Report structured BuilderProblems from compiler diagnostics

pom.xml

@@ -82,7 +82,7 @@ under the License.
 
   <properties>
     <javaVersion>17</javaVersion>
-    <mavenVersion>4.0.0-rc-4</mavenVersion>
+    <mavenVersion>4.1.0-SNAPSHOT</mavenVersion>
 
     <asmVersion>9.10.1</asmVersion>
     <guiceVersion>7.0.0</guiceVersion>

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

@@ -73,6 +73,7 @@
 import org.apache.maven.api.services.DependencyResolver;
 import org.apache.maven.api.services.DependencyResolverRequest;
 import org.apache.maven.api.services.DependencyResolverResult;
+import org.apache.maven.api.services.DiagnosticReporter;
 import org.apache.maven.api.services.MavenException;
 import org.apache.maven.api.services.MessageBuilder;
 import org.apache.maven.api.services.MessageBuilderFactory;
@@ -942,6 +943,17 @@ final void amendincrementalCompilation(EnumSet<IncrementalBuild.Aspect> aspects,
     @Inject
     protected MessageBuilderFactory messageBuilderFactory;
 
+    /**
+     * Service for reporting structured build diagnostics to the build report.
+     * Compiler warnings and errors are reported through this service so they
+     * appear with structured keys, source locations, and suggestions in
+     * {@code mvnlog --diagnostics} and the JSON build report.
+     *
+     * @since 4.0.0-beta-5
+     */
+    @Inject
+    protected DiagnosticReporter diagnosticReporter;
+
     /**
      * The logger for reporting information or warnings to the user.
      * Currently, this is also used for console output.

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

@@ -30,11 +30,14 @@
 import java.util.Optional;
 
 import org.apache.maven.api.plugin.Log;
+import org.apache.maven.api.services.BuilderProblem;
+import org.apache.maven.api.services.DiagnosticReporter;
 import org.apache.maven.api.services.MessageBuilder;
 import org.apache.maven.api.services.MessageBuilderFactory;
 
 /**
- * A Java compiler diagnostic listener which send the messages to the Maven logger.
+ * A Java compiler diagnostic listener which sends the messages to the Maven logger
+ * and reports structured {@link BuilderProblem}s to the {@link DiagnosticReporter}.
  *
  * @author Martin Desruisseaux
  */
@@ -49,6 +52,12 @@ final class DiagnosticLogger implements DiagnosticListener<JavaFileObject> {
      */
     private final MessageBuilderFactory messageBuilderFactory;
 
+    /**
+     * The service for reporting structured diagnostics to the build report.
+     * May be {@code null} if no reporter is available (e.g. Maven 4.0.x).
+     */
+    private final DiagnosticReporter diagnosticReporter;
+
     /**
      * The locale for compiler message.
      */
@@ -75,16 +84,24 @@ final class DiagnosticLogger implements DiagnosticListener<JavaFileObject> {
     private String firstError;
 
     /**
-     * Creates a listener which will send the diagnostics to the given logger.
+     * Creates a listener which will send the diagnostics to the given logger
+     * and to the given diagnostic reporter.
      *
      * @param logger the logger where to send diagnostics
      * @param messageBuilderFactory the factory for creating message builders
+     * @param diagnosticReporter the reporter for structured build diagnostics, or {@code null}
      * @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, Path directory) {
+    DiagnosticLogger(
+            Log logger,
+            MessageBuilderFactory messageBuilderFactory,
+            DiagnosticReporter diagnosticReporter,
+            Locale locale,
+            Path directory) {
         this.logger = logger;
         this.messageBuilderFactory = messageBuilderFactory;
+        this.diagnosticReporter = diagnosticReporter;
         this.locale = locale;
         this.directory = directory;
         codeCount = new LinkedHashMap<>();
@@ -107,6 +124,17 @@ private String relativize(String file) {
         return file;
     }
 
