is similar to the demo I shared but with 31 modules: 17s for the first build, 10s for the second using the command mvn package -pl '-documentation' -DskipTests - note that install does not change this +-0.5s - and validating Nothing to compile - all classes are up to date is actually used.
Finally, doing a bit of simple math the 4mn are ~50% of the build as you can see yourself, now if you add tests and your tests take 10mn I agree with you but it is another problem. If your point is that compilation is always a small amount of time of a full build then this is highly wrong cause you think as a CI but a dev does not always run the full project build but working you often have 'shortcutted' builds to go faster and there, +-1mn is something - human time - so yes 4mn is a lot, means you can build x2 so likely produce way more.

[cstamas]

@rmannibucau could you please build locally this PR and try your private project with that same steps? Just interested in times.... (the "31 modules: 17s for the first build, 10s for the second using the command mvn package -pl '-documentation' -DskipTests")

[rmannibucau]

@cstamas 16s first time then always ~14s so overall the diff logic is worth it even if it costs some time - guess we can make it faster there too tracking files more efficiently in the project but overall this PR has a negative impact on the overall build time.

[cstamas]

@rmannibucau ok, thanks for doing this. As I thought: PR is clearly better at "full recompile" case of m-c-p master

[rmannibucau]

As I thought: PR is clearly better at "full recompile" case of m-c-p master

@cstamas you mean slower and produces the same output? master does by default a full rebuild in 10s whereas your PR does a full rebuild in 14s

[cstamas]

Ok, tried with another project, Maven this time:

maven master: 984f73dc7ca2515ee520fbded3382820f62116e9

Invocations

1st: mvn clean install -DskipTests
2nd: mvn verify -DskipTests

With master

1st: 49.868s
2nd: 36.577s

With PR

1st: 48.978s
2nd: 41.383s

And again, the feature w/ master did not work (!) as I understand, as there are inter-dependencies in reactor?

What's the point of multi module build if this feature does NOT work with it? Am I right that this feature works on multi module build ONLY if there are no inter project dependencies? I just realized what @olamy was saying....

[cstamas]

@cstamas you mean slower and produces the same output? master does by default a full rebuild in 10s whereas your PR does a full rebuild in 14s

The I don't understand this: "17s for the first build, 10s for the second", isn't first the "full"?

[rmannibucau]

The I don't understand this: "17s for the first build, 10s for the second", isn't first the "full"?

@cstamas the first is the run after a clean which means this PR or master will do the same - this is also why it runs in the same time. All consecutive builds are package/install without clean - exact same command than the first run, dev case - and there your PR is significantly slower so you should compare the > 1 build durations and not the first one which is globally the same without the state checking (but this one is almost free as you can see - at least in the error rate).

[cstamas]

Recap what we have so far:

• "incremental" feature may bring 4 minute gain on 300 module build, unless
• there are inter-module dependencies, as in that case this feature would work only on "root" modules?

Is this right?

[rmannibucau]

"incremental" feature may bring 4 minute gain on 300 module build, unless

If we ignore the figures right - highly depends your build, tomee container/openejb-core module for ex, it is 14s -> 4s (1 module so if you do the math you almost save 1h for your example ;)).

there are inter-module dependencies, as in that case this feature would work only on "root" modules?

Kind of, "magic" happens there https://github.com/apache/maven-compiler-plugin/blob/master/src/main/java/org/apache/maven/plugin/compiler/AbstractCompilerMojo.java#L897 and agree the rule can be enhanced to better manage multi-reactor builds, can be a great enhancement/PR to do.

[cstamas]

If we ignore the figures right - highly depends your build, tomee container/openejb-core module for ex, it is 14s -> 4s (1 module so if you do the math you almost save 1h for your example ;)).

Sure, we can come up with numbers as we like, but I will shut up in a moment this very feature saves 1h on a real life project out there.

Kind of, "magic" happens there https://github.com/apache/maven-compiler-plugin/blob/master/src/main/java/org/apache/maven/plugin/compiler/AbstractCompilerMojo.java#L897 and agree the rule can be enhanced to better manage multi-reactor builds, can be a great enhancement/PR to do.

