code: static protected void addCodecOnly(CombinedRegistry registry, String name, ChannelHandler codec){
  		Object oldValue = registry.lookup(name);
  		if (oldValue != null && oldValue != codec){
  			throw new IllegalArgumentException("Codec name '" + name + "' is already in use in at least one of the registries.");
  		}
  		registry.getDefaultSimpleRegistry().put(name, codec);
  	}
docs: Adds codec to Registry only, it will not handle the manipulating of encoders or decoders list in Registry.<br>
  仅仅把codec直接加入到Registry中，而不处理加入到Registry里的encoders或decoders列表中。
  <p>
  If a codec with the same name already exists in the Registry, IllegalArgumentException will be thrown.<br>
  如果Registry中原先已经有同名的别的codec，则抛出IllegalArgumentException。
  
  @param registry	The CombinedRegistry that the codec will be added into.<br>
  codec将要被加入的CombinedRegistry。
  @param name		Name of the codec in Registry.<br>
  codec在Registry中的名字。
  @param codec		The codec that will be used by Netty.<br>
  将被Netty使用的codec。