一. 代碼下載

Netty代碼下載和編譯參考前一篇Netty文章
https://blog.51cto.com/483181/2112163

二. 服務器代碼分析

2.1 服務器代碼編寫

一般Netty服務器端這樣編寫

EventLoopGroup bossGroup = new NioEventLoopGroup(); //1. 實例化NioEventLoopGroup對象EventLoopGroup workerGroup = new NioEventLoopGroup();try {ServerBootstrap b = new ServerBootstrap(); //2. b.group(bossGroup, workerGroup) //3. .channel(NioServerSocketChannel.class).option(ChannelOption.SO_BACKLOG, 100).handler(new LoggingHandler(LogLevel.INFO)).childHandler(new ChannelInitializer<SocketChannel>() {@Overrideprotected void initChannel(SocketChannel ch) throws Exception {ch.pipeline().addLast(new FixedLengthFrameDecoder(20));}});ChannelFuture f = b.bind(port).sync(); //4.f.channel().closeFuture().sync();} catch (Exception e) {e.printStackTrace();} finally {bossGroup.shutdownGracefully();workerGroup.shutdownGracefully();}

2.2 NioEventLoopGroup

2.2.1 NioEventLoopGroup繼承關系

一步步來看，首先看第一個注釋，初始化NioEventLoopGroup對象

EventLoopGroup bossGroup = new NioEventLoopGroup(); //1. 實例化NioEventLoopGroup對象

下圖是NioEventLoopGroup的類繼承圖，包含類成員和方法，比較詳細。 這個功能是IntelliJ 自帶的。
右擊NioEventLoopGroup類名，選擇Diagrams->Show Diagram->上面有f，m的按鈕，分別對應field和method。

如下：

2.2.2 NioEventLoopGroup構造函數

public NioEventLoopGroup() {this(0);}public NioEventLoopGroup(int nThreads) {this(nThreads, (Executor) null);}public NioEventLoopGroup(int nThreads, Executor executor) {this(nThreads, executor, SelectorProvider.provider());} public NioEventLoopGroup(int nThreads, Executor executor, final SelectorProvider selectorProvider) {this(nThreads, executor, selectorProvider, DefaultSelectStrategyFactory.INSTANCE);}public NioEventLoopGroup(int nThreads, Executor executor, final SelectorProvider selectorProvider,final SelectStrategyFactory selectStrategyFactory) {super(nThreads, executor, selectorProvider, selectStrategyFactory, RejectedExecutionHandlers.reject());}

我們可以看到幾點

NioEventLoopGroup采用的是構造函數重載的方式，以適應不同的初始化場景

Executor傳的是null

SelectorProvider用的是SelectorProvider.provider()

然后把構造好的參數都傳給父類MultithreadEventLoopGroup (繼承關系可以看上圖)

2.2.3 SelectorProvider.provider()

private static SelectorProvider provider = null;public static SelectorProvider provider() {synchronized (lock) {if (provider != null)return provider;return AccessController.doPrivileged(new PrivilegedAction<SelectorProvider>() {public SelectorProvider run() {if (loadProviderFromProperty())return provider;if (loadProviderAsService())return provider;provider = sun.nio.ch.DefaultSelectorProvider.create();return provider;}});}}public class DefaultSelectorProvider {private DefaultSelectorProvider() {}public static SelectorProvider create() {return new KQueueSelectorProvider();} }public class KQueueSelectorProvider extends SelectorProviderImpl {public KQueueSelectorProvider() {}public AbstractSelector openSelector() throws IOException {return new KQueueSelectorImpl(this);} }

這段代碼我們也可以看到幾點:

SelectorProvider provider是一個單例,static類型

SelectorProvider.provider的實現，產生了一個KQueueSelectorProvider

KQueueSelectorProvider的openSelector會生成一個KQueueSelectorImpl

這個先記下來，也許后面分析會有用，繼續分析MultithreadEventLoopGroup的構造函數。

2.2.4 MultithreadEventLoopGroup

protected MultithreadEventLoopGroup(int nThreads, ThreadFactory threadFactory, Object... args) {super(nThreads == 0 ? DEFAULT_EVENT_LOOP_THREADS : nThreads, threadFactory, args);}private static final int DEFAULT_EVENT_LOOP_THREADS;static {DEFAULT_EVENT_LOOP_THREADS = Math.max(1, SystemPropertyUtil.getInt("io.netty.eventLoopThreads", NettyRuntime.availableProcessors() * 2));}

