Minor improvements

src/main/java/io/vertx/core/Context.java

@@ -251,20 +251,24 @@ default List<String> processArgs() {
    *
    * @param key  the key of the data
    * @param <T>  the type of the data
-   * @return the data
+   * @return the local data
    */
   @GenIgnore
   <T> T getLocal(ContextKey<T> key);
 
   /**
-   * Get some local data from the context.
+   * Get some local data from the context, when it does not exist the {@code initialValueSupplier} is called to obtain
+   * the initial value.
+   *
+   * <p> The {@code initialValueSupplier} might be called multiple times when multiple threads call this method concurrently.
    *
    * @param key  the key of the data
+   * @param initialValueSupplier the supplier of the initial value optionally called
    * @param <T>  the type of the data
-   * @return the data
+   * @return the local data
    */
   @GenIgnore
-  <T> T getLocal(ContextKey<T> key, Supplier<? extends T> supplier);
+  <T> T getLocal(ContextKey<T> key, Supplier<? extends T> initialValueSupplier);
 
   /**
    * Put some local data in the context.

src/main/java/io/vertx/core/impl/ContextBase.java

@@ -41,19 +41,19 @@ public final <T> T getLocal(ContextKey<T> key) {
     return (T) res;
   }
 
-  public final <T> T getLocal(ContextKey<T> key, Supplier<? extends T> supplier) {
+  public final <T> T getLocal(ContextKey<T> key, Supplier<? extends T> initialValueSupplier) {
     ContextKeyImpl<T> internalKey = (ContextKeyImpl<T>) key;
     int index = internalKey.index;
     if (index >= locals.length) {
-      throw new IllegalArgumentException();
+      throw new IllegalArgumentException("Invalid key index: " + index);
     }
     Object res;
     while (true) {
       res = LOCALS_UPDATER.getVolatile(locals, index);
       if (res != null) {
         break;
       }
-      Object initial = supplier.get();
+      Object initial = initialValueSupplier.get();
       if (initial == null) {
         throw new IllegalStateException();
       }

src/test/java/io/vertx/core/FakeContext.java

@@ -185,7 +185,7 @@ public <T> void putLocal(ContextKey<T> key, T value) {
   }
 
   @Override
-  public <T> T getLocal(ContextKey<T> key, Supplier<? extends T> supplier) {
+  public <T> T getLocal(ContextKey<T> key, Supplier<? extends T> initialValueSupplier) {
     return null;
   }
 }