Sure, but this also means that almost no multi module build benefits from this feature, as the sole idea of multi module build is usually to "depend on each other". I can imagine some "lab" project where you have 300 modules that do not depend on each other, but I guess this cannot be called "majority", unless am mistaken.

[slawekjaranowski]

Master

commit 5be3163

Result 1

• next execution - rebuild is skipped

[INFO] Total time:  01:12 min

Result 2

• after clean

[INFO] Total time:  05:42 min

PR 160

commit 7b59074

Only scenario 2, many times the same command - mvn test-compile on the same workspace,
all modules are recompiled everytime, sample of time:

[INFO] Total time:  06:25 min
[INFO] Total time:  05:34 min
[INFO] Total time:  05:24 min

take similar time like mvn clean ... in my case

so difference from 1 to 5 minutes is important for project which I work at usual day, and many time I need to rebuild

[rmannibucau]

that do not depend on each other, but I guess this cannot be called "majority", unless am mistaken.

Depends how you build the multi-modules projects. Often you have some "core" bottleneck in the build graph then a tons of children (think camel if you want, core then most of the modules are children), then if you run a command bypassing the core - built manually before with install - then you get the benefit from it.
Fully agree it would be better to not have to handle the commands manually but as of today it works and saves time - at least on some big projects I'm working on, agree it can maybe not be the most ones but since code is there I would rather go in the direction to enhance it instead of dropping it otherwise we drop most of the plugin code and plexus abstraction to purely leverage tool provider and be it for the plugin at the end - can be an option but at the opposite of current design which is to make most cases working by default/convention.

[bmarwell]

I agree with @rmannibucau here. It is very dependent of the examples and really changes your output.
Let's keep the default behaviour, but maybe rename it.
If I am not mistaken, it skips entire modules? In that case name it appropriately.

[olamy]

If we ignore the figures right - highly depends your build, tomee container/openejb-core module for ex, it is 14s -> 4s (1 module so if you do the math you almost save 1h for your example ;)).

Sure, we can come up with numbers as we like, but I will shut up in a moment this very feature saves 1h on a real life project out there.

Kind of, "magic" happens there https://github.com/apache/maven-compiler-plugin/blob/master/src/main/java/org/apache/maven/plugin/compiler/AbstractCompilerMojo.java#L897 and agree the rule can be enhanced to better manage multi-reactor builds, can be a great enhancement/PR to do.

Sure, but this also means that almost no multi module build benefits from this feature, as the sole idea of multi module build is usually to "depend on each other". I can imagine some "lab" project where you have 300 modules that do not depend on each other, but I guess this cannot be called "majority", unless am mistaken.

no it depends if you reach install phase or not... I tried to explain previously in this issue and this has already been discussed here https://lists.apache.org/thread/1l87rfqk7k26lgh07vt8w6367qgp3sct

[gnodet]

I agree to drop the "incremental" boolean parameter, but not the default behaviour. Not for performance reasons, but just because the output much more often broken when you're developing : when setting incremental to false, the stale check is not done and the compiler only rebuilds the changed files, even if you changed a dependency. What this means is that if you change a method signature, do a verify, the build works, but it will fail at runtime. This behaviour is explained in the javadoc but I can't see any value in such behaviour.

Example steps on maven:

• change the compiler globally by adding:

      <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-compiler-plugin</artifactId>
        <configuration>
          <useIncrementalCompilation>false</useIncrementalCompilation>
        </configuration>
      </plugin>

in the main pom.xml

• run mvn install -DskipTests
• edit the api/maven-api-core/src/main/java/org/apache/maven/api/Dependency.java and change the return type of the isOptional() method to a String
• run mvn install -DskipTests

The build succeeds, whereas if you run mvn clean install -DskipTests, it will fail.

So I'm +1 to get rid of the useIncrementalCompilation=false behaviour, but definitely not the useIncrementalCompilation=true one. I'm also +1 to stop calling it incremental.

