Netty NioEventLoop 创建过程源码分析

前面 ，我们分析了 Netty 中的 Channel 组件，本篇我们来介绍一下与 Channel 关联的另一个核心的组件 —— EventLoop。

Netty 版本：4.1.30

概述

EventLoop 定义了 Netty 的核心抽象，用于处理网络连接生命周期中所有发生的事件。

我们先来从一个比较高的视角来了解一下 Channels、Thread、EventLoops、EventLoopGroups 之间的关系。

上图是表示了拥有 4 个 EventLoop 的 EventLoopGroup 处理 IO 的流程图。它们之间的关系如下：

• 一个 EventLoopGroup 包含一个或多个 EventLoop
• 一个 EventLoop 在它的生命周期内只和一个 Thread 绑定
• 所有由 EventLoop 处理的 I/O 事件都将在它专有的 Thread 上被处理
• 一个 Channel 在它的生命周期内只注册于一个 EventLoop
• 一个 EventLoop 可能会被分配给一个或多个 Channel

EventLoop 原理

下图是 Netty EventLoop 相关类的 UML 图。从中我们可以看到 EventLoop 相关的类都是实现了 java.util.concurrent 包中的 ExecutorService 接口。我们可以直接将任务 (Runable 或 Callable) 提交给 EventLoop 去立即执行或定时执行。

例如，使用 EventLoop 去执行定时任务，样例代码：

public static void scheduleViaEventLoop() {
    Channel ch = new NioSocketChannel();
    ScheduledFuture<?> future = ch.eventLoop().schedule(
            () -> System.out.println("60 seconds later"), 60, TimeUnit.SECONDS);
}

Thread 管理

Netty 线程模型的高性能主要取决于当前所执行线程的身份的确定。一个线程提交到 EventLoop 执行的流程如下：

• 将 Task 任务提交给 EventLoop 执行
• 在 Task 传递到 execute 方法之后，检查当前要执行的 Task 的线程是否是分配给 EventLoop 的那个线程
• 如果是，则该线程会立即执行
• 如果不是，则将线程放入任务队列中，等待下一次执行

其中，Netty 中的每一个 EventLoop 都有它自己的任务队列，并且和其他的 EventLoop 的任务队列独立开来。

Thread 分配

服务于 Channel 的 I/O 和事件的 EventLoop 包含在 EventLoopGroup 中。根据不同的传输实现，EventLoop 的创建和分配方式也不同。

NIO 传输

在 NIO 传输方式中，使用尽可能少的 EventLoop 就可以服务多个 Channel。如图所示，EventLoopGroup 采用顺序循环的方式负责为每一个新创建的 Channel 分配 EventLoop，每一个 EventLoop 会被分配给多个 Channels。

一旦一个 Channel 被分配给了一个 EventLoop，则这个 Channel 的生命周期内，只会绑定这个 EventLoop。这就让我们在 ChannelHandler 的实现省去了对线程安全和同步问题的担心。

OIO 传输

与 NIO 方式的不同在于，一个 EventLoop 只会服务于一个 Channel。

NioEventLoop & NioEventLoopGroup 创建

初步了解了 EventLoop 以及 EventLoopGroup 的工作机制，接下来我们以 NioEventLoopGroup 为例，来深入分析 NioEventLoopGroup 是如何创建的，又是如何启动的，它的内部执行逻辑又是怎样的等等问题。

MultithreadEventExecutorGroup 构造器

我们从 NioEventLoopGroup 的构造函数开始分析：

EventLoopGroup acceptorEventLoopGroup = new NioEventLoopGroup(1);

NioEventLoopGroup 构造函数会调用到父类 MultithreadEventLoopGroup 的构造函数，默认情况下，EventLoop 的数量 = 处理器数量 x 2：

public abstract class MultithreadEventLoopGroup extends MultithreadEventExecutorGroup implements EventLoopGroup {

    private static final InternalLogger logger = InternalLoggerFactory.getInstance(MultithreadEventLoopGroup.class);

    private static final int DEFAULT_EVENT_LOOP_THREADS;

    // 默认情况下，EventLoop的数量 = 处理器数量 x 2
    static {
        DEFAULT_EVENT_LOOP_THREADS = Math.max(1, SystemPropertyUtil.getInt(
                "io.netty.eventLoopThreads", NettyRuntime.availableProcessors() * 2));

        if (logger.isDebugEnabled()) {
            logger.debug("-Dio.netty.eventLoopThreads: {}", DEFAULT_EVENT_LOOP_THREADS);
        }
    }