+    /**
+     * Maps a {@link Diagnostic.Kind} to a {@link BuilderProblem.Severity}.
+     */
+    private static BuilderProblem.Severity mapSeverity(Diagnostic.Kind kind) {
+        return switch (kind) {
+            case ERROR -> BuilderProblem.Severity.ERROR;
+            case WARNING, MANDATORY_WARNING -> BuilderProblem.Severity.WARNING;
+            default -> BuilderProblem.Severity.INFO;
+        };
+    }
+
     /**
      * Invoked when the compiler emitted a warning.
      *
@@ -176,6 +204,40 @@ public void report(Diagnostic<? extends JavaFileObject> diagnostic) {
         if (code != null) {
             codeCount.merge(code, 1, (old, initial) -> old + 1);
         }
+        // Report structured diagnostic to the build report
+        reportToBuildReport(diagnostic, message, code);
+    }
+
+    /**
+     * Reports a structured {@link BuilderProblem} to the {@link DiagnosticReporter}.
+     * <p>
+     * Each diagnostic is reported with a per-type key ({@code "compiler:<code>"})
+     * so that the build report deduplicates by diagnostic kind. For example, 50
+     * unchecked warnings produce a single entry with count=50 in the summary.
+     * Individual per-file details remain in the build log.
+     */
+    private void reportToBuildReport(Diagnostic<? extends JavaFileObject> diagnostic, String message, String code) {
+        if (diagnosticReporter == null || code == null) {
+            return;
+        }
+        BuilderProblem.Builder builder = BuilderProblem.builder()
+                .severity(mapSeverity(diagnostic.getKind()))
+                .message(message)
+                .key("compiler:" + code);
+        // Attach source location from the first occurrence (collector deduplicates by key)
+        JavaFileObject sourceFile = diagnostic.getSource();
+        if (sourceFile != null) {
+            builder.source(relativize(sourceFile.getName()));
+            long line = diagnostic.getLineNumber();
+            if (line != Diagnostic.NOPOS) {
+                builder.lineNumber((int) line);
+            }
+            long column = diagnostic.getColumnNumber();
+            if (column != Diagnostic.NOPOS) {
+                builder.columnNumber((int) column);
+            }
+        }
+        diagnosticReporter.report(builder.build());
     }
 
     /**

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

@@ -225,7 +225,7 @@ protected ToolExecutor(final AbstractCompilerMojo mojo, DiagnosticListener<? sup
         logger = mojo.logger;
         if (listener == null) {
             Path root = mojo.project.getRootDirectory();
-            listener = new DiagnosticLogger(logger, mojo.messageBuilderFactory, LOCALE, root);
+            listener = new DiagnosticLogger(logger, mojo.messageBuilderFactory, mojo.diagnosticReporter, LOCALE, root);
         }
         this.listener = listener;
         encoding = mojo.charset();

src/test/java/org/apache/maven/plugin/compiler/CompilerMojoTestCase.java

@@ -40,20 +40,21 @@
 import org.apache.maven.api.model.Build;
 import org.apache.maven.api.model.Model;
 import org.apache.maven.api.plugin.Log;
-import org.apache.maven.api.plugin.testing.Basedir;
-import org.apache.maven.api.plugin.testing.InjectMojo;
-import org.apache.maven.api.plugin.testing.MojoExtension;
-import org.apache.maven.api.plugin.testing.MojoParameter;
-import org.apache.maven.api.plugin.testing.MojoTest;
-import org.apache.maven.api.plugin.testing.stubs.ProducedArtifactStub;
-import org.apache.maven.api.plugin.testing.stubs.ProjectStub;
-import org.apache.maven.api.plugin.testing.stubs.SessionMock;
 import org.apache.maven.api.services.ArtifactManager;
+import org.apache.maven.api.services.DiagnosticReporter;
 import org.apache.maven.api.services.MessageBuilderFactory;
 import org.apache.maven.api.services.ToolchainManager;
 import org.apache.maven.impl.DefaultMessageBuilderFactory;
 import org.apache.maven.impl.InternalSession;
 import org.apache.maven.plugin.compiler.stubs.CompilerStub;
+import org.apache.maven.testing.plugin.Basedir;
+import org.apache.maven.testing.plugin.InjectMojo;
+import org.apache.maven.testing.plugin.MojoExtension;
+import org.apache.maven.testing.plugin.MojoParameter;
+import org.apache.maven.testing.plugin.MojoTest;
+import org.apache.maven.testing.plugin.stubs.ProducedArtifactStub;
+import org.apache.maven.testing.plugin.stubs.ProjectStub;
+import org.apache.maven.testing.plugin.stubs.SessionMock;
 import org.junit.jupiter.api.Test;
 
 import static org.junit.jupiter.api.Assertions.assertArrayEquals;
@@ -474,6 +475,14 @@ private static InternalSession createSession() {
         return session;
     }
 
+    @Provides
+    @Singleton
+    @SuppressWarnings("unused")
+    private static DiagnosticReporter createDiagnosticReporter() {
+        // No-op reporter for tests — diagnostics are verified via Log mock
+        return problem -> {};
+    }
+
     @Provides
     @Singleton
     @SuppressWarnings("unused")