Netty源码分析之服务启动

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本文主要分析Netty服务端的启动过程。

Netty是基于Nio实现的，所以也离不开selector、serverSocketChannel、socketChannel和selectKey等，只不过Netty把这些实现都封装在了底层。

从示例可以看出，一切从ServerBootstrap开始。

ServerBootstrap实例中需要两个NioEventLoopGroup实例，按照职责划分成boss和work，有着不同的分工：
1、boss负责请求的accept
2、work负责请求的read、write

NioEventLoopGroup

NioEventLoopGroup主要管理eventLoop的生命周期。
eventLoop是什么？姑且把它看成是内部的一个处理线程，数量默认是处理器个数的两倍。

NioEventLoopGroup构造方法：

public NioEventLoopGroup() {  
    this(0);  
}  
  
public NioEventLoopGroup(int nThreads) {  
    this(nThreads, null); 
}  
  
public NioEventLoopGroup(int nThreads, ThreadFactory threadFactory) {  
    this(nThreads, threadFactory, SelectorProvider.provider());  
}  
  
public NioEventLoopGroup(  
            int nThreads, ThreadFactory threadFactory, final SelectorProvider selectorProvider) {  
    super(nThreads, threadFactory, selectorProvider);  
}  

MultithreadEventLoopGroup是NioEventLoopGroup的父类，构造方法：

protected MultithreadEventLoopGroup(int nThreads, ThreadFactory threadFactory, Object... args) {  
    super(nThreads == 0? DEFAULT_EVENT_LOOP_THREADS : nThreads, threadFactory, args);  
}  

其中 DEFAULT_EVENT_LOOP_THREADS 为处理器数量的两倍。

MultithreadEventExecutorGroup是核心，管理eventLoop的生命周期，先看看其中几个变量。
1、children：EventExecutor数组，保存eventLoop。
2、chooser：从children中选取一个eventLoop的策略。

构造方法：

protected MultithreadEventExecutorGroup(int nThreads, ThreadFactory threadFactory, Object... args) {
    if (nThreads <= 0) {
        throw new IllegalArgumentException(String.format("nThreads: %d (expected: > 0)", nThreads));
    }

    if (threadFactory == null) {
        threadFactory = newDefaultThreadFactory();
    }

    children = new SingleThreadEventExecutor[nThreads];
    if (isPowerOfTwo(children.length)) {
        chooser = new PowerOfTwoEventExecutorChooser();
    } else {
        chooser = new GenericEventExecutorChooser();
    }

    for (int i = 0; i < nThreads; i ++) {
        boolean success = false;
        try {
            children[i] = newChild(threadFactory, args);
            success = true;
        } catch (Exception e) {
            // TODO: Think about if this is a good exception type
            throw new IllegalStateException("failed to create a child event loop", e);
        } finally {
            if (!success) {
                for (int j = 0; j < i; j ++) {
                    children[j].shutdownGracefully();
                }

                for (int j = 0; j < i; j ++) {
                    EventExecutor e = children[j];
                    try {
                        while (!e.isTerminated()) {
                            e.awaitTermination(Integer.MAX_VALUE, TimeUnit.SECONDS);
                        }
                    } catch (InterruptedException interrupted) {
                        Thread.currentThread().interrupt();
                        break;
                    }
                }
            }
        }
    }

    final FutureListener