<!-- If you need to report a security issue please visit https://github.com/apache/dubbo/security/policy -->

ExtensionLoader cannot be resumed after interruption.

```
java.lang.InterruptedException
  at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquireSharedInterruptibly(AbstractQueuedSynchronizer.java:1302)
  at java.util.concurrent.CountDownLatch.await(CountDownLatch.java:231)
  at org.apache.dubbo.common.utils.ClassLoaderResourceLoader.loadResources(ClassLoaderResourceLoader.java:55)
  at org.apache.dubbo.common.extension.ExtensionLoader.loadDirectory(ExtensionLoader.java:1005)
  at org.apache.dubbo.common.extension.ExtensionLoader.loadDirectory(ExtensionLoader.java:943)
  at org.apache.dubbo.common.extension.ExtensionLoader.loadExtensionClasses(ExtensionLoader.java:931)
  at org.apache.dubbo.common.extension.ExtensionLoader.getExtensionClasses(ExtensionLoader.java:913)
  at org.apache.dubbo.common.extension.ExtensionLoader.createExtension(ExtensionLoader.java:752)
  at org.apache.dubbo.common.extension.ExtensionLoader.getExtension(ExtensionLoader.java:548)
  at org.apache.dubbo.common.extension.ExtensionLoader.getExtension(ExtensionLoader.java:523)
  at org.apache.dubbo.rpc.protocol.injvm.InjvmInvoker.<init>(InjvmInvoker.java:74)
```
* In order to trigger extension load, call {@link #getExtension(String)} instead.
     *
     * @see #getExtension(String)
     */
    @SuppressWarnings("unchecked")
    public T getLoadedExtension(String name) {
        checkDestroyed();
        if (StringUtils.isEmpty(name)) {
            throw new IllegalArgumentException("Extension name == null");
        }
        Holder<Object> holder = getOrCreateHolder(name);
        return (T) holder.get();
    }
    private Holder<Object> getOrCreateHolder(String name) {
        Holder<Object> holder = cachedInstances.get(name);
        if (holder == null) {
            cachedInstances.putIfAbsent(name, new Holder<>());
            holder = cachedInstances.get(name);
        }
        return holder;
    }
    /**
     * Return the list of extensions which are already loaded.
     * <p>
     * Usually {@link #getSupportedExtensions()} should be called in order to get all extensions.
     *
     * @see #getSupportedExtensions()
     */
    public Set<String> getLoadedExtensions() {
        return Collections.unmodifiableSet(new TreeSet<>(cachedInstances.keySet()));