上面這段代碼我們可以看到這幾點：

如果我們實例化NioEventLoopGroup沒有傳入參數，也就是沒有nThreads，那么就會采用默認的DEFAULT_EVENT_LOOP_THREADS
DEFAULT_EVENT_LOOP_THREADS如果沒有配置io.netty.eventLoopThreads的話，一般是cpu核數*2

MultithreadEventLoopGroup的實例化方法是繼續調用父類的初始化方法。

繼續父類MultithreadEventExecutorGroup

2.2.5 MultithreadEventExecutorGroup

protected MultithreadEventExecutorGroup(int nThreads, Executor executor,EventExecutorChooserFactory chooserFactory, Object... args) {...children = new EventExecutor[nThreads]; //1. 實例化children數組for (int i = 0; i < nThreads; i ++) { //2. 循環初始化childrenboolean success = false;try {children[i] = newChild(executor, args);success = true;} catch (Exception e) {throw new IllegalStateException("failed to create a child event loop", e);} finally {...}}chooser = chooserFactory.newChooser(children); //3. 實例化chooserfinal FutureListener<Object> terminationListener = new FutureListener<Object>() {@Overridepublic void operationComplete(Future<Object> future) throws Exception {if (terminatedChildren.incrementAndGet() == children.length) {terminationFuture.setSuccess(null);}}};for (EventExecutor e: children) {e.terminationFuture().addListener(terminationListener);}Set<EventExecutor> childrenSet = new LinkedHashSet<EventExecutor>(children.length);Collections.addAll(childrenSet, children);readonlyChildren = Collections.unmodifiableSet(childrenSet);}

上面這段代碼可以從下面幾個點分析：

private final EventExecutor[] children;

children - EventExecutor數組，大小是nThreads，線程數目。

newChild初始化
實例類是NioEventLoopGroup.java，返回NioEventLoop對象 protected EventLoop newChild(Executor executor, Object... args) throws Exception {return new NioEventLoop(this, executor, (SelectorProvider) args[0],((SelectStrategyFactory) args[1]).newSelectStrategy(), (RejectedExecutionHandler) args[2]); }

NioEventLoop的繼承關系是這樣的，繼承于SingleThreadEventLoop，別忘了上面我們看到NioEventLoopGroup繼承自MultithreadEventLoopGroup.(看名字是單線程和多線程的區別？)

繼續看NioEventLoop的構造函數

2.2.6 NioEventLoop

NioEventLoop(NioEventLoopGroup parent, Executor executor, SelectorProvider selectorProvider,SelectStrategy strategy, RejectedExecutionHandler rejectedExecutionHandler) {super(parent, executor, false, DEFAULT_MAX_PENDING_TASKS, rejectedExecutionHandler);provider = selectorProvider;final SelectorTuple selectorTuple = openSelector();selector = selectorTuple.selector;unwrappedSelector = selectorTuple.unwrappedSelector;selectStrategy = strategy;}private SelectorTuple openSelector() {final Selector unwrappedSelector;try {unwrappedSelector = provider.openSelector();} catch (IOException e) {throw new ChannelException("failed to open a new selector", e);}if (DISABLE_KEYSET_OPTIMIZATION) {return new SelectorTuple(unwrappedSelector);}... }

從上面這段代碼我們可以看出這幾點

NioEventLoop里面保存了SelectorProvider selectorProvider, Selector selector, unwrappedSelector(類型是KQueueSelectorImpl)

selector, unwrappedSelector是通過provider.openSelector()打開的.
根據2.3段的介紹，provider之前介紹的類型是KQueueSelectorProvider，然后它的openSelector會生成一個KQueueSelectorImpl
所以provider.openSelector()得到是KQueueSelectorImpl，KQueueSelectorImpl的繼承關系如下：

繼續往回看，看MultithreadEventExecutorGroup的構造函數。

2.2.7 newChooser

EventExecutorChooserFactory.EventExecutorChooser chooser;protected MultithreadEventExecutorGroup(int nThreads, Executor executor, Object... args) {this(nThreads, executor, DefaultEventExecutorChooserFactory.INSTANCE, args);}chooser = chooserFactory.newChooser(children);

