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。
java
/**
* Default select strategy.
*/
final class DefaultSelectStrategy implements SelectStrategy {
static final SelectStrategy INSTANCE = new DefaultSelectStrategy();
private DefaultSelectStrategy() { }
@Override
public int calculateStrategy(IntSupplier selectSupplier, boolean hasTasks) throws Exception {
return hasTasks ? selectSupplier.get() : SelectStrategy.SELECT;
}
}还有 ReadTimeoutException 和 WriteTimeoutException
java
/**
* 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);
}
}
java
/**
* 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
java
/**
* Factory which uses the default select strategy.
*/
public final class DefaultSelectStrategyFactory implements SelectStrategyFactory {
public static final SelectStrategyFactory INSTANCE = new DefaultSelectStrategyFactory();
private DefaultSelectStrategyFactory() { }
@Override
public 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。
java
@Override
public EventExecutorChooser newChooser(EventExecutor[] executors) {
if (isPowerOfTwo(executors.length)) {
return new PowerOfTwoEventExecutorChooser(executors);
} else {
return new GenericEventExecutorChooser(executors);
}
}
java
private static final class PowerOfTwoEventExecutorChooser implements EventExecutorChooser {
private final AtomicInteger idx = new AtomicInteger();
private final EventExecutor[] executors;
PowerOfTwoEventExecutorChooser(EventExecutor[] executors) {
this.executors = executors;
}
@Override
public 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;
}
@Override
public 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。
java
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。
java
/**
* 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>>() {
@Override
protected 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;
}