1. ByteBuf
2. 问题描述
日志记录中报堆外内存溢出。
3. 问题定位及修改
Netty提供了ByteBuf泄露的检测机制。
JVM启动参数中添加： -Dio.netty.leakDetectionLevel=advanced ， log4j2.xml配置io.netty日志记录即可。
禁用（DISABLED） – 完全禁止泄露检测，省点消耗。
简单（SIMPLE） – 默认等级，告诉我们取样的1%的ByteBuf是否发生了泄露，但总共一次只打印一次，看不到就没有了。
高级（ADVANCED） – 告诉我们取样的1%的ByteBuf发生泄露的地方。每种类型的泄漏（创建的地方与访问路径一致）只打印一次。对性能有影响。
偏执（PARANOID） – 跟高级选项类似，但此选项检测所有ByteBuf，而不仅仅是取样的那1%。对性能有绝大的影响。
检测到如下泄露点，
举例1：
13:29:25.273 [MODBUS_MESSAGE_POOL-thread-14] [] [] [] ERROR io.netty.util.ResourceLeakDetector - LEAK: ByteBuf.release() was not called before it's garbage-collected. See http://netty.io/wiki/reference-counted-objects.html for more information.
Created at:
io.netty.buffer.AdvancedLeakAwareByteBuf.writeBytes(AdvancedLeakAwareByteBuf.java:604)
io.netty.buffer.AbstractByteBuf.readBytes(AbstractByteBuf.java:849)
com.pinnet.protocol.hwmodbus.message.ModbusMessageRunnable.run(ModbusMessageRunnable.java:46)
java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
java.lang.Thread.run(Thread.java:748)
可以看到readBytes()创建后，没有释放。
修改：
buffer.readBytes(4).release();
举例2：
2018-06-30 13:29:25.278 [MESSAGE_POOL-thread-14] [] [] [] ERROR io.netty.util.ResourceLeakDetector - LEAK: ByteBuf.release() was not called before it's garbage-collected. See http://netty.io/wiki/reference-counted-objects.html for more information.
#1:
io.netty.handler.codec.ByteToMessageDecoder.channelRead(ByteToMessageDecoder.java:273)
io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:362)
io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:348)
io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:340)
io.netty.handler.timeout.IdleStateHandler.channelRead(IdleStateHandler.java:286)
io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:362)
io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:348)
io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:340)
io.netty.handler.ssl.SslHandler.unwrap(SslHandler.java:1380)
io.netty.handler.ssl.SslHandler.decodeJdkCompatible(SslHandler.java:1159)
io.netty.handler.ssl.SslHandler.decode(SslHandler.java:1194)
io.netty.handler.codec.ByteToMessageDecoder.decodeRemovalReentryProtection(ByteToMessageDecoder.java:489)
#2:
io.netty.buffer.AdvancedLeakAwareByteBuf.getUnsignedShort(AdvancedLeakAwareByteBuf.java:172)
io.netty.handler.codec.LengthFieldBasedFrameDecoder.getUnadjustedFrameLength(LengthFieldBasedFrameDecoder.java:469)
io.netty.handler.codec.LengthFieldBasedFrameDecoder.decode(LengthFieldBasedFrameDecoder.java:417)
com.pinnet.protocol.hwmodbus.tcp.MobusLengthDecoder.decode(MobusLengthDecoder.java:32)
#3:
io.netty.buffer.AdvancedLeakAwareByteBuf.order(AdvancedLeakAwareByteBuf.java:70)
io.netty.handler.codec.LengthFieldBasedFrameDecoder.getUnadjustedFrameLength(LengthFieldBasedFrameDecoder.java:462)
io.netty.handler.codec.LengthFieldBasedFrameDecoder.decode(LengthFieldBasedFrameDecoder.java:417)
com.pinnet.protocol.hwmodbus.tcp.MobusLengthDecoder.decode(MobusLengthDecoder.java:32)
io.netty.handler.codec.LengthFieldBasedFrameDecoder.decode(LengthFieldBasedFrameDecoder.java:343)
io.netty.handler.codec.ByteToMessageDecoder.decodeRemovalReentryProtection(ByteToMessageDecoder.java:489)
io.netty.handler.codec.ByteToMessageDecoder.callDecode(ByteToMessageDecoder.java:428)
io.netty.handler.codec.ByteToMessageDecoder.channelRead(ByteToMessageDecoder.java:265)
#4:
Hint: 'decoder' will handle the message from this point.
#5:
Hint: 'idleHandler' will handle the message from this point.
修改：解码器中释放Bytebuf in
public class MobusLengthDecoder extends LengthFieldBasedFrameDecoder {
public MobusLengthDecoder(ByteOrder byteOrder, int maxFrameLength,
int lengthFieldOffset, int lengthFieldLength, int lengthAdjustment,
int initialBytesToStrip, boolean failFast) {
super(byteOrder, maxFrameLength, lengthFieldOffset, lengthFieldLength,
lengthAdjustment, initialBytesToStrip, failFast);
}
@Override
protected Object decode(ChannelHandlerContext ctx,
io.netty.buffer.ByteBuf in) throws Exception {
ByteBuf buf = (ByteBuf) super.decode(ctx, in);
// LEAK: ByteBuf.release() was not called before it's garbage-collected.
in.release();
...
}
}