netty的异常分析 IllegalReferenceCountException refCnt: 0, decrement: 1

在websocket关闭时经常会抛出如下异常: IllegalReferenceCountException refCnt: 0, decrement: 1

io.netty.util.IllegalReferenceCountException: refCnt: 0, decrement: 1
	at io.netty.buffer.AbstractReferenceCountedByteBuf.release(AbstractReferenceCountedByteBuf.java:101)
	at io.netty.buffer.DefaultByteBufHolder.release(DefaultByteBufHolder.java:73)
	at io.netty.util.ReferenceCountUtil.release(ReferenceCountUtil.java:59)
	at io.netty.channel.SimpleChannelInboundHandler.channelRead(SimpleChannelInboundHandler.java:112)
	at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:318)
	at io.netty.channel.AbstractChannelHandlerContext.access$600(AbstractChannelHandlerContext.java:42)
	at io.netty.channel.AbstractChannelHandlerContext$7.run(AbstractChannelHandlerContext.java:309)
	at io.netty.util.concurrent.DefaultEventExecutor.run(DefaultEventExecutor.java:36)
	at io.netty.util.concurrent.SingleThreadEventExecutor$2.run(SingleThreadEventExecutor.java:112)
	at java.lang.Thread.run(Unknown Source)

原来是在服务器收到CloseWebSocketFrame后，SimpleChannelInboundHandler调用release时，会触发CloseWebSocketFrame.release()

public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception
{
  boolean release = true;
  try {
    if (acceptInboundMessage(msg))
    {
      Object imsg = msg;
      channelRead0(ctx, imsg);
    } else {
      release = false;
      ctx.fireChannelRead(msg);
    }
  } finally {
    if ((this.autoRelease) && (release))
      ReferenceCountUtil.release(msg);//这行


  }
}

通过逐步debug，发现WebSocketServerHandler报异常的代码在handleWebSocketFrame方法中：

		if (frame instanceof CloseWebSocketFrame) {
			CloseWebSocketFrame close=(CloseWebSocketFrame) frame.retain()//这行
			handshaker.close(ctx.channel(),close);
			return;
		}

跟踪发现，它调用了AbstractReferenceCountedByteBuf

public ByteBuf retain()
{
  while (true) {
    int refCnt = this.refCnt;
    if (refCnt == 0) {
      throw new IllegalReferenceCountException(0, 1)//这行

    }
    if (refCnt == 2147483647) {
      throw new IllegalReferenceCountException(2147483647, 1);
    }
    if (refCntUpdater.compareAndSet(this, refCnt, refCnt + 1)) {
      break;
    }
  }
  return this;
}

在netty4中，对象的生命周期由引用计数器控制，ByteBuf就是如此，每个对象的初始化引用计数为1，调用一次release方法，引用计数器会减1，当尝试访问计数器为0的，对象时，会抛出IllegalReferenceCountException，正如ensureAccessible的实现，更加详细的解释可以参考官方文档

所以需要在对象CloseWebSocketFrame的初始化时引用人为+1

在ProtobufVarint32FrameDecoder中添加如下标红代码:

//CloseWebSocketFrame
 /**
*  Be aware that you need to call {@link io.netty.util.ReferenceCounted#retain()} on messages that are just passed through if they
* are of type {@link io.netty.util.ReferenceCounted}. This is needed as the {@link MessageToMessageDecoder} will call
* {@link io.netty.util.ReferenceCounted#release()} on decoded messages.
* 具体参考:
*/
ReferenceCountUtil.retain(frame);//这行
out.add(frame);