netty之NioEventLoopGroup和NioEventLoop

Netty实战抠的图，说明了Channel、EventLoop、Thread以及EventLoopGroup之间的关系。

• 一个EventLoopGroup包含一个或多个EventLoop
• 一个EventLoop在他的生命周期内之和一个Thread绑定
• 所以由EventLoop处理的I/O事件都将在他专有的Thread上被处理
• 一个Channel在他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只定义了一个parent()方法，用于返回到当前EventLoop实现的实例所属的EventLoopGroup的引用

public interface EventLoop extends OrderedEventExecutor, EventLoopGroup {
    EventLoopGroup parent();
}

parent()方法重写了EventExecutor的EventExecutorGroup .parent()方法

public interface EventExecutor extends EventExecutorGroup {
    EventExecutor next();
    EventExecutorGroup parent();
    .....
}

---

我们实际使用NioEventLoopGroup和NioEventLoop，先看看NioEventLoopGroup。是一系列对NioEventLoopGroup一串构造方法的引用。Netty是对NIO的高度封装，NIO里重要部件Selector，这里SelectorProvider.provider()就是根据不同操作系统平台获取相应Selector。

//无参构造方法，线程数为0
public NioEventLoopGroup() {
        this(0);
    }
//nTreads == 0 此时，Executor为null
    public NioEventLoopGroup(int nThreads) {
        this(nThreads, (Executor)null);
    }

    public NioEventLoopGroup(int nThreads, ThreadFactory threadFactory) {
        this(nThreads, threadFactory, SelectorProvider.provider());
    }

    public NioEventLoopGroup(int nThreads, Executor executor) {
        this(nThreads, executor, SelectorProvider.provider());
    }

    public NioEventLoopGroup(int nThreads, ThreadFactory threadFactory, SelectorProvider selectorProvider) {
        this(nThreads, threadFactory, selectorProvider, DefaultSelectStrategyFactory.INSTANCE);
    }

构造方法中，继续调用他的父类MultithreadEventLoopGroup的构造方法。

	public NioEventLoopGroup(int nThreads, ThreadFactory threadFactory, SelectorProvider selectorProvider, SelectStrategyFactory selectStrategyFactory) {
        super(nThreads, threadFactory, new Object[]{selectorProvider, selectStrategyFactory, RejectedExecutionHandlers.reject()});
    }
	public NioEventLoopGroup(int nThreads, Executor executor, SelectorProvider selectorProvider, SelectStrategyFactory selectStrategyFactory) {
        super(nThreads, executor, new Object[]{selectorProvider, selectStrategyFactory, RejectedExecutionHandlers.reject()});
    }

    public NioEventLoopGroup(int nThreads, Executor executor, EventExecutorChooserFactory chooserFactory, SelectorProvider selectorProvider, SelectStrategyFactory selectStrategyFactory) {
        super(nThreads, executor, chooserFactory, new Object[]{selectorProvider, selectStrategyFactory, RejectedExecutionHandlers.reject()});
    }

    public NioEventLoopGroup(int nThreads, Executor executor, EventExecutorChooserFactory chooserFactory, SelectorProvider selectorProvider, SelectStrategyFactory selectStrategyFactory, RejectedExecutionHandler rejectedExecutionHandler) {
        super(nThreads, executor, chooserFactory, new Object[]{selectorProvider, selectStrategyFactory, rejectedExecutionHandler});
    }

让我们看看构造方法中调用的MultithreadEventLoopGroup的父类MultithreadEventExecutorGroup的构造方法

    protected MultithreadEventExecutorGroup(int nThreads, Executor executor, EventExecutorChooserFactory chooserFactory, Object... args) {
        this.terminatedChildren = new AtomicInteger();
        this.terminationFuture = new DefaultPromise(GlobalEventExecutor.INSTANCE);
        if (nThreads <= 0) {
            throw new IllegalArgumentException(String.format("nThreads: %d (expected: > 0)", nThreads));
        } else {
            if (executor == null) {
                executor = new ThreadPerTaskExecutor(this.newDefaultThreadFactory());
            }

            this.children = new EventExecutor[nThreads];

            int j;
            for(int i = 0; i < nThreads; ++i) {
                boolean success = false;
                boolean var18 = false;

                try {
                    var18 = true;
                    this.children[i] = this.newChild((Executor)executor, args);
                    success = true;
                    var18 = false;
                } catch (Exception var19) {
                    throw new IllegalStateException("failed to create a child event loop", var19);
                } finally {
                    if (var18) {
                        if (!success) {
                            int j;
                            for(j = 0; j < i; ++j) {
                                this.children[j].shutdownGracefully();
                            }