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

    ...
}

继续调用父类，会调用到 MultithreadEventExecutorGroup 的构造器，主要做三件事情：

• 创建线程任务执行器 ThreadPerTaskExecutor
• 通过 for 循环创建数量为 nThreads 个的 EventLoop
• 创建 EventLoop 选择器 EventExecutorChooser

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

    // 创建任务执行器 ThreadPerTaskExecutor
    if (executor == null) {
        executor = new ThreadPerTaskExecutor(newDefaultThreadFactory());
    }

    // 创建 EventExecutor 数组
    children = new EventExecutor[nThreads];

    // 通过for循环创建数量为 nThreads 个的 EventLoop
    for (int i = 0; i < nThreads; i ++) {
        boolean success = false;
        try {
            // 调用 newChild 接口
            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;
                    }
                }
            }
        }
    }
	
    // 创建选择器
    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);
}

创建线程任务执行器 ThreadPerTaskExecutor

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

线程任务执行器 ThreadPerTaskExecutor 源码如下，具体的任务都由 ThreadFactory 去执行：

public final class ThreadPerTaskExecutor implements Executor {
    private final ThreadFactory threadFactory;

    public ThreadPerTaskExecutor(ThreadFactory threadFactory) {
        if (threadFactory == null) {
            throw new NullPointerException("threadFactory");
        }
        this.threadFactory = threadFactory;
    }
	
    // 使用 threadFactory 执行任务
    @Override
    public void execute(Runnable command) {
        threadFactory.newThread(command).start();
    }
}

来看看 newDefaultThreadFactory 方法：

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

DefaultThreadFactory

接下来看看 DefaultThreadFactory 这个类，实现了 ThreadFactory 接口，我们可以了解到：

• EventLoopGroup 的命名规则
• 具体的线程为 FastThreadLocalThread

public class DefaultThreadFactory implements ThreadFactory {
    
	// 线程池ID编号自增器
    private static final AtomicInteger poolId = new AtomicInteger();
	// 线程ID自增器
    private final AtomicInteger nextId = new AtomicInteger();
    // 线程名称前缀
    private final String prefix;
    // 是否为守护进程
    private final boolean daemon;
    // 线程优先级
    private final int priority;
    // 线程组
    protected final ThreadGroup threadGroup;

    public DefaultThreadFactory(Class<?> poolType) {
        this(poolType, false, Thread.NORM_PRIORITY);
    }

    ...

    // 获取线程名，返回结果：nioEventLoopGroup
    public static String toPoolName(Class<?> poolType) {
        if (poolType == null) {
            throw new NullPointerException("poolType");
        }

        String poolName = StringUtil.simpleClassName(poolType);
        switch (poolName.length()) {
            case 0:
                return "unknown";
            case 1:
                return poolName.toLowerCase(Locale.US);
            default:
                if (Character.isUpperCase(poolName.charAt(0)) && Character.isLowerCase(poolName.charAt(1))) {
                    return Character.toLowerCase(poolName.charAt(0)) + poolName.substring(1);
                } else {
                    return poolName;
                }
        }
    }

    public DefaultThreadFactory(String poolName, boolean daemon, int priority, ThreadGroup threadGroup) {
        if (poolName == null) {
            throw new NullPointerException("poolName");
        }
        if (priority < Thread.MIN_PRIORITY || priority > Thread.MAX_PRIORITY) {
            throw new IllegalArgumentException(
                    "priority: " + priority + " (expected: Thread.MIN_PRIORITY <= priority <= Thread.MAX_PRIORITY)");
        }
		
        // nioEventLoopGroup-2-
        prefix = poolName + '-' + poolId.incrementAndGet() + '-';
        this.daemon = daemon;
        this.priority = priority;
        this.threadGroup = threadGroup;
    }
	
    public DefaultThreadFactory(String poolName, boolean daemon, int priority) {
        this(poolName, daemon, priority, System.getSecurityManager() == null ?
                Thread.currentThread().getThreadGroup() : System.getSecurityManager().getThreadGroup());
    }
	
    @Override
    public Thread newThread(Runnable r) {
        // 创建新线程 nioEventLoopGroup-2-1
        Thread t = newThread(FastThreadLocalRunnable.wrap(r), prefix + nextId.incrementAndGet());
        try {
            if (t.isDaemon() != daemon) {
                t.setDaemon(daemon);
            }
            if (t.getPriority() != priority) {
                t.setPriority(priority);
            }
        } catch (Exception ignored) {
            // Doesn't matter even if failed to set.
        }
        return t;
    }
	
    // 创建新线程 FastThreadLocalThread
    protected Thread newThread(Runnable r, String name) {
        return new FastThreadLocalThread(threadGroup, r, name);
    }
    
}

创建 NioEventLoop

继续从 MultithreadEventExecutorGroup 构造器开始，创建完任务执行器 ThreadPerTaskExecutor 之后，进入 for 循环，开始创建 NioEventLoop：

for (int i = 0; i < nThreads; i ++) {
    boolean success = false;
    try {
        // 创建 nioEventLoop
        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);
    }
    
    ...
 	
