转入default返回缺省扩展，常量"default"特殊处理 DUBBO-520 ExtensionLoader的getExtensio…

…n名字转入true不应返回缺省扩展

dubbo-common/src/main/java/com/alibaba/dubbo/common/extension/ExtensionLoader.java

@@ -61,6 +61,8 @@
  */
 public class ExtensionLoader<T> {
 
+    public static String DEFAULT_EXTENSION_NAME = "default";
+
     private static final Logger logger = LoggerFactory.getLogger(ExtensionLoader.class);
 
     private static final String SERVICES_DIRECTORY = "META-INF/services/";
@@ -274,6 +276,10 @@ private boolean isActive(Activate activate, URL url) {
     public T getLoadedExtension(String name) {
         if (name == null || name.length() == 0)
             throw new IllegalArgumentException("Extension name == null");
+        if(name.equals(DEFAULT_EXTENSION_NAME)) {
+            name = getDefaultExtensionName();
+        }
+
         Holder<Object> holder = cachedInstances.get(name);
         if (holder == null) {
             cachedInstances.putIfAbsent(name, new Holder<Object>());
@@ -303,6 +309,10 @@ public Set<String> getLoadedExtensions() {
 	public T getExtension(String name) {
 		if (name == null || name.length() == 0)
 		    throw new IllegalArgumentException("Extension name == null");
+        if(name.equals(DEFAULT_EXTENSION_NAME)) {
+            name = getDefaultExtensionName();
+        }
+
 		Holder<Object> holder = cachedInstances.get(name);
 		if (holder == null) {
 		    cachedInstances.putIfAbsent(name, new Holder<Object>());