一. 代碼下載

Netty代碼下載和編譯參考前一篇Netty文章
http://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>() {
                    @Override
                    protected 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繼承關係

一步步來看，首先看第一個註釋，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採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()
然後把構造好的參數都傳給父類MultithreadEventLoopGroup (繼承關係可以看上圖)

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，也許後面分析會有用，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，那麼就會採用默認的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children
            boolean 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chooser

        final FutureListener<Object> terminationListener = new FutureListener<Object>() {
            @Override
            public 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的繼承關係是這樣的，繼承於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裏面保存了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，然後它的openSelector會生成一個KQueueSelectorImpl
所以provider.openSelector()得到是KQueueSelectorImpl，KQueueSelectorImpl的繼承關係如下：

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;
        }

        @Override
        public 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;
        }

        @Override
        public 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，然後+1。再做&運算。
大家自己可以拿到數字舉個例子，比較巧妙。後續自己寫代碼可以借鑑，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的不同之處在於next方法的算法不一樣，作用都是從NioEventLoop數組裏面選出一個NioEventLoop對象來。

但是說實話，我沒有想到這兩種算法有什麼區別，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，數組的大小等於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)，這種關係有點像android裏面ViewGroup和View的關係。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，單線程有沒有關係。

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

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

6. NioEventLoop裏面包含了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() { }

    @Override
    public SelectStrategy newSelectStrategy() {
        return DefaultSelectStrategy.INSTANCE;
    }
}

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;
    }
}

所以SelectStrategy的實現類是DefaultSelectStrategy.

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

56. Netty源代碼分析-服務器初始化 NioEventLoopGroup實例化

一. 代碼下載

二. 服務器代碼分析

2.1 服務器代碼編寫

2.2 NioEventLoopGroup

2.2.1 NioEventLoopGroup繼承關係

2.2.2 NioEventLoopGroup構造函數

2.2.3 SelectorProvider.provider()

2.2.4 MultithreadEventLoopGroup

2.2.5 MultithreadEventExecutorGroup

2.2.6 NioEventLoop

2.2.7 newChooser

2.2.8 newChooser

三. 總結

70.Flutter筆記

67.Python學習筆記

24. iOS基本語法

66.Spring源代碼分析-代碼下載

65.iOS下面LKDatabase 導出和查看

Mac下配置sublime實現LaTeX

https://yachay.unat.edu.pe/blog/index.php?comment_area=format_blog&comment_component=blog&comment_co

linux以太網驅動總結