1 files changed, 23 insertions, 10 deletions

diff --git a/kamon-core/src/main/scala/kamon/context/Storage.scala b/kamon-core/src/main/scala/kamon/context/Storage.scala
index 7bbca111..2b409592 100644
--- a/kamon-core/src/main/scala/kamon/context/Storage.scala
+++ b/kamon-core/src/main/scala/kamon/context/Storage.scala
@@ -27,23 +27,36 @@ object Storage {
     def close(): Unit
   }
 
-
+  /**
+    * Wrapper that implements optimized {@link ThreadLocal} access pattern ideal for heavily used
+    * ThreadLocals.
+    *
+    * <p> 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().
+    *
+    * <p> Credit to @trask from the FastThreadLocal in glowroot.
+    *
+    * <p> One small change is that we don't use an kamon-defined holder object as that would prevent class unloading.
+    *
+    * */
   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[Array[AnyRef]]() {
+      override def initialValue(): Array[AnyRef] =
+        Array(Context.Empty)
     }
 
     override def current(): Context =
-      tls.get()
+      tls.get()(0).asInstanceOf[Context]
 
-    override def store(context: Context): Scope = {
-      val newContext = context
-      val previousContext = tls.get()
-      tls.set(newContext)
+    override def store(newContext: Context): Scope = {
+      val ref = tls.get()
+      val previousContext = ref(0)
+      ref(0) = newContext
 
       new Scope {
         override def context: Context = newContext
-        override def close(): Unit = tls.set(previousContext)
+        override def close(): Unit = ref(0) = previousContext
       }
     }
   }
@@ -51,4 +64,4 @@ object Storage {
   object ThreadLocal {
     def apply(): ThreadLocal = new ThreadLocal()
   }
-}
\ No newline at end of file
+}