Use relative paths when possible in the javac.args file that is generated.

apache#193

Author: desruisseaux

src/main/java/org/apache/maven/plugin/compiler/AbstractCompilerMojo.java

@@ -1787,25 +1787,47 @@ private void writeDebugFile(
         try (BufferedWriter out = Files.newBufferedWriter(path)) {
             compilerConfiguration.format(commandLine, out);
             for (Map.Entry<PathType, List<Path>> entry : dependencies.entrySet()) {
+                List<Path> files = entry.getValue();
+                files = files.stream().map(this::relativize).toList();
                 String separator = "";
-                for (String element : entry.getKey().option(entry.getValue())) {
+                for (String element : entry.getKey().option(files)) {
                     out.write(separator);
                     out.write(element);
                     separator = " ";
                 }
                 out.newLine();
             }
             out.write("-d \"");
-            out.write(getOutputDirectory().toString());
+            out.write(relativize(getOutputDirectory()).toString());
             out.write('"');
             out.newLine();
             for (SourceFile sf : sourceFiles) {
                 out.write('"');
-                out.write(sf.file.toString());
+                out.write(relativize(sf.file).toString());
                 out.write('"');
                 out.newLine();
             }
         }
         tipForCommandLineCompilation = commandLine.append(" @").append(path).toString();
     }
+
+    /**
+     * Makes the given file relative to the base directory if the path is inside the project directory tree.
+     * The check for the project directory tree (starting from the root of all sub-projects) is for avoiding
+     * to relativize the paths to JAR files in the Maven local repository for example.
+     *
+     * @param  file  the path to make relative to the base directory
+     * @return the given path, potentially relative to the base directory
+     */
+    private Path relativize(Path file) {
+        Path root = project.getRootDirectory();
+        if (root != null && file.startsWith(root)) {
+            try {
+                file = basedir.relativize(file);
+            } catch (IllegalArgumentException e) {
+                // Ignore, keep the absolute path.
+            }
+        }
+        return file;
+    }
 }

src/main/java/org/apache/maven/plugin/compiler/SourceDirectory.java

@@ -59,7 +59,11 @@ final class SourceDirectory {
     static final String CLASS_FILE_SUFFIX = ".class";
 
     /**
-     * The root directory of all source files.
+     * The root directory of all source files. Whether the path is relative or absolute depends on the paths given to
+     * the {@link #fromProject fromProject(…)} or {@link #fromPluginConfiguration fromPluginConfiguration(…)} methods.
+     * This class preserves the relative/absolute characteristic of the user-specified directories in order to behave
+     * as intended by users in operations such as {@linkplain Path#relativize relativization}, especially in regard of
+     * symbolic links. In practice, this path is often an absolute path.
      */
     final Path root;