}    

NioEventLoopGroup 类中的 newChild() 方法：

@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]);
}

NioEventLoop 构造器：

public final class NioEventLoop extends SingleThreadEventLoop{
    
    ...
    
    NioEventLoop(NioEventLoopGroup parent, Executor executor, SelectorProvider selectorProvider, SelectStrategy strategy, RejectedExecutionHandler rejectedExecutionHandler) {
        // 调用父类 SingleThreadEventLoop 构造器
        super(parent, executor, false, DEFAULT_MAX_PENDING_TASKS, rejectedExecutionHandler);
        if (selectorProvider == null) {
            throw new NullPointerException("selectorProvider");
        }
        if (strategy == null) {
            throw new NullPointerException("selectStrategy");
        }
        // 设置 selectorProvider
        provider = selectorProvider;
        // 获取 SelectorTuple 对象，里面封装了原生的selector和优化过的selector
        final SelectorTuple selectorTuple = openSelector();
        // 设置优化过的selector
        selector = selectorTuple.selector;
        // 设置原生的selector
        unwrappedSelector = selectorTuple.unwrappedSelector;
        // 设置select策略
        selectStrategy = strategy;
    }
	
	...
    
}

接下来我们看看 获取多路复用选择器 方法 —— openSelector () ，

// selectKey 优化选项flag
private static final boolean DISABLE_KEYSET_OPTIMIZATION =
    SystemPropertyUtil.getBoolean("io.netty.noKeySetOptimization", false);

private SelectorTuple openSelector() {
    // JDK原生的selector
    final Selector unwrappedSelector;
    try {
        // 通过 SelectorProvider 创建获得selector
        unwrappedSelector = provider.openSelector();
    } catch (IOException e) {
        throw new ChannelException("failed to open a new selector", e);
    }

    // 如果不优化，则直接返回
    if (DISABLE_KEYSET_OPTIMIZATION) {
        return new SelectorTuple(unwrappedSelector);
    }

    // 通过反射创建 sun.nio.ch.SelectorImpl 对象
    Object maybeSelectorImplClass = AccessController.doPrivileged(new PrivilegedAction<Object>() {
        @Override
        public Object run() {
            try {
                return Class.forName(
                        "sun.nio.ch.SelectorImpl",
                        false,
                        PlatformDependent.getSystemClassLoader());
            } catch (Throwable cause) {
                return cause;
            }
        }
    });
    
    // 如果 maybeSelectorImplClass 不是 selector 的一个实现，则直接返回原生的Selector 
    if (!(maybeSelectorImplClass instanceof Class) ||
        // ensure the current selector implementation is what we can instrument.
        // 确保当前的选择器实现是我们可以检测的
        !((Class<?>) maybeSelectorImplClass).isAssignableFrom(unwrappedSelector.getClass())) {
        if (maybeSelectorImplClass instanceof Throwable) {
            Throwable t = (Throwable) maybeSelectorImplClass;
            logger.trace("failed to instrument a special java.util.Set into: {}", unwrappedSelector, t);
        }
        return new SelectorTuple(unwrappedSelector);
    }
    // maybeSelectorImplClass 是selector的实现，则转化为 selector 实现类
    final Class<?> selectorImplClass = (Class<?>) maybeSelectorImplClass;
    // 创建新的 SelectionKey 集合 SelectedSelectionKeySet,内部采用的是 SelectionKey 数组的形
    // 式，而非 set 集合
    final SelectedSelectionKeySet selectedKeySet = new SelectedSelectionKeySet();

