[jnigen] Propagate Javadoc deprecation messages

pkgs/jnigen/lib/src/bindings/dart_generator.dart

@@ -1312,7 +1312,9 @@ ${modifier}final _$idName = $_protectedExtension
     }
     final params = defArgs.delimited(', ');
     if (node.isDeprecated) {
-      s.writeln("  @core\$_.Deprecated('This Java method is deprecated.')");
+      final message =
+          node.javadoc?.deprecatedMessage ?? 'This Java method is deprecated.';
+      s.writeln('  @core\$_.Deprecated(${jsonEncode(message)})');
     }
     if (node.methodKind == MethodKind.getter) {
       s.write('  $ifStatic$returnType get $name ');

pkgs/jnigen/lib/src/elements/elements.dart

@@ -973,6 +973,40 @@ class JavaDocComment implements Element<JavaDocComment> {
 
   final String comment;
 
+  String? get deprecatedMessage {
+    final lines = comment.split('\n');
+    final messageLines = <String>[];
+    var readingDeprecatedTag = false;
+
+    for (final line in lines) {
+      final trimmed = line.trim();
+
+      if (!readingDeprecatedTag) {
+        if (trimmed.startsWith('@deprecated')) {
+          readingDeprecatedTag = true;
+
+          final firstLine = trimmed.substring('@deprecated'.length).trim();
+          if (firstLine.isNotEmpty) {
+            messageLines.add(firstLine);
+          }
+        }
+        continue;
+      }
+
+      // A new Javadoc block tag marks the end of the deprecated message.
+      if (trimmed.startsWith('@')) {
+        break;
+      }
+
+      if (trimmed.isNotEmpty) {
+        messageLines.add(trimmed);
+      }
+    }
+
+    final message = messageLines.join('\n').trim();
+    return message.isEmpty ? null : message;
+  }
+
   factory JavaDocComment.fromJson(Map<String, dynamic> json) =>
       _$JavaDocCommentFromJson(json);
 

pkgs/jnigen/test/simple_package_test/bindings/simple_package.dart

@@ -1283,7 +1283,7 @@ extension Example$$Methods on Example {
 
   /// from: `public java.lang.String deprecatedMethod()`
   /// The returned object must be released after use, by calling the [release] method.
-  @core$_.Deprecated('This Java method is deprecated.')
+  @core$_.Deprecated("This Java method is deprecated.")
   jni$_.JString? deprecatedMethod() {
     final _$$selfRef = reference;
     return _deprecatedMethod(_$$selfRef.pointer, _id_deprecatedMethod.pointer)

pkgs/jnigen/test/simple_package_test/java/com/github/dart_lang/jnigen/simple_package/Example.java

@@ -147,6 +147,11 @@ public String joinStrings(List<String> values, String delim) {
     return null;
   }
 
+  /**
+   * This method is retained for compatibility.
+   *
+   * @deprecated Use methodWithSeveralParams instead.
+   */
   @Deprecated
   public String deprecatedMethod() {
     return "deprecated";