ChannelHandlerAdapter与@Sharable

Sharable的应用案例

先直观感受一下该注解的影响：

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public static class Handler extends SimpleChannelInboundHandler<String> {
    @Override
    protected void channelRead0(ChannelHandlerContext ctx, String msg) throws Exception {
        System.out.println(msg);
    }
}

public static void main(String[] args) {

    Handler handler = new Handler();

    BootstrapUtils.runServer(new ChannelInitializer<SocketChannel>() {
        @Override
        protected void initChannel(SocketChannel ch) throws Exception {
            ch.pipeline().addLast(handler);
        }
    });
}

如上代码，在第二个客户端连接的时候，会报如下错误：


io.netty.channel.ChannelPipelineException: shareable.Server$Handler is not a @Sharable handler, so can't be added or removed multiple times.
	at io.netty.channel.DefaultChannelPipeline.checkMultiplicity(DefaultChannelPipeline.java:600)
	at io.netty.channel.DefaultChannelPipeline.addLast(DefaultChannelPipeline.java:202)
	at io.netty.channel.DefaultChannelPipeline.addLast(DefaultChannelPipeline.java:381)
	at io.netty.channel.DefaultChannelPipeline.addLast(DefaultChannelPipeline.java:370)
	at shareable.Server$1.initChannel(Server.java:27)
	at shareable.Server$1.initChannel(Server.java:24)
	at io.netty.channel.ChannelInitializer.initChannel(ChannelInitializer.java:129)
	at io.netty.channel.ChannelInitializer.handlerAdded(ChannelInitializer.java:112)
	at io.netty.channel.AbstractChannelHandlerContext.callHandlerAdded(AbstractChannelHandlerContext.java:938)
	at io.netty.channel.DefaultChannelPipeline.callHandlerAdded0(DefaultChannelPipeline.java:609)
	at io.netty.channel.DefaultChannelPipeline.access$100(DefaultChannelPipeline.java:46)
	at io.netty.channel.DefaultChannelPipeline$PendingHandlerAddedTask.execute(DefaultChannelPipeline.java:1463)
	at io.netty.channel.DefaultChannelPipeline.callHandlerAddedForAllHandlers(DefaultChannelPipeline.java:1115)
	at io.netty.channel.DefaultChannelPipeline.invokeHandlerAddedIfNeeded(DefaultChannelPipeline.java:650)
	at io.netty.channel.AbstractChannel$AbstractUnsafe.register0(AbstractChannel.java:514)
	at io.netty.channel.AbstractChannel$AbstractUnsafe.access$200(AbstractChannel.java:429)
	at io.netty.channel.AbstractChannel$AbstractUnsafe$1.run(AbstractChannel.java:486)
	at io.netty.util.concurrent.AbstractEventExecutor.safeExecute$$$capture(AbstractEventExecutor.java:164)
	at io.netty.util.concurrent.AbstractEventExecutor.safeExecute(AbstractEventExecutor.java)
	at io.netty.util.concurrent.SingleThreadEventExecutor.runAllTasks(SingleThreadEventExecutor.java:469)
	at io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:500)
	at io.netty.util.concurrent.SingleThreadEventExecutor$4.run(SingleThreadEventExecutor.java:986)
	at io.netty.util.internal.ThreadExecutorMap$2.run(ThreadExecutorMap.java:74)
	at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30)
	at java.lang.Thread.run(Thread.java:748)

如果在Handler上增加@Shareable注解，就不会存在这个问题。

ChannelHandlerAdapter对其的支持

ChannelHandlerAdapter仅包含如下两个有价值的实现：

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protected void ensureNotSharable() { }

public boolean isSharable() { }

这两个方法在什么时候会用到呢？在pipeline的addLast方法中有如下代码：

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public final ChannelPipeline addLast(EventExecutorGroup group, String name, ChannelHandler handler) {
    final AbstractChannelHandlerContext newCtx;
    synchronized (this) {
        checkMultiplicity(handler);

        // other code
    }
}

private static void checkMultiplicity(ChannelHandler handler) {
    if (handler instanceof ChannelHandlerAdapter) {
        ChannelHandlerAdapter h = (ChannelHandlerAdapter) handler;
        if (!h.isSharable() && h.added) {
            throw new ChannelPipelineException(
                    h.getClass().getName() +
                    " is not a @Sharable handler, so can't be added or removed multiple times.");
        }
        h.added = true;
    }
}

isSharable的实现如下：

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public boolean isSharable() {
    Class<?> clazz = getClass();
    Map<Class<?>, Boolean> cache = InternalThreadLocalMap.get().handlerSharableCache();
    Boolean sharable = cache.get(clazz);
    if (sharable == null) {
        sharable = clazz.isAnnotationPresent(Sharable.class);
        cache.put(clazz, sharable);
    }
    return sharable;
}

InternalThreadLocalMap.get().handlerSharableCache()为一个空Map，其资源是在cache.put(clazz, sharable)中放置进去的。搞SpringBoot多了，总感觉有些东西是在启动时通过反射之类的方法加载的，我还在找@Sharable的实现类，没想到用了这么简单的一个方案处理。

ChannelHandlerAdapter中其他实现

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@Skip
@Override
@Deprecated
public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
    ctx.fireExceptionCaught(cause);
}

已经废除了，研究价值不是很大，@Skip比较新颖，有点意思。

参考资料

Netty 之 @Sharable