    Object maybeException = AccessController.doPrivileged(new PrivilegedAction<Object>() {
       @Override
       public Object run() {
           try {
            // 通过反射的方式获取 sun.nio.ch.SelectorImpl 的成员变量 selectedKeys
            Field selectedKeysField = selectorImplClass.getDeclaredField("selectedKeys");
            // 通过反射的方式获取 sun.nio.ch.SelectorImpl 的成员变量 publicSelectedKeys
            Field publicSelectedKeysField = selectorImplClass.getDeclaredField("publicSelectedKeys");

             if (PlatformDependent.javaVersion() >= 9 && PlatformDependent.hasUnsafe()) {
                 // Let us try to use sun.misc.Unsafe to replace the SelectionKeySet.
                 // This allows us to also do this in Java9+ without any extra flags.
           long selectedKeysFieldOffset = 	PlatformDependent.objectFieldOffset(selectedKeysField);
           long publicSelectedKeysFieldOffset =
                            PlatformDependent.objectFieldOffset(publicSelectedKeysField);

         if (selectedKeysFieldOffset != -1 && publicSelectedKeysFieldOffset != -1) {
             PlatformDependent.putObject( unwrappedSelector, selectedKeysFieldOffset, selectedKeySet);
             PlatformDependent.putObject(unwrappedSelector, publicSelectedKeysFieldOffset, selectedKeySet);
                        return null;
          }
                    // We could not retrieve the offset, lets try reflection as last-resort.
                }

               // 设置字段 selectedKeys  Accessible 为true
                Throwable cause = ReflectionUtil.trySetAccessible(selectedKeysField, true);
                if (cause != null) {
                    return cause;
                }
                // 设置字段 publicSelectedKeys  Accessible 为true
                cause = ReflectionUtil.trySetAccessible(publicSelectedKeysField, true);
                if (cause != null) {
                    return cause;
                }

                selectedKeysField.set(unwrappedSelector, selectedKeySet);
                publicSelectedKeysField.set(unwrappedSelector, selectedKeySet);
                return null;
            } catch (NoSuchFieldException e) {
                return e;
            } catch (IllegalAccessException e) {
                return e;
            }
        }
    });

    if (maybeException instanceof Exception) {
        selectedKeys = null;
        Exception e = (Exception) maybeException;
        logger.trace("failed to instrument a special java.util.Set into: {}", unwrappedSelector, e);
        return new SelectorTuple(unwrappedSelector);
    }
    
    // 设置 SelectedSelectionKeySet
    selectedKeys = selectedKeySet;
    logger.trace("instrumented a special java.util.Set into: {}", unwrappedSelector);
    // 返回包含了原生selector和优化过的selector的SelectorTuple
    return new SelectorTuple(unwrappedSelector,
                             new SelectedSelectionKeySetSelector(unwrappedSelector, selectedKeySet));
}

优化后的 SelectedSelectionKeySet 对象，内部采用 SelectionKey 数组的形式：

final class SelectedSelectionKeySet extends AbstractSet<SelectionKey> {
    SelectionKey[] keys;
    int size;

    SelectedSelectionKeySet() {
        keys = new SelectionKey[1024];
    }

    // 使用数组，来替代HashSet,可以降低时间复杂度为O(1)
    @Override
    public boolean add(SelectionKey o) {
        if (o == null) {
            return false;
        }

        keys[size++] = o;
        if (size == keys.length) {
            increaseCapacity();
        }

        return true;
    }

    @Override
    public boolean remove(Object o) {
        return false;
    }

    @Override
    public boolean contains(Object o) {
        return false;
    }

    @Override
    public int size() {
        return size;
    }

    @Override
    public Iterator<SelectionKey> iterator() {
        return new Iterator<SelectionKey>() {
            private int idx;

            @Override
            public boolean hasNext() {
                return idx < size;
            }

            @Override
            public SelectionKey next() {
                if (!hasNext()) {
                    throw new NoSuchElementException();
                }
                return keys[idx++];
            }

            @Override
            public void remove() {
                throw new UnsupportedOperationException();
            }
        };
    }

    void reset() {
        reset(0);
    }

    void reset(int start) {
        Arrays.fill(keys, start, size, null);
        size = 0;
    }

    // 扩容
    private void increaseCapacity() {
        SelectionKey[] newKeys = new SelectionKey[keys.length << 1];
        System.arraycopy(keys, 0, newKeys, 0, size);
        keys = newKeys;
    }
}

SingleThreadEventLoop 构造器

public abstract class SingleThreadEventLoop extends SingleThreadEventExecutor implements EventLoop {
    
    ...
    
	protected SingleThreadEventLoop(EventLoopGroup parent, Executor executor,
                                boolean addTaskWakesUp, int maxPendingTasks,
                                RejectedExecutionHandler rejectedExecutionHandler) {
        // 调用 SingleThreadEventExecutor 构造器
        super(parent, executor, addTaskWakesUp, maxPendingTasks, rejectedExecutionHandler);
        tailTasks = newTaskQueue(maxPendingTasks);
	}
    
