【Netty源码学习】EventLoopGroup

在上一篇博客【Netty源码解析】入门示例中我们介绍了一个Netty入门的示例代码，接下来的博客我们会分析一下整个demo工程运行过程的运行机制。 无论在Netty应用的客户端还是服务端都首先会初始化EventLoopGroup，其封装了线程的创建，以及loop和线程的关联等内容。

EventLoopGroup group = new NioEventLoopGroup();  

接下来我们一步一步看NioEventLoopGroup在初始化过程中都做了什么事情，其源码如下：

/**
 * {@link MultithreadEventLoopGroup} implementations which is used for NIO {@link Selector} based {@link Channel}s.
 */
public class NioEventLoopGroup extends MultithreadEventLoopGroup {
    /**
     * Create a new instance using the default number of threads, the default {@link ThreadFactory} and
     * the {@link SelectorProvider} which is returned by {@link SelectorProvider#provider()}.
     */
    public NioEventLoopGroup() {
        this(0);
    }
    /**
     * Create a new instance using the specified number of threads, {@link ThreadFactory} and the
     * {@link SelectorProvider} which is returned by {@link SelectorProvider#provider()}.
     */
    public NioEventLoopGroup(int nThreads) {
        this(nThreads, (Executor) null);
    }
    /**
     * Create a new instance using the specified number of threads, the given {@link ThreadFactory} and the
     * {@link SelectorProvider} which is returned by {@link SelectorProvider#provider()}.
     */
    public NioEventLoopGroup(int nThreads, ThreadFactory threadFactory) {
        this(nThreads, threadFactory, SelectorProvider.provider());
    }
    public NioEventLoopGroup(int nThreads, Executor executor) {
        this(nThreads, executor, SelectorProvider.provider());
    }
    /**
     * Create a new instance using the specified number of threads, the given {@link ThreadFactory} and the given
     * {@link SelectorProvider}.
     */
    public NioEventLoopGroup(
            int nThreads, ThreadFactory threadFactory, final SelectorProvider selectorProvider) {
        this(nThreads, threadFactory, selectorProvider, DefaultSelectStrategyFactory.INSTANCE);
    }
    public NioEventLoopGroup(int nThreads, ThreadFactory threadFactory,
        final SelectorProvider selectorProvider, final SelectStrategyFactory selectStrategyFactory) {
        super(nThreads, threadFactory, selectorProvider, selectStrategyFactory, RejectedExecutionHandlers.reject());
    }
    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());
    }
    public NioEventLoopGroup(int nThreads, Executor executor, EventExecutorChooserFactory chooserFactory,
                             final SelectorProvider selectorProvider,
                             final SelectStrategyFactory selectStrategyFactory) {
        super(nThreads, executor, chooserFactory, selectorProvider, selectStrategyFactory,
                RejectedExecutionHandlers.reject());
    }
    public NioEventLoopGroup(int nThreads, Executor executor, EventExecutorChooserFactory chooserFactory,
                             final SelectorProvider selectorProvider,
                             final SelectStrategyFactory selectStrategyFactory,
                             final RejectedExecutionHandler rejectedExecutionHandler) {
        super(nThreads, executor, chooserFactory, selectorProvider, selectStrategyFactory, rejectedExecutionHandler);
    }
    /**
     * Sets the percentage of the desired amount of time spent for I/O in the child event loops.  The default value is
     * {@code 50}, which means the event loop will try to spend the same amount of time for I/O as for non-I/O tasks.
     */
    public void setIoRatio(int ioRatio) {
        for (EventExecutor e: this) {
            ((NioEventLoop) e).setIoRatio(ioRatio);
        }
    }
    /**
     * Replaces the current {@link Selector}s of the child event loops with newly created {@link Selector}s to work
     * around the  infamous epoll 100% CPU bug.
     */
    public void rebuildSelectors() {
        for (EventExecutor e: this) {
            ((NioEventLoop) e).rebuildSelector();
        }
    }
    @Override
    protected EventLoop newChild(Executor executor, Object... args) throws Exception {
        return new NioEventLoop(this, executor, (SelectorProvider) args[0],
            ((SelectStrategyFactory) args[1]).newSelectStrategy(), (RejectedExecutionHandler) args[2]);
    }
}

由上面的源码我们了解到NioEventLoopGroup构造函数初始化的结果是调用父类MultithreadEventLoopGroup的构造函数，接下来我们看一下MultithreadEventLoopGroup的构造函数做了什么事情。

public abstract class MultithreadEventExecutorGroup extends AbstractEventExecutorGroup {

    private final EventExecutor[] children;
    private final Set<EventExecutor> readonlyChildren;
    private final AtomicInteger terminatedChildren = new AtomicInteger();
    private final Promise<?> terminationFuture = new DefaultPromise(GlobalEventExecutor.INSTANCE);
    private final EventExecutorChooserFactory.EventExecutorChooser chooser;

    protected MultithreadEventExecutorGroup(int nThreads, ThreadFactory threadFactory, Object... args) {
        this(nThreads, threadFactory == null ? null : new ThreadPerTaskExecutor(threadFactory), args);
    }

    protected MultithreadEventExecutorGroup(int nThreads, Executor executor, Object... args) {
        this(nThreads, executor, DefaultEventExecutorChooserFactory.INSTANCE, args);
    }

