netty的异常 IllegalReferenceCountException refCnt: 0

这是因为Netty有引用计数器的原因,自从Netty 4开始,对象的生命周期由它们的引用计数(reference counts)管理,而不是由垃圾收集器(garbage collector)管理了。ByteBuf是最值得注意的,它使用了引用计数来改进分配内存和释放内存的性能。

在我们创建ByteBuf对象后,它的引用计数是1,当你释放(release)引用计数对象时,它的引用计数减1,如果引用计数为0,这个引用计数对象会被释放(deallocate),并返回对象池。

当尝试访问引用计数为0的引用计数对象会抛出IllegalReferenceCountException异常:

/**
* Should be called by every method that tries to access the buffers content to check
* if the buffer was released before.
*/
protected final void ensureAccessible() {
if (checkAccessible && refCnt() == 0) {
throw new IllegalReferenceCountException(0);
}
}

  

这个问题产生的最要原因是在第一次发送完心跳请求后,ctx.write 等一系列方法调用了ByteBuf的release方法,将其引用计数减为了0 导致的:

我们主要看其方法栈中调用信息,得到一个结论,是每次调用ctx.write/writeAndFlush, pipeline.write/writeAndFlush , 等一系列方法时,被封装的ByteBuf对象的引用计数会减一。导致第二次使用同样对象的包装对象时,出现引用计数的问题。

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.

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:
    @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:
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~
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();
}
}

  

05-04 03:50