    ...
}

SingleThreadEventExecutor 构造器，主要做两件事情：

• 设置线程任务执行器。
• 设置任务队列。前面讲到 EventLoop 对于不能立即执行的 Task 会放入一个队列中，就是这里设置的。

public abstract class SingleThreadEventExecutor extends AbstractScheduledEventExecutor implements OrderedEventExecutor {
	
    ...
    
    protected SingleThreadEventExecutor(EventExecutorGroup parent, Executor executor,
                                    boolean addTaskWakesUp, int maxPendingTasks,
                                    RejectedExecutionHandler rejectedHandler) {
        super(parent);
        this.addTaskWakesUp = addTaskWakesUp;
        this.maxPendingTasks = Math.max(16, maxPendingTasks);
        // 设置线程任务执行器
        this.executor = ObjectUtil.checkNotNull(executor, "executor");
        // 设置任务队列
        taskQueue = newTaskQueue(this.maxPendingTasks);
        rejectedExecutionHandler = ObjectUtil.checkNotNull(rejectedHandler, "rejectedHandler");
    
	}    
    
    ...
    
}

NioEventLoop 中对 newTaskQueue 接口的实现，返回的是 JCTools 工具包 Mpsc 队列。后面我们写文章单独介绍 JCTools 中的相关队列。

Mpsc：Multi Producer Single Consumer (Lock less, bounded and unbounded)

多个生产者对单个消费者（无锁、有界和无界都有实现）

public final class NioEventLoop extends SingleThreadEventLoop {

    ...

    @Override
    protected Queue<Runnable> newTaskQueue(int maxPendingTasks) {
        // This event loop never calls takeTask()
        return maxPendingTasks == Integer.MAX_VALUE ? PlatformDependent.<Runnable>newMpscQueue()
                                                    : PlatformDependent.<Runnable>newMpscQueue(maxPendingTasks);
    }
    
    ...
}

创建线程执行选择器 chooser

接下来，我们看看 MultithreadEventExecutorGroup 构造器的最后一个部分内容，创建线程执行选择器 chooser，它的主要作用就是 EventLoopGroup 用于从 EventLoop 数组中选择一个 EventLoop 去执行任务。

// 创建选择器
chooser = chooserFactory.newChooser(children);

EventLoopGroup 中定义的 next() 接口：

public interface EventLoopGroup extends EventExecutorGroup {
	
	...
	
	// 选择下一个 EventLoop 用于执行任务
    @Override
    EventLoop next();
	
	...

}

MultithreadEventExecutorGroup 中对 next () 的实现：

@Override
public EventExecutor next() {
    // 调用 DefaultEventExecutorChooserFactory 中的next()
    return chooser.next();
}

DefaultEventExecutorChooserFactory 对于如何从数组中选择任务执行器，也做了巧妙的优化。

public final class DefaultEventExecutorChooserFactory implements EventExecutorChooserFactory {

    public static final DefaultEventExecutorChooserFactory INSTANCE = new DefaultEventExecutorChooserFactory();

    private DefaultEventExecutorChooserFactory() { }

    @SuppressWarnings("unchecked")
    @Override
    public EventExecutorChooser newChooser(EventExecutor[] executors) {
        if (isPowerOfTwo(executors.length)) {
            return new PowerOfTwoEventExecutorChooser(executors);
        } else {
            return new GenericEventExecutorChooser(executors);
        }
    }
	
    // 判断线程任务执行的个数是否为 2 的幂次方。e.g: 2、4、8、16
    private static boolean isPowerOfTwo(int val) {
        return (val & -val) == val;
    }
	
    // 幂次方选择器
    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() {
            // 通过二级制进行 & 运算，效率更高
            return executors[idx.getAndIncrement() & executors.length - 1];
        }
    }

    // 普通选择器
    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() {
            // 按照最普通的取模的方式从index=0开始向后开始选择
            return executors[Math.abs(idx.getAndIncrement() % executors.length)];
        }
    }
}

小结

通过本节内容，我们了解到了 EventLoop 与 EventLoopGroup 的基本原理，EventLoopGroup 与 EventLoop 的创建过程：

• 创建线程任务执行器 ThreadPerTaskExecutor
• 创建 EventLoop
• 创建任务选择器 EventExecutorChooser

参考资料

• Java 读源码之 Netty 深入剖析
• 《Netty in action》