Create a doclet that generates a markdown file for a class annotation

with @configuration

Scan files with @configuration and generate .md files:

```
/**
* A useful workflow
*/
@configuration
public class MyWorkflow {

}
```

A mvn site or more specifically `mvn javadoc:javadoc ` would generate
target/site/MyWorkflow.md:
```
A useful workflow
```

Signed-off-by: Roy Golan <rgolan@redhat.com>

workflow-examples/pom.xml

@@ -208,6 +208,15 @@
                 <groupId>org.apache.maven.plugins</groupId>
                 <artifactId>maven-checkstyle-plugin</artifactId>
             </plugin>
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-javadoc-plugin</artifactId>
+                <configuration>
+                    <docletPath>${project.basedir}/target/classes</docletPath>
+                    <doclet>com.redhat.parodos.examples.doclet.Markdown</doclet>
+                    <useStandardDocletOptions>false</useStandardDocletOptions>
+                </configuration>
+            </plugin>
         </plugins>
     </build>
 </project>

workflow-examples/src/main/java/com/redhat/parodos/examples/doclet/Markdown.java

@@ -0,0 +1,141 @@
+package com.redhat.parodos.examples.doclet;
+
+import java.io.IOException;
+import java.io.PrintStream;
+import java.nio.file.Files;
+import java.nio.file.Paths;
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Locale;
+import java.util.Map;
+import java.util.Set;
+import java.util.TreeMap;
+
+import javax.lang.model.SourceVersion;
+import javax.lang.model.element.Element;
+import javax.lang.model.util.ElementScanner9;
+
+import com.sun.source.doctree.DocCommentTree;
+import com.sun.source.doctree.UnknownBlockTagTree;
+import com.sun.source.util.DocTrees;
+import com.sun.source.util.SimpleDocTreeVisitor;
+import jdk.javadoc.doclet.Doclet;
+import jdk.javadoc.doclet.DocletEnvironment;
+import jdk.javadoc.doclet.Reporter;
+
+public class Markdown implements Doclet {
+
+	private DocTrees treeUtils;
+
+	public Markdown() {
+		var foo = "";
+	}
+
+	@Override
+
+	public void init(Locale locale, Reporter reporter) {
+
+	}
+
+	@Override
+	public String getName() {
+		return getClass().getSimpleName();
+	}
+
+	@Override
+	public Set<? extends Option> getSupportedOptions() {
+		return Set.of();
+	}
+
+	@Override
+	public SourceVersion getSupportedSourceVersion() {
+		return SourceVersion.latest();
+	}
+
+	private static final boolean OK = true;
+
+	@Override
+	public boolean run(DocletEnvironment environment) {
+		treeUtils = environment.getDocTrees();
+		ShowTags st = new ShowTags(System.out);
+		st.show(environment.getSpecifiedElements());
+		return OK;
+	}
+
+	/**
+	 * A scanner to search for elements with documentation comments, and to examine those
+	 * comments for custom tags.
+	 */
+	class ShowTags extends ElementScanner9<Void, Integer> {
+
+		final PrintStream out;
+
+		ShowTags(PrintStream out) {
+			this.out = out;
+		}
+
+		void show(Set<? extends Element> elements) {
+			scan(elements, 0);
+		}
+
+		@Override
+		public Void scan(Element e, Integer depth) {
+			DocCommentTree dcTree = treeUtils.getDocCommentTree(e);
+			if (dcTree != null) {
+				String indent = "  ".repeat(depth);
+				out.println(indent + "| " + e.getKind() + " " + e + " annotations has configuration "
+						+ e.getAnnotationMirrors().toArray().toString());
+
+				if (e.getAnnotationMirrors().stream()
+						.anyMatch(a -> a.toString().equals("@org.springframework.context.annotation.Configuration"))) {
+					try {
+						Files.write(Paths.get(e.getSimpleName() + ".md"), dcTree.getFullBody().toString().getBytes());
+					}
+					catch (IOException ex) {
+						throw new RuntimeException(ex);
+					}
+				}
+				Map<String, List<String>> tags = new TreeMap<>();
+				new TagScanner(tags).visitDocComment(dcTree, null);
+				tags.forEach((t, l) -> {
+					out.println(indent + "  @" + t);
+					l.forEach(c -> out.println(indent + "    " + c));
+				});
+			}
+			// records are not supported by the tool for some reason. Probably a javadoc
+			// bug.
+			if (e.getKind().toString().equals("RECORD")) {
+				return null;
+			}
+			return super.scan(e, depth + 1);
+		}
+
+	}
+
+	/**
+	 * A visitor to gather the block tags found in a comment.
+	 */
+	class TagScanner extends SimpleDocTreeVisitor<Void, Void> {
+
+		private final Map<String, List<String>> tags;
+
+		TagScanner(Map<String, List<String>> tags) {
+			this.tags = tags;
+		}
+
+		@Override
+		public Void visitDocComment(DocCommentTree tree, Void p) {
+			return visit(tree.getBlockTags(), null);
+		}
+
+		@Override
+		public Void visitUnknownBlockTag(UnknownBlockTagTree tree, Void p) {
+			String name = tree.getTagName();
+			String content = tree.getContent().toString();
+			tags.computeIfAbsent(name, n -> new ArrayList<>()).add(content);
+			return null;
+		}
+
+	}
+
+}