Fix a bug in ConstructorCache when classes are GC'ed but not removed from cache (#6186)

* Fix a bug in ConstructorCache when classes are GC'ed but not removed from cache

* Added a changelog

* Address PR feedback

Author: alextwoods

.changes/next-release/bugfix-AWSSDKforJavaV2-a136845.json

@@ -0,0 +1,6 @@
+{
+    "type": "bugfix",
+    "category": "AWS SDK for Java V2",
+    "contributor": "",
+    "description": "Fix a bug in ConstructorCache when classes are GC'ed but not removed from cache"
+}

core/checksums/src/main/java/software/amazon/awssdk/checksums/internal/ConstructorCache.java

@@ -63,7 +63,17 @@ private Optional<Class<?>> getClass(String className) {
                 return Optional.empty();
             }
         });
-        return classRef.map(WeakReference::get);
+
+        // if the WeakReference to the class has been garbage collected, remove it from the cache and try again
+        if (classRef.isPresent()) {
+            Class<?> clazz = classRef.get().get();
+            if (clazz != null) {
+                return Optional.of(clazz);
+            }
+            classesByClassLoader.remove(classLoader);
+            return getClass(className);
+        }
+        return Optional.empty();
     }
 
     /**