之前做的東華車管數據采集平臺總是發生數據丟失的情況，雖然不頻繁但是還是要關注一下原因，于是今天提高了Netty的Log級別，打算查找一下問題出在哪了，提高級別代碼：

ServerBootstrap b =new ServerBootstrap(); b.group(bossGroup,workerGroup).channel(NioServerSocketChannel.class).option(ChannelOption.SO_BACKLOG, 2048).handler(new LoggingHandler(LogLevel.DEBUG)).childHandler(new ChildChannelHandler());

將Loglevel設置成DEBUG模式就OK了。
于是開始安心的觀察日志：

2017-01-19 10:04:46 [ nioEventLoopGroup-1-0:1625429 ] - [ INFO ] 消息主體:60160308049620860021010707190117020453395443491162627407087d081f00002e37008801008c00f9 2017-01-19 10:04:49 [ nioEventLoopGroup-1-0:1628830 ] - [ ERROR ] LEAK: ByteBuf.release() was not called before it's garbage-collected. Enable advanced leak reporting to find out where the leak occurred. To enable advanced leak reporting, specify the JVM option '-Dio.netty.leakDetectionLevel=advanced' or call ResourceLeakDetector.setLevel() See http://netty.io/wiki/reference-counted-objects.html for more information. 2017-01-19 10:04:49 [ nioEventLoopGroup-1-0:1628845 ] - [ INFO ] 入緩存隊列操作結果:9 2017-01-19 10:04:49 [ nioEventLoopGroup-1-0:1628845 ] - [ INFO ] 消息主體:601603080496208600210107071901170204573954434611626262170f88091f00002e37008801008c00fa 2017-01-19 10:04:53 [ nioEventLoopGroup-1-0:1632839 ] - [ INFO ] 入緩存隊列操作結果:9 2017-01-19 10:04:53 [ nioEventLoopGroup-1-0:1632839 ] - [ INFO ] 消息主體:60160308049620860021010707190117020501395443581162624817108a091f00002e37008801008c00fb 2017-01-19 10:04:55 [ nioEventLoopGroup-1-0:1634196 ] - [ INFO ] 入緩存隊列操作結果:9 2017-01-19 10:04:55 [ nioEventLoopGroup-1-0:1634196 ] - [ INFO ] 消息主體:601603080496208600210107071901170205023954436011626244571288091f00002e37008801008c00fc 2017-01-19 10:04:56 [ nioEventLoopGroup-1-0:1635288 ] - [ INFO ] 入緩存隊列操作結果:9 2017-01-19 10:04:56 [ nioEventLoopGroup-1-0:1635288 ] - [ INFO ] 消息主體:60160308049620860021010707190117020503395443651162624107118a091f00002e37008801008c00fd 2017-01-19 10:04:57 [ nioEventLoopGroup-1-0:1636443 ] - [ INFO ] 入緩存隊列操作結果:9 2017-01-19 10:04:57 [ nioEventLoopGroup-1-0:1636443 ] - [ INFO ] 消息主體:601603080496208600210107071901170205053954437111626234671088091f00002e37008801008c00fe

注意這句話：

LEAK: ByteBuf.release() was not called before it's garbage-collected. Enable advanced leak reporting to find out where the leak occurred. To enable advanced leak reporting, specify the JVM option '-Dio.netty.leakDetectionLevel=advanced' or call ResourceLeakDetector.setLevel() See http://netty.io/wiki/reference-counted-objects.html for more information.

通過這句話我們可以得知，只要加入

ResourceLeakDetector.setLevel(ResourceLeakDetector.Level.ADVANCED);

