extract method to cache default extension name (#3456)

Author: wanghbxxxx

dubbo-common/src/main/java/org/apache/dubbo/common/extension/ExtensionLoader.java

@@ -624,6 +624,22 @@ private Map<String, Class<?>> getExtensionClasses() {
 
     // synchronized in getExtensionClasses
     private Map<String, Class<?>> loadExtensionClasses() {
+        cacheDefaultExtensionName();
+
+        Map<String, Class<?>> extensionClasses = new HashMap<>();
+        loadDirectory(extensionClasses, DUBBO_INTERNAL_DIRECTORY, type.getName());
+        loadDirectory(extensionClasses, DUBBO_INTERNAL_DIRECTORY, type.getName().replace("org.apache", "com.alibaba"));
+        loadDirectory(extensionClasses, DUBBO_DIRECTORY, type.getName());
+        loadDirectory(extensionClasses, DUBBO_DIRECTORY, type.getName().replace("org.apache", "com.alibaba"));
+        loadDirectory(extensionClasses, SERVICES_DIRECTORY, type.getName());
+        loadDirectory(extensionClasses, SERVICES_DIRECTORY, type.getName().replace("org.apache", "com.alibaba"));
+        return extensionClasses;
+    }
+
+    /**
+     * extract and cache default extension name if exists
+     */
+    private void cacheDefaultExtensionName() {
         final SPI defaultAnnotation = type.getAnnotation(SPI.class);
         if (defaultAnnotation != null) {
             String value = defaultAnnotation.value();
@@ -638,15 +654,6 @@ private Map<String, Class<?>> loadExtensionClasses() {
                 }
             }
         }
-
-        Map<String, Class<?>> extensionClasses = new HashMap<String, Class<?>>();
-        loadDirectory(extensionClasses, DUBBO_INTERNAL_DIRECTORY, type.getName());
-        loadDirectory(extensionClasses, DUBBO_INTERNAL_DIRECTORY, type.getName().replace("org.apache", "com.alibaba"));
-        loadDirectory(extensionClasses, DUBBO_DIRECTORY, type.getName());
-        loadDirectory(extensionClasses, DUBBO_DIRECTORY, type.getName().replace("org.apache", "com.alibaba"));
-        loadDirectory(extensionClasses, SERVICES_DIRECTORY, type.getName());
-        loadDirectory(extensionClasses, SERVICES_DIRECTORY, type.getName().replace("org.apache", "com.alibaba"));
-        return extensionClasses;
     }
 
     private void loadDirectory(Map<String, Class<?>> extensionClasses, String dir, String type) {