Reuse unionmaps for thread bound MDCs

Author: amarziali

dd-java-agent/agent-tooling/src/main/java/datadog/trace/agent/tooling/log/UnionMap.java

@@ -14,16 +14,31 @@
  * Lazy deduplication occurs once: before iterating over entries/values, or when combining sizes.
  */
 public final class UnionMap<K, V> extends AbstractMap<K, V> implements Serializable {
-  private final Map<K, V> primaryMap;
-  private final Map<K, V> secondaryMap;
+  private Map<K, V> primaryMap;
+  private Map<K, V> secondaryMap;
   private transient Set<Map.Entry<K, V>> entrySet;
   private transient volatile boolean deduped;
+  private static final ThreadLocal<UnionMap<?, ?>> TL = new ThreadLocal<>();
 
   public UnionMap(Map<K, V> primaryMap, Map<K, V> secondaryMap) {
     this.primaryMap = primaryMap;
     this.secondaryMap = secondaryMap;
   }
 
+  @SuppressWarnings({"unchecked"})
+  public static <K, V> UnionMap<K, V> create(Map<K, V> primaryMap, Map<K, V> secondaryMap) {
+    UnionMap ret = TL.get();
+    if (ret == null) {
+      ret = new UnionMap(primaryMap, secondaryMap);
+      TL.set(ret);
+    } else {
+      ret.primaryMap = primaryMap;
+      ret.secondaryMap = secondaryMap;
+      ret.deduped = false;
+    }
+    return ret;
+  }
+
   private void dedup() {
     if (!deduped) {
       if (primaryMap.isEmpty()) {

dd-java-agent/instrumentation/logback-1/src/main/java/datadog/trace/instrumentation/logback/LoggingEventInstrumentation.java

@@ -110,7 +110,7 @@ public static void onExit(
         correlationValues.put(Tags.DD_VERSION, version);
       }
 
-      mdc = null != mdc ? new UnionMap<>(mdc, correlationValues) : correlationValues;
+      mdc = null != mdc ? UnionMap.create(mdc, correlationValues) : correlationValues;
     }
   }
 }

dd-java-agent/instrumentation/tinylog-2.0/src/main/java/datadog/trace/instrumentation/tinylog2/LogEntryInstrumentation.java

@@ -103,7 +103,7 @@ public static void onExit(
         correlationValues.put(Tags.DD_VERSION, version);
       }
 
-      mdc = null != mdc ? new UnionMap<>(mdc, correlationValues) : correlationValues;
+      mdc = null != mdc ? UnionMap.create(mdc, correlationValues) : correlationValues;
     }
   }
 }