                            for(j = 0; j < i; ++j) {
                                EventExecutor e = this.children[j];

                                try {
                                    while(!e.isTerminated()) {
                                        e.awaitTermination(2147483647L, TimeUnit.SECONDS);
                                    }
                                } catch (InterruptedException var20) {
                                    Thread.currentThread().interrupt();
                                    break;
                                }
                            }
                        }

                    }
                }

                if (!success) {
                    for(j = 0; j < i; ++j) {
                        this.children[j].shutdownGracefully();
                    }

                    for(j = 0; j < i; ++j) {
                        EventExecutor e = this.children[j];

                        try {
                            while(!e.isTerminated()) {
                                e.awaitTermination(2147483647L, TimeUnit.SECONDS);
                            }
                        } catch (InterruptedException var22) {
                            Thread.currentThread().interrupt();
                            break;
                        }
                    }
                }
            }

            this.chooser = chooserFactory.newChooser(this.children);
            FutureListener<Object> terminationListener = new FutureListener<Object>() {
                public void operationComplete(Future<Object> future) throws Exception {
                    if (MultithreadEventExecutorGroup.this.terminatedChildren.incrementAndGet() == MultithreadEventExecutorGroup.this.children.length) {
                        MultithreadEventExecutorGroup.this.terminationFuture.setSuccess((Object)null);
                    }

                }
            };
            EventExecutor[] var24 = this.children;
            j = var24.length;

            for(int var26 = 0; var26 < j; ++var26) {
                EventExecutor e = var24[var26];
                e.terminationFuture().addListener(terminationListener);
            }

            Set<EventExecutor> childrenSet = new LinkedHashSet(this.children.length);
            Collections.addAll(childrenSet, this.children);
            this.readonlyChildren = Collections.unmodifiableSet(childrenSet);
        }
    }

在该构造方法中，首先对terminatedChildren初始化，对terminationFuture的初始化使用DefaultPromise。executor初始化一个线程池。
然后对children进行操作，保证nThreads大于0，再看children，是一个EventExecutor的数组，数组的长度是传入的线程数量。使用newChild方法给每个元素初始化。newChild的实现在NIOEventLoopGroup中。

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

这里通过executor, (SelectorProvider)args[0], 【放在args数组中的SelectorProvider】((SelectStrategyFactory)args[1]).newSelectStrategy()， (RejectedExecutionHandler)args[2]产生了一个NioEventLoop对象。
接着我们去看NioEventLoop

	private Selector selector;
    private Selector unwrappedSelector;
    private SelectedSelectionKeySet selectedKeys;

    private final SelectorProvider provider;

    private final AtomicBoolean wakenUp = new AtomicBoolean();

    private final SelectStrategy selectStrategy;

    private volatile int ioRatio = 50;
    private int cancelledKeys;
    private boolean needsToSelectAgain;

    NioEventLoop(NioEventLoopGroup parent, Executor executor, SelectorProvider selectorProvider,
                 SelectStrategy strategy, RejectedExecutionHandler rejectedExecutionHandler) {
        super(parent, executor, false, DEFAULT_MAX_PENDING_TASKS, rejectedExecutionHandler);
        if (selectorProvider == null) {
            throw new NullPointerException("selectorProvider");
        }
        if (strategy == null) {
            throw new NullPointerException("selectStrategy");
        }
        provider = selectorProvider;
        final SelectorTuple selectorTuple = openSelector();
        selector = selectorTuple.selector;
        unwrappedSelector = selectorTuple.unwrappedSelector;
        selectStrategy = strategy;
    }