[gnodet]

On the performance side, I've been trying to ensure that plugins don't modify any output file without any actual change. So having the compiler plugin behave in this way is really a big win. If the build is setup correctly, the second maven run should not modify any artifact at all (including packaged jars). I would -1 any change that does not keep such a behaviour. That does not mean any kind of incremental compilation : the current behaviour is all or nothing, i.e. any change will recompile the whole module, but I'm fine with that.

[gnodet]

-1 on the idea to drop the current behaviour

[hunterpayne]

Incremental building is an important feature. If you need it, you need it. Most projects don't but some very important projects do and they need something like this the most. I think the right solution is a compromise where we scale back how this works. The problem is that computing the right set of code to recompile is hard. So let's just make that part easy.

Perhaps make the incremental based upon something higher level that where we are now. If that's not possible, then the solution is to actually maintain a map of files to classes to dependent classes/files from the last build and to use that to compute what is built. I think that is the buggy part as that that map is hard to maintain. Perhaps make that map manually editable by devs. We only need this feature in dev environments anyway.

TLDR This is an important feature so let's find a way to reduce the complexity to make it work and be maintainable.

[cstamas]

I agree that "incremental" is very important feature. My stance is that this implementation is bad/wrong.

[hunterpayne]

I agree.  I am asking if there is a way we can relax the feature/requirement to make it easier to implement and maintain. On Monday, April 22, 2024 at 12:07:46 PM PDT, Tamas Cservenak ***@***.***> wrote: I agree that "incremental" is very important feature. My stance is that this implementation is bad/wrong. — Reply to this email directly, view it on GitHub, or unsubscribe. You are receiving this because you commented.Message ID: ***@***.***>

[cstamas]

Today, if one needs fully incremental (and build avoidance) with current stable Maven 3.9.x, one should use Takari Lifecycle plugin. But even then you should use Eclipse JDT not javac. But, in turn, with JDT, you get other nice things like enforcing dependency usage and so on...

http://takari.io/book/index.html

[hunterpayne]

Great, so we have 2 buckets to break these issues into.  1) issues with the difference in behavior with javac (vs JDT) and 2) issues with the difference in behavior with this plugin and the Takari one.  I'm curious, what are the issues in the first bucket?  Perhaps the target should be to get the maven compiler to work on JDT as a requirement instead of javac and JDT.  Maybe that's a goal we can achieve.

[desruisseaux]

While incremental builds is important, my reading of the source code inherited from Maven 3 suggests that in its current state, incremental build has bugs causing the opposite effect than what the plugin wanted to achieve. We could try to fix the Maven 3 compiler plugin, but I do not know if it is worth the effort given that the plugin is getting a significant rewrite for Maven 4.

The current useIncrementalCompilation parameter is a boolean flag which is actually mixing at least two aspects: whether to check if a dependency changed, and how to detect which source files changed. I suggest to deprecate that parameter and replace it with a new incrementalCompilation parameter (same name but without use prefix). The value of that parameter would be a comma-separated list of the following:

• dependencies: check whether a dependency changed.
• sources: check whether a source file has been updated, or whether source files were added or removed.
• classes: check whether a source file is more recent than its .class file.
• modules (new): don't try to do incremental build in Maven, but delegate that task to javac instead. This is possible only with modular projects. It uses the --module compiler option, which is an alternative to enumerating all source files on the command-line.

Equivalences with the current parameter:

• useIncrementalCompilation = true is equivalent to incrementalCompilation = dependencies,sources (except maybe for the cases of newly added source files).
• useIncrementalCompilation = false is equivalent to incrementalCompilation = classes.

For an initial version of the Maven 4 compiler plugin, detection of changes would be based only on file timestamps. This is the same strategy than the current plugin. This approach would be unable to detect that a class needs to be recompiled because it depends on another class which has been changed. However, the current plugin cannot detect such cases neither, so we don't lost anything. If we support that feature in the future, it would be one new value in the above-cited list of comma-separated values.

