add @GuardedBy and remove redundant synchronized

Author: dugenkui03

src/main/java/org/dataloader/DataLoaderHelper.java

@@ -1,5 +1,6 @@
 package org.dataloader;
 
+import org.dataloader.annotations.GuardedBy;
 import org.dataloader.annotations.Internal;
 import org.dataloader.impl.CompletableFutureKit;
 import org.dataloader.stats.StatisticsCollector;
@@ -287,6 +288,7 @@ private void possiblyClearCacheEntriesOnExceptions(List<K> keys) {
         }
     }
 
+    @GuardedBy("dataLoader")
     private CompletableFuture<V> loadFromCache(K key, Object loadContext, boolean batchingEnabled) {
         final Object cacheKey = loadContext == null ? getCacheKey(key) : getCacheKeyWithContext(key, loadContext);
 
@@ -296,15 +298,12 @@ private CompletableFuture<V> loadFromCache(K key, Object loadContext, boolean ba
             return futureCache.get(cacheKey);
         }
 
-        CompletableFuture<V> loadCallFuture;
-        synchronized (dataLoader) {
-            loadCallFuture = queueOrInvokeLoader(key, loadContext, batchingEnabled, true);
-        }
-
+        CompletableFuture<V> loadCallFuture = queueOrInvokeLoader(key, loadContext, batchingEnabled, true);
         futureCache.set(cacheKey, loadCallFuture);
         return loadCallFuture;
     }
 
+    @GuardedBy("dataLoader")
     private CompletableFuture<V> queueOrInvokeLoader(K key, Object loadContext, boolean batchingEnabled, boolean cachingEnabled) {
         if (batchingEnabled) {
             CompletableFuture<V> loadCallFuture = new CompletableFuture<>();

src/main/java/org/dataloader/annotations/GuardedBy.java

@@ -0,0 +1,21 @@
+package org.dataloader.annotations;
+
+import static java.lang.annotation.ElementType.FIELD;
+import static java.lang.annotation.ElementType.METHOD;
+import static java.lang.annotation.RetentionPolicy.CLASS;
+
+import java.lang.annotation.Retention;
+import java.lang.annotation.Target;
+
+/**
+ * Indicates that the annotated element should be used only while holding the specified lock.
+ */
+@Target({FIELD, METHOD})
+@Retention(CLASS)
+public @interface GuardedBy {
+
+  /**
+   * The lock that should be held.
+   */
+  String value();
+}