[Java知识库]Netty Demo报错

报错信息

五月 04, 2022 4:37:18 下午 io.netty.channel.DefaultChannelPipeline onUnhandledInboundException
警告: 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:1489)
	at io.netty.buffer.AbstractByteBuf.checkIndex(AbstractByteBuf.java:1418)
	at io.netty.buffer.UnsafeByteBufUtil.getBytes(UnsafeByteBufUtil.java:481)
	at io.netty.buffer.PooledUnsafeDirectByteBuf.getBytes(PooledUnsafeDirectByteBuf.java:130)
	at io.netty.buffer.ByteBufUtil.decodeString(ByteBufUtil.java:834)
	at io.netty.buffer.AbstractByteBuf.toString(AbstractByteBuf.java:1247)
	at io.netty.buffer.AbstractByteBuf.toString(AbstractByteBuf.java:1242)
	at server.HandlerServerHello.channelRead(HandlerServerHello.java:17)
	at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:379)
	at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:365)
	at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:357)
	at io.netty.channel.DefaultChannelPipeline$HeadContext.channelRead(DefaultChannelPipeline.java:1410)
	at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:379)
	at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:365)
	at io.netty.channel.DefaultChannelPipeline.fireChannelRead(DefaultChannelPipeline.java:919)
	at io.netty.channel.nio.AbstractNioByteChannel$NioByteUnsafe.read(AbstractNioByteChannel.java:163)
	at io.netty.channel.nio.NioEventLoop.processSelectedKey(NioEventLoop.java:714)
	at io.netty.channel.nio.NioEventLoop.processSelectedKeysOptimized(NioEventLoop.java:650)
	at io.netty.channel.nio.NioEventLoop.processSelectedKeys(NioEventLoop.java:576)
	at io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:493)
	at io.netty.util.concurrent.SingleThreadEventExecutor$4.run(SingleThreadEventExecutor.java:989)
	at io.netty.util.internal.ThreadExecutorMap$2.run(ThreadExecutorMap.java:74)
	at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30)
	at java.lang.Thread.run(Thread.java:748)

解决方案

把super方法注释掉

原因

1.netty4中，对象的生命周期由引用计数器控制
2.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异常
3.ByteBuf对象就是如此
4.查看AbstractByteBuf.java 源码

5.在Server端进行debug
调用栈为：invokeChannelRead

最终会调用ReferenceCountUtil类的release方法

因此：

1. 每次super.channelRead(ctx, msg);后，ByteBuf就会调用release()方法，计数器减一。
2. 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。
3. 解决方案，可以将super()注释掉。

demo代码

代码放置在github上，链接如下：github代码

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