Netty中常见的IllegalReferenceCountException异常原因及解决

转载自：https://emacsist.github.io/2018/04/28/netty%E4%B8%AD%E5%B8%B8%E8%A7%81%E7%9A%84illegalreferencecountexception%E5%BC%82%E5%B8%B8%E5%8E%9F%E5%9B%A0%E5%8F%8A%E8%A7%A3%E5%86%B3/

 

Netty中常见的IllegalReferenceCountException异常原因及解决

 

问题代码

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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 { @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(); } }

上面的代码原目的是回显的, 但请求一下, 就会报:

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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) ...

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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@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 && release) { ReferenceCountUtil.release(msg); } } }

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).

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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@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); }

即上面的 echoMsg.retain() 方法.

• 构造 response 对象时, 不要复用 echoMsg 对象, 例如:

1	DefaultFullHttpResponse response = new DefaultFullHttpResponse(HttpVersion.HTTP_1_1, HttpResponseStatus.OK, Unpooled.copiedBuffer(echoMsg));

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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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(); } }