RestConfig: added fallback */* converter and more explanation comments

model/rest-impl/src/main/java/com/evolveum/midpoint/rest/impl/RestConfig.java

@@ -6,10 +6,21 @@
  */
 package com.evolveum.midpoint.rest.impl;
 
+import java.io.IOException;
+import java.nio.charset.StandardCharsets;
+import java.util.List;
+
+import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.ComponentScan;
 import org.springframework.context.annotation.Configuration;
+import org.springframework.http.HttpInputMessage;
+import org.springframework.http.HttpOutputMessage;
 import org.springframework.http.MediaType;
+import org.springframework.http.converter.AbstractHttpMessageConverter;
+import org.springframework.http.converter.HttpMessageConverter;
+import org.springframework.http.converter.HttpMessageNotWritableException;
 import org.springframework.web.servlet.config.annotation.ContentNegotiationConfigurer;
 import org.springframework.web.servlet.config.annotation.WebMvcConfigurer;
 
@@ -26,10 +37,16 @@
 public class RestConfig
         implements WebMvcConfigurer {
 
-    // TODO probably @Override addArgumentResolvers for @Converter processing?
-
+    /**
+     * Registers content-types for path extension and request parameter usage.
+     * Not needed for header-based content negotiation and midPoint typically
+     * doesn't use this, but it doesn't hurt and may be handy for download URLs.
+     * <p>
+     * See <a href="https://www.baeldung.com/spring-mvc-content-negotiation-json-xml">this
+     * tutorial for more about content negotiation</a>.
+     */
     @Override
-    public void configureContentNegotiation(ContentNegotiationConfigurer configurer) {
+    public void configureContentNegotiation(@NotNull ContentNegotiationConfigurer configurer) {
         for (MediaType mediaType : MidpointYamlHttpMessageConverter.MEDIA_TYPES) {
             configurer.mediaType(mediaType.getSubtype(), mediaType);
         }
@@ -53,4 +70,52 @@ public MidpointJsonHttpMessageConverter jsonConverter(
             PrismContext prismContext, LocalizationService localizationService) {
         return new MidpointJsonHttpMessageConverter(prismContext, localizationService);
     }
+
+    /**
+     * All beans above will be first in the converter list and other Spring converters
+     * will be available as well.
+     * We want to add "catch-all" converter for cases like error output for any (even unsupported)
+     * content type.
+     */
+    @Override
+    public void extendMessageConverters(List<HttpMessageConverter<?>> converters) {
+        converters.add(new FallbackConverter());
+    }
+
+    private static class FallbackConverter extends AbstractHttpMessageConverter<Object> {
+        /**
+         * Supports all media types - that's the purpose of this converter.
+         */
+        protected FallbackConverter() {
+            super(MediaType.ALL);
+        }
+
+        /**
+         * Supports all object types - that's the purpose of this converter.
+         */
+        @Override
+        protected boolean supports(@NotNull Class<?> clazz) {
+            return true;
+        }
+
+        /**
+         * Only for output, this can't read anything.
+         */
+        @Override
+        public boolean canRead(@NotNull Class<?> clazz, @Nullable MediaType mediaType) {
+            return false;
+        }
+
+        @Override
+        protected @NotNull Object readInternal(
+                @NotNull Class<?> clazz, @NotNull HttpInputMessage inputMessage) {
+            throw new UnsupportedOperationException("FallbackConverter is write-only");
+        }
+
+        @Override
+        protected void writeInternal(@NotNull Object o, @NotNull HttpOutputMessage outputMessage)
+                throws IOException, HttpMessageNotWritableException {
+            outputMessage.getBody().write(o.toString().getBytes(StandardCharsets.UTF_8));
+        }
+    }
 }