將警告級別設置成Advaced即可查到更詳細的泄漏信息，之后再度查看日志：

2017-01-19 10:35:59 [ nioEventLoopGroup-1-0:665092 ] - [ ERROR ] LEAK: ByteBuf.release() was not called before it's garbage-collected. See http://netty.io/wiki/reference-counted-objects.html for more information. Recent access records: 5 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#4:Hint: 'ObdServerHandler#0' will handle the message from this 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#1:io.netty.buffer.AdvancedLeakAwareByteBuf.skipBytes(AdvancedLeakAwareByteBuf.java:465)io.netty.handler.codec.DelimiterBasedFrameDecoder.decode(DelimiterBasedFrameDecoder.java:272)io.netty.handler.codec.DelimiterBasedFrameDecoder.decode(DelimiterBasedFrameDecoder.java:216)io.netty.handler.codec.ByteToMessageDecoder.callDecode(ByteToMessageDecoder.java:316)io.netty.handler.codec.ByteToMessageDecoder.channelRead(ByteToMessageDecoder.java:230)io.netty.channel.ChannelHandlerInvokerUtil.invokeChannelReadNow(ChannelHandlerInvokerUtil.java:84)io.netty.channel.DefaultChannelHandlerInvoker.invokeChannelRead(DefaultChannelHandlerInvoker.java:153)io.netty.channel.PausableChannelEventExecutor.invokeChannelRead(PausableChannelEventExecutor.java:86)io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:389)io.netty.channel.DefaultChannelPipeline.fireChannelRead(DefaultChannelPipeline.java:956)io.netty.channel.nio.AbstractNioByteChannel$NioByteUnsafe.read(AbstractNioByteChannel.java:127)io.netty.channel.nio.NioEventLoop.processSelectedKey(NioEventLoop.java:514)io.netty.channel.nio.NioEventLoop.processSelectedKeysOptimized(NioEventLoop.java:471)io.netty.channel.nio.NioEventLoop.processSelectedKeys(NioEventLoop.java:385)io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:351)io.netty.util.concurrent.SingleThreadEventExecutor$2.run(SingleThreadEventExecutor.java:116)io.netty.util.internal.chmv8.ForkJoinTask$RunnableExecuteAction.exec(ForkJoinTask.java:1412)io.netty.util.internal.chmv8.ForkJoinTask.doExec(ForkJoinTask.java:280)io.netty.util.internal.chmv8.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:877)io.netty.util.internal.chmv8.ForkJoinPool.scan(ForkJoinPool.java:1706)io.netty.util.internal.chmv8.ForkJoinPool.runWorker(ForkJoinPool.java:1661)io.netty.util.internal.chmv8.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:126) Created at:io.netty.buffer.PooledByteBufAllocator.newDirectBuffer(PooledByteBufAllocator.java:250)io.netty.buffer.AbstractByteBufAllocator.directBuffer(AbstractByteBufAllocator.java:155)io.netty.buffer.AbstractByteBufAllocator.directBuffer(AbstractByteBufAllocator.java:146)io.netty.buffer.AbstractByteBufAllocator.ioBuffer(AbstractByteBufAllocator.java:107)io.netty.channel.AdaptiveRecvByteBufAllocator$HandleImpl.allocate(AdaptiveRecvByteBufAllocator.java:104)io.netty.channel.nio.AbstractNioByteChannel$NioByteUnsafe.read(AbstractNioByteChannel.java:113)io.netty.channel.nio.NioEventLoop.processSelectedKey(NioEventLoop.java:514)io.netty.channel.nio.NioEventLoop.processSelectedKeysOptimized(NioEventLoop.java:471)io.netty.channel.nio.NioEventLoop.processSelectedKeys(NioEventLoop.java:385)io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:351)io.netty.util.concurrent.SingleThreadEventExecutor$2.run(SingleThreadEventExecutor.java:116)io.netty.util.internal.chmv8.ForkJoinTask$RunnableExecuteAction.exec(ForkJoinTask.java:1412)io.netty.util.internal.chmv8.ForkJoinTask.doExec(ForkJoinTask.java:280)io.netty.util.internal.chmv8.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:877)io.netty.util.internal.chmv8.ForkJoinPool.scan(ForkJoinPool.java:1706)io.netty.util.internal.chmv8.ForkJoinPool.runWorker(ForkJoinPool.java:1661)io.netty.util.internal.chmv8.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:126)

定位到我的代碼中為：

ByteBuf buff=(ByteBuf) msg; byte[] req=new byte[buff.readableBytes()];

于是可以確定是ByteBuff內存泄漏導致的問題，于是從這方面著手調查，發現netty5默認的分配bytebuff的方式是PooledByteBufAllocator,所以要手動回收，要不然會造成內存泄漏。
于是釋放ByteBuff即可

ReferenceCountUtil.release(buff);

這里引入一個網友對于這行代碼的說明：

ReferenceCountUtil.release()其實是ByteBuf.release()方法（從ReferenceCounted接口繼承而來）的包裝。netty4中的ByteBuf使用了引用計數（netty4實現了一個可選的ByteBuf池），每一個新分配的ByteBuf>>的引用計數值為1，每對這個ByteBuf對象增加一個引用，需要調用ByteBuf.retain()方法，而每減少一個引用，需要調用ByteBuf.release()方法。當這個ByteBuf對象的引用計數值為0時，表示此對象可回收。我這只是用ByteBuf說明，還有其他對象實現了ReferenceCounted接口，此時同理。

在檢查問題的過程中，我還懷疑是不是我的Netty使用了UDP協議導致的數據丟失，于是這里附上Netty使用的是TCP還是UDP的判斷方法:

關于TCP和UDP
socket可以基于TCP，也可以基于UDP。區別在于UDP的不保證數據包都正確收到，所以性能更好，但容錯不高。TCP保證不錯，所以性能沒那么好。
UDP基本只適合做在線視頻傳輸之類，我們的需求應該會是TCP。

那這2種方式在寫法上有什么不同？網上搜到這樣的說法：

在ChannelFactory 的選擇上，UDP的通信選擇 NioDatagramChannelFactory，TCP的通信我們選擇的是NioServerSocketChannelFactory；
在Bootstrap的選擇上，UDP選擇的是ConnectionlessBootstrap，而TCP選擇的是ServerBootstrap。

對于編解碼器decoder和Encoder，以及ChannelPipelineFactory，UDP開發與TCP并沒有什么區別，在此不做詳細介紹。

對于ChannelHandler，是UDP與TCP區別的核心所在。大家都知道UDP是無連接的，也就是說你通過 MessageEvent 參數對象的 getChannel() 方法獲取當前會話連接，但是其 isConnected() 永遠都返回 false。
UDP 開發中在消息獲取事件回調方法中，獲取了當前會話連接 channel 對象后可直接通過 channel 的 write 方法發送數據給對端 channel.write(message, remoteAddress)，第一個參數仍然是要發送的消息對象，
第二個參數則是要發送的對端 SocketAddress 地址對象。
這里最需要注意的一點是SocketAddress，在TCP通信中我們可以通過channel.getRemoteAddress()獲得，但在UDP通信中，我們必須從MessageEvent中通過調用getRemoteAddress()方法獲得對端的SocketAddress 地址。

?

總結

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