1.在NettyClientHandler中的ChannelRead()读取数据时报IllegalReferenceCountException异常
问题：NettyClientHandler继承的SimpleChannelInboundHandler<Object>类，在执行ChannelRead()中的buf.readBytes(bs)时报以下错误：
DefaultChannelPipeline: An exceptionCaught() event was fired, and it reached at the tail of the pipeline. It usually means the last handler in the pipeline did not handle the exception.
io.netty.util.IllegalReferenceCountException: refCnt: 0
at io.netty.buffer.AbstractByteBuf.ensureAccessible(AbstractByteBuf.java:1150)
at io.netty.buffer.AbstractByteBuf.checkReadableBytes(AbstractByteBuf.java:1133)
at io.netty.buffer.AbstractByteBuf.readBytes(AbstractByteBuf.java:648)
at io.netty.buffer.AbstractByteBuf.readBytes(AbstractByteBuf.java:656)
at com.xianfan.jt808demo.connect.NettyClientHandler.channelRead(NettyClientHandler.java:59)
at io.netty.channel.ChannelHandlerInvokerUtil.invokeChannelReadNow(ChannelHandlerInvokerUtil.java:84)
at io.netty.channel.DefaultChannelHandlerInvoker.invokeChannelRead(DefaultChannelHandlerInvoker.java:153)
at io.netty.channel.PausableChannelEventExecutor.invokeChannelRead(PausableChannelEventExecutor.java:86)
at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:389)
at io.netty.channel.DefaultChannelPipeline.fireChannelRead(DefaultChannelPipeline.java:956)
at io.netty.channel.nio.AbstractNioByteChannel$NioByteUnsafe.read(AbstractNioByteChannel.java:127)
at io.netty.channel.nio.NioEventLoop.processSelectedKey(NioEventLoop.java:514)
at io.netty.channel.nio.NioEventLoop.processSelectedKeysPlain(NioEventLoop.java:433)
at io.netty.channel.nio.NioEventLoop.processSelectedKeys(NioEventLoop.java:387)
at io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:351)
at io.netty.util.concurrent.SingleThreadEventExecutor$2.run(SingleThreadEventExecutor.java:116)
at io.netty.util.internal.chmv8.ForkJoinTask$RunnableExecuteAction.exec(ForkJoinTask.java:1412)
at io.netty.util.internal.chmv8.ForkJoinTask.doExec(ForkJoinTask.java:280)
at io.netty.util.internal.chmv8.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:877)
at io.netty.util.internal.chmv8.ForkJoinPool.scan(ForkJoinPool.java:1706)
at io.netty.util.internal.chmv8.ForkJoinPool.runWorker(ForkJoinPool.java:1661)
at io.netty.util.internal.chmv8.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:126)
原因：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
AbstractByteBuf.java
protected final void ensureAccessible() {
if (refCnt() == 0) {
throw new IllegalReferenceCountException(0);
}
}
NettyClientHandler类中的 super.channelRead(ctx, msg);这行代码。追踪调用路径
private void invokeChannelRead(Object msg) {
try {
((ChannelInboundHandler) handler()).channelRead(this, msg);
} catch (Throwable t) {
notifyHandlerException(t);
}
}
最终调用的代码是：ReferenceCountUtil.release(msg)
public static boolean release(Object msg) {
if (msg instanceof ReferenceCounted) {
return ((ReferenceCounted) msg).release();
}
return false;
}
也就是每次super.channelRead(ctx, msg);后，ByteBuf就会调用release()方法，计数器减一，然后在 buf.readBytes(butfs);这行代码就会校验ensureAccessible(),计数器为0，netty认为ByteBuf对象已经释放，就抛出异常。
解决方法：去掉NettyClientHandler中这行代码 super.channelRead(ctx, msg);
ByteBuf对象谁处理谁释放。
