
replace code 
```java
  ExtensionLoader<T> loader = (ExtensionLoader<T>) EXTENSION_LOADERS.get(type);
    if (loader == null) {
        EXTENSION_LOADERS.putIfAbsent(type, new ExtensionLoader<T>(type));
        loader = (ExtensionLoader<T>) EXTENSION_LOADERS.get(type);
    }
  return loader;
```

to

```java
return (ExtensionLoader<T>) EXTENSION_LOADERS.computeIfAbsent(type, k -> new ExtensionLoader<T>(type));
```


throw new IllegalArgumentException("Extension name == null");
        }
        return getExtensionClasses().get(name);
    }
    private Map<String, Class<?>> getExtensionClasses() {
        Map<String, Class<?>> classes = cachedClasses.get();
        if (classes == null) {
            synchronized (cachedClasses) {
                classes = cachedClasses.get();
                if (classes == null) {
                    classes = loadExtensionClasses();
                    cachedClasses.set(classes);
                }
            }
        }
        return classes;
    }
    /**
     * synchronized in getExtensionClasses
     * */
    private Map<String, Class<?>> loadExtensionClasses() {
        cacheDefaultExtensionName();
        Map<String, Class<?>> extensionClasses = new HashMap<>();
        for (LoadingStrategy strategy : strategies) {
            loadDirectory(extensionClasses, strategy.directory(), type.getName(), strategy.preferExtensionClassLoader(), strategy.excludedPackages());
            loadDirectory(extensionClasses, strategy.directory(), type.getName().replace("org.apache", "com.alibaba"), strategy.preferExtensionClassLoader(), strategy.excludedPackages());
        }
        return extensionClasses;
    }
    /**