[hunterpayne]

Thank you for the summary of the state of the code, that is very helpful. I can tell you with 100% certainty that only tracking file timestamps won't work and will be a bug farm.  You must know which .class files are generated from which .java files and in addition know which .class files depend upon which other .class files.  If this isn't possible, then it isn't possible to implement this feature in a maintainable way.  So either we need to track this additional information, or relax the requirement to for example the module level or package level and track dependencies at that level.  I am OK with either path but we shouldn't go ahead with only tracking file timestamps and acting like that is a working incremental build feature. EDITED for clarity and to remove all the cc includes from my email browser.

[desruisseaux]

I can tell you with 100% certainty that only tracking file timestamps won't work and will be a bug farm.

It is incomplete, but this is what the current plugin and the javac command line do. Implementing a better incremental build would probably be a multi-steps process. In medium term, my goal is to migrate the plugin from Maven 3 / Plexus API to Maven 4 / javax.tools API in order to improve JPMS support. Fixing some of the incremental build issues is a side effect of this work. The goal for now is only to preserve the existing functionalities (with bug fixes).

After a refactored compiler plugin become stable enough, it would be possible to investigate how to do better. The proposal to use comma-separated values in incrementalCompilation parameters leaves room for that.

[hunterpayne]

I agree it is a lot of work. But you can't say this implementation will work either. It will be a bug farm. Inner classes, enums, and interface inheritance will all probably have weird and unexpected issues in more complex cases. And if there are classes compiled by other plugins like scalac? That's why bugs like this exist, because the mapping between java files and class files isn't clean.

[olamy]

I can tell you with 100% certainty that only tracking file timestamps won't work and will be a bug farm.

It is incomplete, but this is what the current plugin and the javac command line do. Implementing a better incremental build would probably be a multi-steps process. In medium term, my goal is to migrate the plugin from Maven 3 / Plexus API to Maven 4 / javax.tools API in order to improve JPMS support. Fixing some of the incremental build issues is a side effect of this work. The goal for now is only to preserve the existing functionalities (with bug fixes).

remove plexus-compiler layer but what about other compilers?

[desruisseaux]

remove plexus-compiler layer but what about other compilers?

javax.tools.JavaCompiler is an interface. From a quick search on internet, I think (but did not verified closely) that the Eclipse compiler implements this interface. If there is still a need for the Plexus compiler, we can write a wrapper. The javax.tools API is preferred to the Plexus API because it has some JPMS specific methods that I didn't saw in Plexus API. It also has a method telling us whether an option is valid, and a caching mechanism.

For languages other than Java, we would need a separated plugin. With JPMS support, multi-releases support, incremental build (even incomplete), etc., this plugin is very Java-specific.

[rmannibucau]

@desruisseaux +1 to drop plexus but also +1 to keep the current incremental feature support by default - whatever name is given if incremental is not considered accurate.

[gnodet]

remove plexus-compiler layer but what about other compilers?

javax.tools.JavaCompiler is an interface. From a quick search on internet, I think (but did not verified closely) that the Eclipse compiler implements this interface. If there is still a need for the Plexus compiler, we can write a wrapper. The javax.tools API is preferred to the Plexus API because it has some JPMS specific methods that I didn't saw in Plexus API. It also has a method telling us whether an option is valid, and a caching mechanism.

For languages other than Java, we would need a separated plugin. With JPMS support, multi-releases support, incremental build (even incomplete), etc., this plugin is very Java-specific.

The point is that this interface may not be sufficient. Ideally, we'd have a list of output files, that would help with the heuristic.
I'm not sure the eclipse compiler is required for full incremental build, I think the requirement for the eclipse compiler is when only exposing direct dependencies for compiled classes (i.e. hide transitive dependencies so that the user is forced to declare those when the code is actually using them)

[desruisseaux]

The use of Eclipse compiler would not be mandated. If no tool chain or compilerId parameter is specified, the plugin uses whatever compiler is returned by ToolProvider.getSystemJavaCompiler(). The Eclipse compiler was mentioned only for saying that its use should still be possible.

The JavaCompiler API seems to have some support for giving the output files with JavaFileManager.getJavaFileForOutput(StandardLocation.CLASS_OUTPUT, <class name>, JavaFileObject.Kind.CLASS, <source file>) (I just tested it). However, it does not return the *.class files for inner (nested) classes, so some heuristic would still be needed.