Use relative paths in compiler error and warning messages, for shorter and more readable output.

Give a hint when a change of directory is needed for using the relative paths.

Author: desruisseaux

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

@@ -29,6 +29,7 @@
 
 import java.io.BufferedReader;
 import java.io.BufferedWriter;
+import java.io.File;
 import java.io.IOException;
 import java.io.InputStream;
 import java.io.StreamTokenizer;
@@ -1600,10 +1601,18 @@ private void writeDebugFile(final ToolExecutor executor, final Options configura
             logger.warn("The <debugFileName> parameter should not be empty.");
             return;
         }
-        final var commandLine = new StringBuilder("For trying to compile from the command-line, use:")
-                .append(System.lineSeparator())
-                .append("    ")
-                .append(executable != null ? executable : compilerId);
+        final var commandLine = new StringBuilder("For trying to compile from the command-line, use:");
+        final var chdir =
+                Path.of(System.getProperty("user.dir")).relativize(basedir).toString();
+        if (!chdir.isEmpty()) {
+            boolean isWindows = (File.separatorChar == '\\');
+            commandLine
+                    .append(System.lineSeparator())
+                    .append("    ")
+                    .append(isWindows ? "chdir " : "cd ")
+                    .append(chdir);
+        }
+        commandLine.append(System.lineSeparator()).append("    ").append(executable != null ? executable : compilerId);
         try (BufferedWriter out = Files.newBufferedWriter(path)) {
             configuration.format(commandLine, out);
             for (Map.Entry<PathType, List<Path>> entry : executor.dependencies.entrySet()) {

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

@@ -22,6 +22,7 @@
 import javax.tools.DiagnosticListener;
 import javax.tools.JavaFileObject;
 
+import java.nio.file.Path;
 import java.util.Arrays;
 import java.util.LinkedHashMap;
 import java.util.Locale;
@@ -53,6 +54,11 @@ final class DiagnosticLogger implements DiagnosticListener<JavaFileObject> {
      */
     private final Locale locale;
 
+    /**
+     * The base directory with which to relativize the paths to source files.
+     */
+    private final Path directory;
+
     /**
      * Number of errors or warnings.
      */
@@ -74,14 +80,31 @@ final class DiagnosticLogger implements DiagnosticListener<JavaFileObject> {
      * @param logger the logger where to send diagnostics
      * @param messageBuilderFactory the factory for creating message builders
      * @param locale the locale for compiler message
+     * @param directory the base directory with which to relativize the paths to source files
      */
-    DiagnosticLogger(Log logger, MessageBuilderFactory messageBuilderFactory, Locale locale) {
+    DiagnosticLogger(Log logger, MessageBuilderFactory messageBuilderFactory, Locale locale, Path directory) {
         this.logger = logger;
         this.messageBuilderFactory = messageBuilderFactory;
         this.locale = locale;
+        this.directory = directory;
         codeCount = new LinkedHashMap<>();
     }
 
+    /**
+     * Makes the given file relative to the base directory.
+     *
+     * @param  file  the path to make relative to the base directory
+     * @return the given path, potentially relative to the base directory
+     */
+    private String relativize(String file) {
+        try {
+            return directory.relativize(Path.of(file)).toString();
+        } catch (IllegalArgumentException e) {
+            // Ignore, keep the absolute path.
+            return file;
+        }
+    }
+
     /**
      * Invoked when the compiler emitted a warning.
      *
@@ -95,6 +118,7 @@ public void report(Diagnostic<? extends JavaFileObject> diagnostic) {
         }
         MessageBuilder record = messageBuilderFactory.builder();
         record.a(message);
+        JavaFileObject source = diagnostic.getSource();
         Diagnostic.Kind kind = diagnostic.getKind();
         String style;
         switch (kind) {
@@ -107,11 +131,13 @@ public void report(Diagnostic<? extends JavaFileObject> diagnostic) {
                 break;
             default:
                 style = ".info:-bold,f:blue";
+                if (diagnostic.getLineNumber() == Diagnostic.NOPOS) {
+                    source = null; // Some messages are generic, e.g. "Recompile with -Xlint:deprecation".
+                }
                 break;
         }
-        JavaFileObject source = diagnostic.getSource();
         if (source != null) {
-            record.newline().a("    at ").a(source.getName());
+            record.newline().a("    at ").a(relativize(source.getName()));
             long line = diagnostic.getLineNumber();
             long column = diagnostic.getColumnNumber();
             if (line != Diagnostic.NOPOS || column != Diagnostic.NOPOS) {

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

@@ -204,7 +204,8 @@ protected ToolExecutor(final AbstractCompilerMojo mojo, DiagnosticListener<? sup
 
         logger = mojo.logger;
         if (listener == null) {
-            listener = new DiagnosticLogger(logger, mojo.messageBuilderFactory, LOCALE);
+            listener =
+                    new DiagnosticLogger(logger, mojo.messageBuilderFactory, LOCALE, mojo.project.getRootDirectory());
         }
         this.listener = listener;
         encoding = mojo.charset();