Support Log4J2 MultiFormatStringBuilderFormattable structured messages

Update Log4J2 `ElasticCommonSchemaStructuredLogFormatter` and
`LogstashStructuredLogFormatter` to support Log4J2 JSON structured
messages (typically `MapMessage`)

Closes gh-42034

Author: philwebb

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/logging/log4j2/ElasticCommonSchemaStructuredLogFormatter.java

@@ -22,7 +22,6 @@
 import org.apache.logging.log4j.core.LogEvent;
 import org.apache.logging.log4j.core.impl.ThrowableProxy;
 import org.apache.logging.log4j.core.time.Instant;
-import org.apache.logging.log4j.message.Message;
 import org.apache.logging.log4j.util.ReadOnlyStringMap;
 
 import org.springframework.boot.json.JsonWriter;
@@ -54,7 +53,7 @@ private static void jsonMembers(Environment environment, JsonWriter.Members<LogE
 		members.add("process.thread.name", LogEvent::getThreadName);
 		ElasticCommonSchemaService.get(environment).jsonMembers(members);
 		members.add("log.logger", LogEvent::getLoggerName);
-		members.add("message", LogEvent::getMessage).as(Message::getFormattedMessage);
+		members.add("message", LogEvent::getMessage).as(StructuredMessage::get);
 		members.from(LogEvent::getContextData)
 			.whenNot(ReadOnlyStringMap::isEmpty)
 			.usingPairs((contextData, pairs) -> contextData.forEach(pairs::accept));

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/logging/log4j2/LogstashStructuredLogFormatter.java

@@ -27,7 +27,6 @@
 import org.apache.logging.log4j.core.LogEvent;
 import org.apache.logging.log4j.core.impl.ThrowableProxy;
 import org.apache.logging.log4j.core.time.Instant;
-import org.apache.logging.log4j.message.Message;
 import org.apache.logging.log4j.util.ReadOnlyStringMap;
 
 import org.springframework.boot.json.JsonWriter;
@@ -51,7 +50,7 @@ class LogstashStructuredLogFormatter extends JsonWriterStructuredLogFormatter<Lo
 	private static void jsonMembers(JsonWriter.Members<LogEvent> members) {
 		members.add("@timestamp", LogEvent::getInstant).as(LogstashStructuredLogFormatter::asTimestamp);
 		members.add("@version", "1");
-		members.add("message", LogEvent::getMessage).as(Message::getFormattedMessage);
+		members.add("message", LogEvent::getMessage).as(StructuredMessage::get);
 		members.add("logger_name", LogEvent::getLoggerName);
 		members.add("thread_name", LogEvent::getThreadName);
 		members.add("level", LogEvent::getLevel).as(Level::name);

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/logging/log4j2/StructuredMessage.java

@@ -0,0 +1,66 @@
+/*
+ * Copyright 2012-2024 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.logging.log4j2;
+
+import java.io.IOException;
+
+import org.apache.logging.log4j.message.Message;
+import org.apache.logging.log4j.util.MultiFormatStringBuilderFormattable;
+
+import org.springframework.boot.json.JsonWriter.WritableJson;
+
+/**
+ * Helper used to adapt {@link Message} for structured writing.
+ *
+ * @author Phillip Webb
+ */
+final class StructuredMessage {
+
+	private static final String JSON2 = "JSON";
+
+	private static final String[] JSON = { JSON2 };
+
+	private StructuredMessage() {
+	}
+
+	static Object get(Message message) {
+		if (message instanceof MultiFormatStringBuilderFormattable multiFormatMessage
+				&& hasJsonFormat(multiFormatMessage)) {
+			return WritableJson.of((out) -> formatTo(multiFormatMessage, out));
+		}
+		return message.getFormattedMessage();
+	}
+
+	private static boolean hasJsonFormat(MultiFormatStringBuilderFormattable message) {
+		for (String format : message.getFormats()) {
+			if (JSON2.equalsIgnoreCase(format)) {
+				return true;
+			}
+		}
+		return false;
+	}
+
+	private static void formatTo(MultiFormatStringBuilderFormattable message, Appendable out) throws IOException {
+		if (out instanceof StringBuilder stringBuilder) {
+			message.formatTo(JSON, stringBuilder);
+		}
+		else {
+			out.append(message.getFormattedMessage(JSON));
+		}
+	}
+
+}

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/logging/log4j2/ElasticCommonSchemaStructuredLogFormatterTests.java

@@ -20,6 +20,7 @@
 
 import org.apache.logging.log4j.core.impl.JdkMapAdapterStringMap;
 import org.apache.logging.log4j.core.impl.MutableLogEvent;
+import org.apache.logging.log4j.message.MapMessage;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 
@@ -78,4 +79,18 @@ void shouldFormatException() {
 						java.lang.RuntimeException: Boom\\n\\tat org.springframework.boot.logging.log4j2.ElasticCommonSchemaStructuredLogFormatterTests.shouldFormatException""");
 	}
 
+	@Test
+	void shouldFormatStructuredMessage() {
+		MutableLogEvent event = createEvent();
+		event.setMessage(new MapMessage<>().with("foo", true).with("bar", 1.0));
+		String json = this.formatter.format(event);
+		assertThat(json).endsWith("\n");
+		Map<String, Object> deserialized = deserialize(json);
+		Map<String, Object> expectedMessage = Map.of("foo", true, "bar", 1.0);
+		assertThat(deserialized).containsExactlyInAnyOrderEntriesOf(map("@timestamp", "2024-07-02T08:49:53Z",
+				"log.level", "INFO", "process.pid", 1, "process.thread.name", "main", "service.name", "name",
+				"service.version", "1.0.0", "service.environment", "test", "service.node.name", "node-1", "log.logger",
+				"org.example.Test", "message", expectedMessage, "ecs.version", "8.11"));
+	}
+
 }

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/logging/log4j2/LogstashStructuredLogFormatterTests.java

@@ -25,6 +25,7 @@
 import org.apache.logging.log4j.MarkerManager.Log4jMarker;
 import org.apache.logging.log4j.core.impl.JdkMapAdapterStringMap;
 import org.apache.logging.log4j.core.impl.MutableLogEvent;
+import org.apache.logging.log4j.message.MapMessage;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 
@@ -77,4 +78,19 @@ void shouldFormatException() {
 						java.lang.RuntimeException: Boom\\n\\tat org.springframework.boot.logging.log4j2.LogstashStructuredLogFormatterTests.shouldFormatException""");
 	}
 
+	@Test
+	void shouldFormatStructuredMessage() {
+		MutableLogEvent event = createEvent();
+		event.setMessage(new MapMessage<>().with("foo", true).with("bar", 1.0));
+		String json = this.formatter.format(event);
+		assertThat(json).endsWith("\n");
+		Map<String, Object> deserialized = deserialize(json);
+		Map<String, Object> expectedMessage = Map.of("foo", true, "bar", 1.0);
+		String timestamp = DateTimeFormatter.ISO_OFFSET_DATE_TIME
+			.format(OffsetDateTime.ofInstant(EVENT_TIME, ZoneId.systemDefault()));
+		assertThat(deserialized).containsExactlyInAnyOrderEntriesOf(
+				map("@timestamp", timestamp, "@version", "1", "message", expectedMessage, "logger_name",
+						"org.example.Test", "thread_name", "main", "level", "INFO", "level_value", 400));
+	}
+
 }