From f36a5f9645aada4313d3a7ef94641699673d0580 Mon Sep 17 00:00:00 2001
From: Jerry Lee <oldratlee@gmail.com>
Date: Sat, 7 Jul 2018 19:49:03 +0800
Subject: [PATCH] add readability note, since holder map is used as set

more discussion see #95
---
 src/main/java/com/alibaba/ttl/TransmittableThreadLocal.java | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/src/main/java/com/alibaba/ttl/TransmittableThreadLocal.java b/src/main/java/com/alibaba/ttl/TransmittableThreadLocal.java
index 7cf2f9a8..0ec4e180 100644
--- a/src/main/java/com/alibaba/ttl/TransmittableThreadLocal.java
+++ b/src/main/java/com/alibaba/ttl/TransmittableThreadLocal.java
@@ -97,6 +97,10 @@ public class TransmittableThreadLocal<T> extends InheritableThreadLocal<T> {
         return copy(get());
     }
 
+    // Note about holder:
+    // 1. The value of holder is type Map<TransmittableThreadLocal<?>, ?> (WeakHashMap implementation),
+    //    but it is used as *set*.
+    // 2. WeakHashMap support null value.
     private static InheritableThreadLocal<Map<TransmittableThreadLocal<?>, ?>> holder =
             new InheritableThreadLocal<Map<TransmittableThreadLocal<?>, ?>>() {
                 @Override
-- 
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