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Netty作为一个高性能的网络通信框架，里面有很多优秀的代码值得我们学习，今天我们一起看下Netty中用到了哪些设计模式。

一、单例模式

Netty通过 NioEventLoop 将通道注册到选择器，并在事件循环中多路复用它们。其中提供了一个选择策略对象 SelectStrategy，它只有一个默认实现：DefaultSelectStrategy。

/*** Default select strategy.*/
final class DefaultSelectStrategy implements SelectStrategy {static final SelectStrategy INSTANCE = new DefaultSelectStrategy();private DefaultSelectStrategy() { }@Overridepublic int calculateStrategy(IntSupplier selectSupplier, boolean hasTasks) throws Exception {return hasTasks ? selectSupplier.get() : SelectStrategy.SELECT;}
}

还有 ReadTimeoutException 和 WriteTimeoutException

/*** A {@link TimeoutException} raised by {@link ReadTimeoutHandler} when no data* was read within a certain period of time.*/
public final class ReadTimeoutException extends TimeoutException {private static final long serialVersionUID = 169287984113283421L;public static final ReadTimeoutException INSTANCE = PlatformDependent.javaVersion() >= 7 ?new ReadTimeoutException(true) : new ReadTimeoutException();ReadTimeoutException() { }private ReadTimeoutException(boolean shared) {super(shared);}
}

/*** A {@link TimeoutException} raised by {@link WriteTimeoutHandler} when a write operation* cannot finish in a certain period of time.*/
public final class WriteTimeoutException extends TimeoutException {private static final long serialVersionUID = -144786655770296065L;public static final WriteTimeoutException INSTANCE = PlatformDependent.javaVersion() >= 7 ?new WriteTimeoutException(true) : new WriteTimeoutException();private WriteTimeoutException() { }private WriteTimeoutException(boolean shared) {super(shared);}
}

二、工厂模式

工厂模式是非常常见的一种模式，Netty中也使用到，比如 上面提到的选择策略工厂： DefaultSelectStrategyFactory

/*** Factory which uses the default select strategy.*/
public final class DefaultSelectStrategyFactory implements SelectStrategyFactory {public static final SelectStrategyFactory INSTANCE = new DefaultSelectStrategyFactory();private DefaultSelectStrategyFactory() { }@Overridepublic SelectStrategy newSelectStrategy() {return DefaultSelectStrategy.INSTANCE;}
}

三、策略模式

在默认的事件执行选择工厂 DefaultEventExecutorChooserFactory 的 newChooser 方法中，根据数组参数的长度是否是2的幂 来选择不同的 EventExecutorChooser。两种方式都是简单的轮询方式，只是方式不同。

    @Overridepublic EventExecutorChooser newChooser(EventExecutor[] executors) {if (isPowerOfTwo(executors.length)) {return new PowerOfTwoEventExecutorChooser(executors);} else {return new GenericEventExecutorChooser(executors);}}

private static final class PowerOfTwoEventExecutorChooser implements EventExecutorChooser {private final AtomicInteger idx = new AtomicInteger();private final EventExecutor[] executors;PowerOfTwoEventExecutorChooser(EventExecutor[] executors) {this.executors = executors;}@Overridepublic EventExecutor next() {return executors[idx.getAndIncrement() & executors.length - 1];}}private static final class GenericEventExecutorChooser implements EventExecutorChooser {// Use a 'long' counter to avoid non-round-robin behaviour at the 32-bit overflow boundary.// The 64-bit long solves this by placing the overflow so far into the future, that no system// will encounter this in practice.private final AtomicLong idx = new AtomicLong();private final EventExecutor[] executors;GenericEventExecutorChooser(EventExecutor[] executors) {this.executors = executors;}@Overridepublic EventExecutor next() {return executors[(int) Math.abs(idx.getAndIncrement() % executors.length)];}}

四、装饰者模式

WrappedByteBuf 就是对 ByteBuf的装饰，来实现对它的增加。

class WrappedByteBuf extends ByteBuf {protected final ByteBuf buf;protected WrappedByteBuf(ByteBuf buf) {if (buf == null) {throw new NullPointerException("buf");}this.buf = buf;}......
}

五、责任链模式

ChannelPipeline 就是用到了责任链模式，所谓的责任链模式是指：它允许多个对象在处理请求时形成一条链，每个对象都有机会处理请求，将请求沿着链传递，直到有一个对象处理它为止。

/*** The default {@link ChannelPipeline} implementation.  It is usually created* by a {@link Channel} implementation when the {@link Channel} is created.*/
public class DefaultChannelPipeline implements ChannelPipeline {static final InternalLogger logger = InternalLoggerFactory.getInstance(DefaultChannelPipeline.class);private static final String HEAD_NAME = generateName0(HeadContext.class);private static final String TAIL_NAME = generateName0(TailContext.class);private static final FastThreadLocal<Map<Class<?>, String>> nameCaches =new FastThreadLocal<Map<Class<?>, String>>() {@Overrideprotected Map<Class<?>, String> initialValue() {return new WeakHashMap<Class<?>, String>();}};private static final AtomicReferenceFieldUpdater<DefaultChannelPipeline, MessageSizeEstimator.Handle> ESTIMATOR =AtomicReferenceFieldUpdater.newUpdater(DefaultChannelPipeline.class, MessageSizeEstimator.Handle.class, "estimatorHandle");final HeadContext head;final TailContext tail;private final Channel channel;private final ChannelFuture succeededFuture;private final VoidChannelPromise voidPromise;private final boolean touch = ResourceLeakDetector.isEnabled();private Map<EventExecutorGroup, EventExecutor> childExecutors;private volatile MessageSizeEstimator.Handle estimatorHandle;private boolean firstRegistration = true;/*** This is the head of a linked list that is processed by {@link #callHandlerAddedForAllHandlers()} and so process* all the pending {@link #callHandlerAdded0(AbstractChannelHandlerContext)}.** We only keep the head because it is expected that the list is used infrequently and its size is small.* Thus full iterations to do insertions is assumed to be a good compromised to saving memory and tail management* complexity.*/private PendingHandlerCallback pendingHandlerCallbackHead;/*** Set to {@code true} once the {@link AbstractChannel} is registered.Once set to {@code true} the value will never* change.*/private boolean registered;protected DefaultChannelPipeline(Channel channel) {this.channel = ObjectUtil.checkNotNull(channel, "channel");succeededFuture = new SucceededChannelFuture(channel, null);voidPromise =  new VoidChannelPromise(channel, true);tail = new TailContext(this);head = new HeadContext(this);head.next = tail;tail.prev = head;}