上面代碼我們可以看到：

chooserFactory的類型是DefaultEventExecutorChooserFactory，所以newChooser調用的是DefaultEventExecutorChooserFactory.newChooser方法。
如下：

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

傳入的參數是children，也就是NioEventLoop數組

DefaultEventExecutorChooserFactory INSTANCE是一個static final類型對象，也就是一種餓漢式的單例模式，如下：

public static final DefaultEventExecutorChooserFactory INSTANCE = new DefaultEventExecutorChooserFactory();

繼續看newChooser的實現

2.2.8 newChooser

newChooser的代碼就不貼了，上面就有，從上面代碼可以看到：

isPowerOfTwo是用來判斷一個整數是否是2的冪，比如(2,4, 8,16,32等等)，它的實現方式如下：

private static boolean isPowerOfTwo(int val) {return (val & -val) == val;}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 {private final AtomicInteger idx = new AtomicInteger();private final EventExecutor[] executors;GenericEventExecutorChooser(EventExecutor[] executors) {this.executors = executors;}@Overridepublic EventExecutor next() {return executors[Math.abs(idx.getAndIncrement() % executors.length)];}}

這種實現方法感覺比較優雅和高效，首先拿到-val，也就是val的二進制倒轉，然后+1。再做&運算。
大家自己可以拿到數字舉個例子，比較巧妙。后續自己寫代碼可以借鑒，這是讀源碼的一個好處，可以學習到別人很多優秀的寫法。

PowerOfTwoEventExecutorChooser和GenericEventExecutorChooser的不同之處在于next方法的算法不一樣，作用都是從NioEventLoop數組里面選出一個NioEventLoop對象來。

但是說實話，我沒有想到這兩種算法有什么區別，如果誰知道，請告訴我，謝謝。

return executors[idx.getAndIncrement() & executors.length - 1];return executors[Math.abs(idx.getAndIncrement() % executors.length)];

繼續往回走，MultithreadEventExecutorGroup的構造函數就基本看完了。

三. 總結

我們來總結下NioEventLoopGroup的實例化過程，可以得到以下幾點。

1. NioEventLoopGroup的父類MultithreadEventExecutorGroup包含一個NioEventLoop數組children，數組的大小等于nThreads線程數目。如果沒有指定，默認一般是cpu核數 x 2

2. NioEventLoopGroup和NioEventLoop一樣都是繼承自Executor，但是NioEventLoopGroup又包含多個NioEventLoop(children數組)，這種關系有點像android里面ViewGroup和View的關系。或者裝飾者模式？

3. NioEventLoopGroup繼承自MultithreadEventLoopGroup，而NioEventLoop繼承自SingleThreadEventLoop，從名字看，不知道和多線程，單線程有沒有關系。

4. MultithreadEventLoopGroup有個chooser，執行next方法的時候，會選擇下一個NioEventLoop對象，雖然并不知道兩個chooser算法有何區別。

5. NioEventLoopGroup里面重寫了newChild方法，里面實例化NioEventLoop。

6. NioEventLoop里面包含了Selector，類型是KQueueSelectorImpl

SelectorProvider provider
SelectStrategy selectStrategy

SelectStrategy這個我們上面我們沒有關注，其實它是NioEventLoopGroup構造函數傳進去的，如下：

public NioEventLoopGroup(int nThreads, Executor executor, final SelectorProvider selectorProvider) {this(nThreads, executor, selectorProvider, DefaultSelectStrategyFactory.INSTANCE);}public final class DefaultSelectStrategyFactory implements SelectStrategyFactory {public static final SelectStrategyFactory INSTANCE = new DefaultSelectStrategyFactory();private DefaultSelectStrategyFactory() { }@Overridepublic SelectStrategy newSelectStrategy() {return DefaultSelectStrategy.INSTANCE;} }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;} }

所以SelectStrategy的實現類是DefaultSelectStrategy.

在理清楚NioEventLoopGroup實例化的過程之后，我們下一篇繼續按照源代碼分析Netty服務器端的源代碼。

轉載于:https://blog.51cto.com/483181/2118817

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