feat(jdk-codemodel): expose DiagnosticListener on JdkInitializer

jdk-codemodel/src/main/java/build/codemodel/jdk/JdkInitializer.java

@@ -61,6 +61,7 @@
 import javax.lang.model.element.TypeElement;
 import javax.lang.model.element.VariableElement;
 import javax.tools.Diagnostic;
+import javax.tools.DiagnosticListener;
 import javax.tools.JavaCompiler;
 import javax.tools.JavaFileObject;
 import javax.tools.StandardJavaFileManager;
@@ -90,6 +91,8 @@ public class JdkInitializer
      */
     private Predicate<URI> registrationFilter = null;
 
+    private DiagnosticListener<JavaFileObject> diagnosticListener = d -> {};
+
     private boolean initialized = false;
 
     // Set at the start of initialize() for use by helper methods
@@ -156,6 +159,11 @@ public static JdkInitializer ofDirectory(final Path directory) {
         return new JdkInitializer(List.of(), List.of(directory), List.of());
     }
 
+    public JdkInitializer withDiagnosticListener(final DiagnosticListener<JavaFileObject> listener) {
+        this.diagnosticListener = listener;
+        return this;
+    }
+
     /**
      * Restricts descriptor registration to compilation units whose source URI satisfies the given
      * predicate. Units that do not match are still compiled (providing type resolution context)
@@ -185,15 +193,14 @@ public void initialize(final CodeModel codeModel) {
         this.codeModel = codeModel;
         this.nameProvider = codeModel.getNameProvider();
         final JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
-        try (StandardJavaFileManager fileManager = compiler.getStandardFileManager(null, null, null)) {
+        try (StandardJavaFileManager fileManager = compiler.getStandardFileManager(diagnosticListener, null, null)) {
 
             final List<JavaFileObject> combined = collectSources(fileManager);
             if (combined.isEmpty()) {
                 return;
             }
 
-            final var task = compiler.getTask(null, fileManager, diagnostic -> {
-            }, buildOptions(), null, combined);
+            final var task = compiler.getTask(null, fileManager, diagnosticListener, buildOptions(), null, combined);
             final var javacTask = (JavacTask) task;
             final var compilationUnits = javacTask.parse();
             javacTask.analyze();

jdk-codemodel/src/test/java/build/codemodel/jdk/JdkInitializerTests.java

@@ -24,8 +24,11 @@
 import java.io.PrintStream;
 import java.nio.file.Files;
 import java.nio.file.Path;
+import java.util.ArrayList;
 import java.util.List;
 import java.util.Optional;
+import javax.tools.Diagnostic;
+import javax.tools.JavaFileObject;
 import javax.tools.ToolProvider;
 
 import static org.assertj.core.api.Assertions.assertThat;
@@ -191,6 +194,27 @@ public class Foo {
         assertThat(field.type()).isInstanceOf(UnknownTypeUsage.class);
     }
 
+    @Test
+    void shouldForwardDiagnosticsToCustomListener() {
+        final var source = JavaFileObjects.forSourceString(
+            "com.example.Broken",
+            """
+                package com.example;
+                public class Broken {
+                    private com.example.Missing dep;
+                }
+                """);
+
+        final List<Diagnostic<? extends JavaFileObject>> captured = new ArrayList<>();
+        runInternal(
+            new JdkInitializer(List.of(), List.of(), List.of(source))
+                .withDiagnosticListener(captured::add));
+
+        assertThat(captured)
+            .as("listener installed via withDiagnosticListener must receive javac diagnostics")
+            .isNotEmpty();
+    }
+
     @Test
     void shouldResolveTypesFromClasspath() throws Exception {
         // Compile a helper class to a temp directory, then pass that directory as the classpath.