Netty中常见的IllegalReferenceCountException异常原因及解决

淩亂°似流年 2022-12-31 14:24 336阅读 0赞

# Netty中常见的IllegalReferenceCountException异常原因及解决 #

# 问题代码 #

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&nbsp;</pre> package hello.in; import io.netty.buffer.ByteBuf; import io.netty.buffer.ByteBufUtil; import io.netty.channel.ChannelHandlerContext; import io.netty.channel.SimpleChannelInboundHandler; import io.netty.handler.codec.http.DefaultFullHttpResponse; import io.netty.handler.codec.http.HttpContent; import io.netty.handler.codec.http.HttpResponseStatus; import io.netty.handler.codec.http.HttpVersion; public class EchoHandler extends SimpleChannelInboundHandler&lt;HttpContent&gt; { @Override protected void channelRead0(final ChannelHandlerContext ctx, final HttpContent msg) { System.out.println("收到" + msg); ByteBuf echoMsg = msg.content(); System.out.println(new String(ByteBufUtil.getBytes(echoMsg))); DefaultFullHttpResponse response = new DefaultFullHttpResponse(HttpVersion.HTTP_1_1, HttpResponseStatus.OK, echoMsg); response.headers().set("Content-Type", "text/plain"); ctx.write(response).addListener(ChannelFutureListener.CLOSE); } @Override public void channelReadComplete(final ChannelHandlerContext ctx) { ctx.flush(); } @Override public void exceptionCaught(final ChannelHandlerContext ctx, final Throwable cause) { cause.printStackTrace(); ctx.close(); } } </td> 
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上面的代码原目的是回显的, 但请求一下, 就会报:

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&nbsp;</pre> io.netty.util.IllegalReferenceCountException: refCnt: 0, increment: 1 at io.netty.buffer.AbstractReferenceCountedByteBuf.release0(AbstractReferenceCountedByteBuf.java:100) at io.netty.buffer.AbstractReferenceCountedByteBuf.release(AbstractReferenceCountedByteBuf.java:84) at io.netty.handler.codec.http.DefaultHttpContent.release(DefaultHttpContent.java:94) at io.netty.util.ReferenceCountUtil.release(ReferenceCountUtil.java:88) ... </td> 
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# 原因 #

首先要重点强调的是: `SimpleChannelInboundHandler` 它会自动进行一次释放(即引用计数减1). 源码如下:

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&nbsp;</pre> @Override public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception { boolean release = true; try { if (acceptInboundMessage(msg)) { @SuppressWarnings("unchecked") I imsg = (I) msg; channelRead0(ctx, imsg); } else { release = false; ctx.fireChannelRead(msg); } } finally { if (autoRelease &amp;&amp; release) { ReferenceCountUtil.release(msg); } } } </td> 
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我们继承这个类, 一般是要重写 `channelRead0()` 这个方法的, 但实际上 Netty 内部处理的是 `channelRead()` 方法, 只是它通过模板模式来进行调用而已.

然后, 我们的代码里 `DefaultFullHttpResponse response = new DefaultFullHttpResponse(HttpVersion.HTTP_1_1, HttpResponseStatus.OK, echoMsg);` 这里使用了原 `msg.content()` 的 `ByteBuf`, 然后调用 `ctx.write(response)` 后, 就会导致 `msg` 的引用计数减1了.(这时, 引用计数变成0了~)

> 可以调用 `echoMsg.refCnt();` 来获取当前引用计数值. 在 `ctx.write(...)` 前后加一行打印, 就可以发现, `ctx.write(...)` 完之后, 引用计数减少了1.

然后最后 `channelRead0()` 执行完毕返回了, `SimpleChannelInboundHandler` 的模板方法还会再一次进行释放 `release`, 这时就会触发 `IllegalReferenceCountException` 异常了.(参考 [`[翻译]Netty中的引用计数对象`][Netty]).

# 解决 #

* 如果不想创建新的数据, 则可以直接在原对象里调用 `echoMsg.retain()` 进行引用计数加1.例如:
 
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 &nbsp;</pre> @Override protected void channelRead0(final ChannelHandlerContext ctx, final HttpContent msg) { System.out.println("收到" + msg); ByteBuf echoMsg = msg.content(); echoMsg.retain(); System.out.println(new String(ByteBufUtil.getBytes(echoMsg))); DefaultFullHttpResponse response = new DefaultFullHttpResponse(HttpVersion.HTTP_1_1, HttpResponseStatus.OK, echoMsg); response.headers().set("Content-Type", "text/plain"); ctx.write(response).addListener(ChannelFutureListener.CLOSE); } </td> 
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即上面的 `echoMsg.retain()` 方法.

* 构造 response 对象时, 不要复用 `echoMsg` 对象, 例如:
 
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 &nbsp;</pre> DefaultFullHttpResponse response = new DefaultFullHttpResponse(HttpVersion.HTTP_1_1, HttpResponseStatus.OK, Unpooled.copiedBuffer(echoMsg)); </td> 
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即, 使用 `Unpooled.copiedBuffer(...)` 来复制多一份内存数据~

* 直接使用 `ChannelInboundHandlerAdapter` 自动手动处理释放, 以免像 `SimpleChannelInboundHandler` 那样导致多次释放引用计数对象~
 
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 &nbsp;</pre> package hello.in; import io.netty.buffer.ByteBuf; import io.netty.buffer.ByteBufUtil; import io.netty.buffer.Unpooled; import io.netty.channel.ChannelFutureListener; import io.netty.channel.ChannelHandlerContext; import io.netty.channel.ChannelInboundHandlerAdapter; import io.netty.handler.codec.http.DefaultFullHttpResponse; import io.netty.handler.codec.http.HttpContent; import io.netty.handler.codec.http.HttpResponseStatus; import io.netty.handler.codec.http.HttpVersion; public class EchoHandler extends ChannelInboundHandlerAdapter { @Override public void channelRead(final ChannelHandlerContext ctx, final Object msg) { if (msg instanceof HttpContent) { manual(ctx, (HttpContent) msg); } } protected void manual(final ChannelHandlerContext ctx, final HttpContent msg) { System.out.println("收到" + msg); ByteBuf echoMsg = msg.content(); System.out.println(new String(ByteBufUtil.getBytes(echoMsg))); DefaultFullHttpResponse response = new DefaultFullHttpResponse(HttpVersion.HTTP_1_1, HttpResponseStatus.OK, echoMsg); response.headers().set("Content-Type", "text/plain"); ctx.write(response).addListener(ChannelFutureListener.CLOSE); } @Override public void channelReadComplete(final ChannelHandlerContext ctx) { ctx.flush(); } @Override public void exceptionCaught(final ChannelHandlerContext ctx, final Throwable cause) { cause.printStackTrace(); ctx.close(); } } </td> 
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[Netty]: https://emacsist.github.io/2018/04/28/%E7%BF%BB%E8%AF%91netty%E4%B8%AD%E7%9A%84%E5%BC%95%E7%94%A8%E8%AE%A1%E6%95%B0%E5%AF%B9%E8%B1%A1/