In Buildship: how can I substitute a Gradle project with a built jar?

Question

I would like to be a able to get Eclipse to ignore one Gradle project, and instead use a pre-built version of it.

Background

I have a project "parser" written in Scala, and a dozen others written in Java. The weakest link in my tool-set is Scala IDE. I use this plugin to edit & compile Scala code, but unfortunately it breaks the Java (JDT) tooling quite badly in mixed-language projects*.

• Specifically: Call-hierarchy is missing results, searches crash and so on. Also Scala IDE appears to have lost funding and the issues sound fairly fundamental, so I'm not holding my breath for these issues to be fixed.

With Maven (m2e) I had a workaround I was quite happy with:

1. Build as a .jar put into my local .m2 repository:

   cd parser; mvn install
   

2. In Eclipse, close the "parser" project

"Like magic", m2e simply picked up the most recent 'installed' .jar and used it in place of the closed project.

An awesome answer would be how to get Gradle to do that!

However all I wish for is any solution that meets these...

Requirements

1. That I can open Project parser when necessary (which is seldom), to edit and build changes via the Gradle command-line. I will close it when done.
2. Other projects use the built .jar from my local .m2 repo. (It's fine if they always do so.)
3. The change must not affect others who don't use Eclipse
4. (ideally) the change can be used by other Eclipse users

Approaches

A similar question had this good answer by @lance-java with a number of general suggestions. I think I can rule out these ideas:

• composite build support / multiple repos. Other team members wouldn't think it makes sense to build this project separately, as it is quite closely integrated with the others.
• dependency substitution rules - doesn't appear to meet requirement 3.

Something along the lines of lance-java's idea #4 sounds viable. Paraphrasing...

• "use the eclipse plugin [in conjunction with] Buildship, e.g. using the whenMerged hook to tweak the generated .classpath [of all the Java projects]."

  UPDATE: [18 Apr]: I had hit a brick wall in this approach. Buildship was not putting the built .jar onto the runtime classpath. (UPDATE 2: Now resolved - see my answer.)

Questions

The main question: How can I structure a solution to this, that will actually work & avoid any major pitfalls?

Note that the project itself has a few dependencies, specifically:

dependencies {
  compile 'org.scala-lang:scala-library:2.12.4'
  compileOnly  'com.google.code.findbugs:jsr305:1.3.9'
  antlr 'org.antlr:antlr4:4.5.3'
}

So a sub-question may be: How to pull these in into the other projects without duplicating the definition? (If that doesn't work automatically.)

Answer 1

So the solution was a bit involved. After adding 'maven-publish' to create the library, I then implemented the following to force Eclipse to use the prebuilt library:

subprojects {
    // Additional configuration to manipulate the Eclipse classpaths
    configurations {
        parserSubstitution
    }
    dependencies {
        parserSubstitution module("com.example:parser:${project.version}")
    }

    apply plugin: 'eclipse'
    eclipse {
        classpath {
        plusConfigurations += [ configurations.pseLangSubstitution ]
        file {
            whenMerged { cp ->

                // Get Gradle to add the depedency upon
                // parser-xxx.jar via 'plusConfigurations' above.
                // Then this here if we have a dependency on Project(':parser')
                //  - If so, remove it (completing the project -> jar substitution).
                //  - If not, remove the .jar dependency: it wasn't needed.
                def usesParser = entries.removeAll {
                    it instanceof ProjectDependency && it.path.startsWith('/parser')
                }
                def parserJar =
                    cp.entries.find { it instanceof Library && it.path.contains('parser-') }
                if (usesParser) {
                    // This trick stops Buildship deleting it from the runtime classpath
                    parserJar ?. entryAttributes ?. remove("gradle_used_by_scope")
                } else {
                    cp.entries.remove { parserJar }
                }
            }
        }
    }

So there are 2 parts to this:

1. Using 'plusConfigurations' felt a bit round-about. I ended up doing this because I could not see how to construct class Library classpath entries directly. However it could well be that this is required to implement the 'transient dependencies' correctly anyway. (See the end of the question.)
2. The trick to stop Buildship removing the .jar from the runtime classpath (thus deviating from a Gradle command-line launch) was provided to me by a Gradle developer in this discussion.

Usage

The solution works just as I hoped. Every time some code in this library is modified, I execute the following task of mine on the command line (which also does some other code & resource generation steps, in addition to building the parser jar):

./gradlew generateEclipse

Then in Eclipse I press keyboard shortcuts for "Gradle -> Refresh Gradle Projects", Build.

And harmony is restored. :-)

• Navigating to the (prebuilt) source of parser works.
• If I need to edit the source, I can open the parser project and edit it. Scala-IDE still does a good job for this.
• When I'm done I execute the command, close the project and my Java tools are happy.

Answer 2

In parser project

You shoud use the maven-publish plugin with the publishToMavenLocal task

apply plugin: 'maven-publish'

group = 'your.company'
version = '1.0.0'

publishing {    
    publications {
        mavenJava(MavenPublication) {
            from components.java

            pom.withXml {
                def root = asNode()
                root.appendNode('name', 'Your parser project name')
                root.appendNode('description', 'Your parser project description')
            }
        }
    }
}

Everytime you make a modification, just change the version number if necessary and go with gradle publishToMavenLocal

In other java project using parser

Just use parser as a regular dependency :

repositories {
    mavenLocal()
    ...
}

compile 'your.company:parser:1.0.0'

If my understanding of your situation is good, it should do the trick.