feat: support customization for MDC serializer

Author: aliaksandr-lavishak-tde

log4j2-ecs-layout/src/main/java/co/elastic/logging/log4j2/MdcSerializer.java

@@ -24,27 +24,42 @@
  */
 package co.elastic.logging.log4j2;
 
+import java.util.ServiceLoader;
+
 import co.elastic.logging.EcsJsonSerializer;
 import co.elastic.logging.JsonUtils;
 import org.apache.logging.log4j.core.LogEvent;
 import org.apache.logging.log4j.util.TriConsumer;
 
-interface MdcSerializer {
+public interface MdcSerializer {
 
     void serializeMdc(LogEvent event, StringBuilder builder);
 
     class Resolver {
 
         public static MdcSerializer resolve() {
+            MdcSerializer serializer = resolveUserDefinedMdcSerializer();
+            if (serializer != null) {
+                return serializer;
+            }
             try {
                 LogEvent.class.getMethod("getContextData");
                 return (MdcSerializer) Class.forName("co.elastic.logging.log4j2.MdcSerializer$UsingContextData").getEnumConstants()[0];
-            } catch (Exception ignore) {
-            } catch (LinkageError ignore) {
+            } catch (Exception | LinkageError ignore) {
             }
             return UsingContextMap.INSTANCE;
         }
 
+        private static MdcSerializer resolveUserDefinedMdcSerializer() {
+            for (MdcSerializerFactory factory : ServiceLoader.load(MdcSerializerFactory.class)) {
+                MdcSerializer serializer = factory.create();
+                if (serializer != null) {
+                    return serializer;
+                }
+            }
+            return null;
+        }
+
     }
 
     /**

log4j2-ecs-layout/src/main/java/co/elastic/logging/log4j2/MdcSerializerFactory.java

@@ -0,0 +1,34 @@
+/*-
+ * #%L
+ * Java ECS logging
+ * %%
+ * Copyright (C) 2019 - 2020 Elastic and contributors
+ * %%
+ * Licensed to Elasticsearch B.V. under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch B.V. licenses this file to you 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
+ *
+ *   http://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.
+ * #L%
+ */
+package co.elastic.logging.log4j2;
+
+/**
+ * To customize the {@link MdcSerializer} used for serializing MDC (Mapped Diagnostic Context) data,
+ * implement this interface and add the fully qualified class name to
+ * {@code src/main/resources/META-INF/services/co.elastic.logging.log4j2.MdcSerializerFactory}.
+ */
+public interface MdcSerializerFactory {
+    MdcSerializer create();
+}

log4j2-ecs-layout/src/test/java/co/elastic/logging/log4j2/AbstractLog4j2EcsLayoutTest.java

@@ -37,6 +37,7 @@
 
 import java.util.List;
 
+import static co.elastic.logging.log4j2.TestMdcSerializerFactory.CUSTOM_MDC_SERIALIZER_TEST_KEY;
 import static org.assertj.core.api.Assertions.assertThat;
 
 abstract class AbstractLog4j2EcsLayoutTest extends AbstractEcsLoggingTest {
@@ -52,6 +53,7 @@ void tearDown() throws Exception {
     void testAdditionalFieldsWithLookup() throws Exception {
         putMdc("trace.id", "foo");
         putMdc("foo", "bar");
+        putMdc(CUSTOM_MDC_SERIALIZER_TEST_KEY, "some_text_lower_case");
         debug("test");
         assertThat(getAndValidateLastLogLine().get("cluster.uuid").textValue()).isEqualTo("9fe9134b-20b0-465e-acf9-8cc09ac9053b");
         assertThat(getAndValidateLastLogLine().get("node.id").textValue()).isEqualTo("foo");
@@ -60,6 +62,7 @@ void testAdditionalFieldsWithLookup() throws Exception {
         assertThat(getAndValidateLastLogLine().get("clazz").textValue()).startsWith(getClass().getPackageName());
         assertThat(getAndValidateLastLogLine().get("404")).isNull();
         assertThat(getAndValidateLastLogLine().get("foo").textValue()).isEqualTo("bar");
+        assertThat(getAndValidateLastLogLine().get(CUSTOM_MDC_SERIALIZER_TEST_KEY).textValue()).isEqualTo("SOME_TEXT_LOWER_CASE");
     }
 
     @Test

log4j2-ecs-layout/src/test/java/co/elastic/logging/log4j2/TestMdcSerializerFactory.java

@@ -0,0 +1,70 @@
+/*-
+ * #%L
+ * Java ECS logging
+ * %%
+ * Copyright (C) 2019 - 2020 Elastic and contributors
+ * %%
+ * Licensed to Elasticsearch B.V. under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch B.V. licenses this file to you 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
+ *
+ *   http://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.
+ * #L%
+ */
+package co.elastic.logging.log4j2;
+
+import org.apache.logging.log4j.util.TriConsumer;
+
+import co.elastic.logging.EcsJsonSerializer;
+import co.elastic.logging.JsonUtils;
+
+public class TestMdcSerializerFactory implements MdcSerializerFactory {
+
+    protected static final String  CUSTOM_MDC_SERIALIZER_TEST_KEY = "SPECIAL_TEST_CUSTOM_KEY";
+
+    @Override
+    public MdcSerializer create() {
+        return (event, builder) -> event.getContextData()
+                .forEach((key, value) -> getWriteFunctionForKey(key).accept(key, value, builder));
+    }
+
+    // Default function for serializing MDC entries
+    private static final TriConsumer<String, Object, StringBuilder> DEFAULT_WRITE_MDC_FUNCTION = (key, value, stringBuilder) -> {
+        stringBuilder.append('\"');
+        JsonUtils.quoteAsString(key, stringBuilder);
+        stringBuilder.append("\":\"");
+        JsonUtils.quoteAsString(EcsJsonSerializer.toNullSafeString(String.valueOf(value)), stringBuilder);
+        stringBuilder.append("\",");
+    };
+
+    // Custom function for handling a specific key
+    private static final TriConsumer<String, Object, StringBuilder> CUSTOM_KEY_WRITE_MDC_FUNCTION = (key, value, stringBuilder) -> DEFAULT_WRITE_MDC_FUNCTION.accept(
+            key,
+            value.toString().toUpperCase(),
+            stringBuilder
+    );
+
+    /**
+     * Returns the appropriate function to write an MDC entry based on the key.
+     *
+     * @param key MDC key.
+     * @return The function to serialize the MDC entry value.
+     */
+    private TriConsumer<String, Object, StringBuilder> getWriteFunctionForKey(String key) {
+        if (CUSTOM_MDC_SERIALIZER_TEST_KEY.equals(key)) {
+            return CUSTOM_KEY_WRITE_MDC_FUNCTION;
+        }
+        return DEFAULT_WRITE_MDC_FUNCTION;
+    }
+}

log4j2-ecs-layout/src/test/resources/META-INF/services/co.elastic.logging.log4j2.MdcSerializerFactory

@@ -0,0 +1 @@
+co.elastic.logging.log4j2.TestMdcSerializerFactory