1 files changed, 26 insertions, 7 deletions

diff --git a/kamon-core/src/main/scala/kamon/context/Storage.scala b/kamon-core/src/main/scala/kamon/context/Storage.scala
index 4f4e6cbb..f116142d 100644
--- a/kamon-core/src/main/scala/kamon/context/Storage.scala
+++ b/kamon-core/src/main/scala/kamon/context/Storage.scala
@@ -15,6 +15,8 @@
 
 package kamon.context
 
+import kamon.context.Storage.ThreadLocal.ContextHolder
+
 trait Storage {
   def current(): Context
   def store(context: Context): Storage.Scope
@@ -27,28 +29,45 @@ object Storage {
     def close(): Unit
   }
 
-
+  /**
+    * Wrapper that implements optimized {@link ThreadLocal} access pattern ideal for heavily used
+    * ThreadLocals.
+    *
+    * It is faster to use a mutable holder object and always perform ThreadLocal.get() and never use
+    * ThreadLocal.set(), because the value is more likely to be found in the ThreadLocalMap direct hash
+    * slot and avoid the slow path ThreadLocalMap.getEntryAfterMiss().
+    *
+    * Important: this thread local will live in ThreadLocalMap forever, so use with care.
+    */
   class ThreadLocal extends Storage {
-    private val tls = new java.lang.ThreadLocal[Context]() {
-      override def initialValue(): Context = Context.Empty
+    private val tls = new java.lang.ThreadLocal[ContextHolder]() {
+      override def initialValue() = new ContextHolder(Context.Empty)
     }
 
     override def current(): Context =
-      tls.get()
+      get()
 
     override def store(context: Context): Scope = {
       val newContext = context
-      val previousContext = tls.get()
-      tls.set(newContext)
+      val previousContext = get()
+      set(newContext)
 
       new Scope {
         override def context: Context = newContext
-        override def close(): Unit = tls.set(previousContext)
+        override def close(): Unit = set(previousContext)
       }
     }
+
+    private def get():Context =
+      tls.get().value
+
+    private def set(value:Context) : Unit =
+      tls.get().value = value
   }
 
   object ThreadLocal {
     def apply(): ThreadLocal = new ThreadLocal()
+
+    final class ContextHolder(var value:Context)
   }
 }
\ No newline at end of file