    /**
     * Create a new instance.
     *
     * @param nThreads          the number of threads that will be used by this instance.
     * @param executor          the Executor to use, or {@code null} if the default should be used.
     * @param chooserFactory    the {@link EventExecutorChooserFactory} to use.
     * @param args              arguments which will passed to each {@link #newChild(Executor, Object...)} call
     */
    protected MultithreadEventExecutorGroup(int nThreads, Executor executor,
                                            EventExecutorChooserFactory chooserFactory, Object... args) {
        if (nThreads <= 0) {
            throw new IllegalArgumentException(String.format("nThreads: %d (expected: > 0)", nThreads));
        }

        if (executor == null) {
            executor = new ThreadPerTaskExecutor(newDefaultThreadFactory());
        }

        children = new EventExecutor[nThreads];

        for (int i = 0; i < nThreads; i ++) {
            boolean success = false;
            try {
                children[i] = newChild(executor, 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) {
                            // Let the caller handle the interruption.
                            Thread.currentThread().interrupt();
                            break;
                        }
                    }
                }
            }
        }

        chooser = chooserFactory.newChooser(children);

        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);
    }

    protected ThreadFactory newDefaultThreadFactory() {
        return new DefaultThreadFactory(getClass());
    }

    @Override
    public EventExecutor next() {
        return chooser.next();
    }

    @Override
    public Iterator<EventExecutor> iterator() {
        return readonlyChildren.iterator();
    }

    /**
     * Return the number of {@link EventExecutor} this implementation uses. This number is the maps
     * 1:1 to the threads it use.
     */
    public final int executorCount() {
        return children.length;
    }

    /**
     * Create a new EventExecutor which will later then accessible via the {@link #next()}  method. This method will be
     * called for each thread that will serve this {@link MultithreadEventExecutorGroup}.
     *
     */
    protected abstract EventExecutor newChild(Executor executor, Object... args) throws Exception;

    @Override
    public Future<?> shutdownGracefully(long quietPeriod, long timeout, TimeUnit unit) {
        for (EventExecutor l: children) {
            l.shutdownGracefully(quietPeriod, timeout, unit);
        }
        return terminationFuture();
    }

    @Override
    public Future<?> terminationFuture() {
        return terminationFuture;
    }

    @Override
    @Deprecated
    public void shutdown() {
        for (EventExecutor l: children) {
            l.shutdown();
        }
    }

    @Override
    public boolean isShuttingDown() {
        for (EventExecutor l: children) {
            if (!l.isShuttingDown()) {
                return false;
            }
        }
        return true;
    }

    @Override
    public boolean isShutdown() {
        for (EventExecutor l: children) {
            if (!l.isShutdown()) {
                return false;
            }
        }
        return true;
    }

    @Override
    public boolean isTerminated() {
        for (EventExecutor l: children) {
            if (!l.isTerminated()) {
                return false;
            }
        }
        return true;
    }

    @Override
    public boolean awaitTermination(long timeout, TimeUnit unit)
            throws InterruptedException {
        long deadline = System.nanoTime() + unit.toNanos(timeout);
        loop: for (EventExecutor l: children) {
            for (;;) {
                long timeLeft = deadline - System.nanoTime();
                if (timeLeft <= 0) {
                    break loop;
                }
                if (l.awaitTermination(timeLeft, TimeUnit.NANOSECONDS)) {
                    break;
                }
            }
        }
        return isTerminated();
    }
}

从上面代码的构造函数中我们发现了真正处理功能

（1）executor = new ThreadPerTaskExecutor(newDefaultThreadFactory()); 首先创建一个单线程执行的线程池

（2）children = new EventExecutor[nThreads];  创建处理器两倍数目的处理线程数组

（3）children[i] = newChild(executor, args)  实例化线程数组，具体实现在NioEventLoopGroup

@Override
    protected EventLoop newChild(Executor executor, Object... args) throws Exception {
        return new NioEventLoop(this, executor, (SelectorProvider) args[0],
            ((SelectStrategyFactory) args[1]).newSelectStrategy(), (RejectedExecutionHandler) args[2]);
    }

这样EventLoopGroup的主要功能就完成了，其主要机制就是创建了一定数目的NioEventLoop。

EventLoopGroup类图结构：