Simplify logic surrounding building of Java & Kotlin parsers.

[timtebeek]

What's changed?

• Simplify logic surrounding building Java & Kotlin parsers when there are no source files to parse.
• Also simplify logic related to log statements, markers & return value.
• Avoid calling parsers when there are no source files #660

What's your motivation?

Readability suffered a bit when mentally trying to parse what we had before.

Stream<? extends SourceFile> cus = mainJavaSources.isEmpty() ? Stream.empty()
        : Stream.of(javaParserBuilder).map(JavaParser.Builder::build)
                .flatMap(parser -> parser.parse(mainJavaSources, baseDir, ctx));

Then further improvements from there on as spotted.

Have you considered any alternatives or workarounds?

Could have merely gone for javaParserBuilder.build() in ternary, but that didn't yet help readability much.

[knutwannheden]

Seems like it would be even better if we could filter out these generated sources before parsing them.

[timtebeek]

I had thought we parse them such that we have their types available for attribution, as indicated on line 302 above. What improvement would you make here then?

[knutwannheden]

I suppose we have to test this, but these sources should still not need to be parsed, because the type attribution in other sources is via class files.

[timtebeek]

I think it boils down to the question if getCompileClasspathElements() returns the classes compiled fom generated sources as well, which could depend on a number of different factors.

rewrite-maven-plugin/src/main/java/org/openrewrite/maven/MavenMojoProjectParser.java

Lines 317 to 324 in 0b8fc33

	logInfo(mavenProject, "Parsing source files");
	List<Path> dependencies = mavenProject.getCompileClasspathElements().stream()
	.distinct()
	.map(Paths::get)
	.collect(toList());
	JavaTypeCache typeCache = new JavaTypeCache();
	javaParserBuilder.classpath(dependencies).typeCache(typeCache);
	kotlinParserBuilder.classpath(dependencies).typeCache(new JavaTypeCache());

Right now we define generated sources as anything we find in the target/ directory, and parse those as source rather than compiled classes.

rewrite-maven-plugin/src/main/java/org/openrewrite/maven/MavenMojoProjectParser.java

Lines 302 to 308 in 0b8fc33

	// Some annotation processors output generated sources to the /target directory. These are added for parsing but
	// should be filtered out of the final SourceFile list.
	List<Path> generatedSourcePaths = listJavaSources(mavenProject.getBasedir().toPath().resolve(mavenProject.getBuild().getDirectory()));
	List<Path> mainJavaSources = Stream.concat(
	generatedSourcePaths.stream(),
	listJavaSources(mavenProject.getBasedir().toPath().resolve(mavenProject.getBuild().getSourceDirectory())).stream()
	).collect(toList());

While I agree it might be interesting to not parse those generated sources as source files but rather only as classpath entries, I'm not entirely sure that would work in all cases, and verifying that would take more time than I currently have available. Are you ok merging this PR as-is to at least simplify the logic?