NioEventLoop调用其父类SingleThreadEventLoop的构造进行成员的赋值。

	protected SingleThreadEventLoop(EventLoopGroup parent, ThreadFactory threadFactory, boolean addTaskWakesUp) {
        this(parent, threadFactory, addTaskWakesUp, DEFAULT_MAX_PENDING_TASKS, RejectedExecutionHandlers.reject());
    }

    protected SingleThreadEventLoop(EventLoopGroup parent, Executor executor, boolean addTaskWakesUp) {
        this(parent, executor, addTaskWakesUp, DEFAULT_MAX_PENDING_TASKS, RejectedExecutionHandlers.reject());
    }

    protected SingleThreadEventLoop(EventLoopGroup parent, ThreadFactory threadFactory,
                                    boolean addTaskWakesUp, int maxPendingTasks,
                                    RejectedExecutionHandler rejectedExecutionHandler) {
        super(parent, threadFactory, addTaskWakesUp, maxPendingTasks, rejectedExecutionHandler);
        tailTasks = newTaskQueue(maxPendingTasks);
    }

    protected SingleThreadEventLoop(EventLoopGroup parent, Executor executor,
                                    boolean addTaskWakesUp, int maxPendingTasks,
                                    RejectedExecutionHandler rejectedExecutionHandler) {
        super(parent, executor, addTaskWakesUp, maxPendingTasks, rejectedExecutionHandler);
        tailTasks = newTaskQueue(maxPendingTasks);
    }

SingleThreadEventLoop调用其父类AbstractScheduledEventExecutor的构造进行成员的赋值。

	protected SingleThreadEventExecutor(EventExecutorGroup parent, Executor executor,
                                        boolean addTaskWakesUp, int maxPendingTasks,
                                        RejectedExecutionHandler rejectedHandler) {
        super(parent);
        this.addTaskWakesUp = addTaskWakesUp;
        this.maxPendingTasks = Math.max(16, maxPendingTasks);
        this.executor = ObjectUtil.checkNotNull(executor, "executor");
        taskQueue = newTaskQueue(this.maxPendingTasks);
        rejectedExecutionHandler = ObjectUtil.checkNotNull(rejectedHandler, "rejectedHandler");
    }

AbstractScheduledEventExecutor调用其父类AbstractEventExecutor的构造进行成员的赋值。

    protected AbstractScheduledEventExecutor(EventExecutorGroup parent) {
        super(parent);
    }

    protected AbstractEventExecutor(EventExecutorGroup parent) {
        this.parent = parent;
    }

经过继承链操作后，再回到NioEventLoop

        provider = selectorProvider;
        final SelectorTuple selectorTuple = openSelector();
        selector = selectorTuple.selector;
        unwrappedSelector = selectorTuple.unwrappedSelector;
        selectStrategy = strategy;

将selectorProvider赋值给provider。使用openSelector()方法，provider是WindowsSelectorProvider类的openSelector()，最终创建JDK原生的Selector

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);
        }

        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;
                }
            }
        });

        if (!(maybeSelectorImplClass instanceof Class) ||
                // ensure the current selector implementation is what we can instrument.
                !((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);
        }

        final Class<?> selectorImplClass = (Class<?>) maybeSelectorImplClass;
        final SelectedSelectionKeySet selectedKeySet = new SelectedSelectionKeySet();

        Object maybeException = AccessController.doPrivileged(new PrivilegedAction<Object>() {
            @Override
            public Object run() {
                try {
                    Field selectedKeysField = selectorImplClass.getDeclaredField("selectedKeys");
                    Field publicSelectedKeysField = selectorImplClass.getDeclaredField("publicSelectedKeys");

                    Throwable cause = ReflectionUtil.trySetAccessible(selectedKeysField, true);
                    if (cause != null) {
                        return cause;
                    }
                    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);
        }
        selectedKeys = selectedKeySet;
        logger.trace("instrumented a special java.util.Set into: {}", unwrappedSelector);
        return new SelectorTuple(unwrappedSelector,
                                 new SelectedSelectionKeySetSelector(unwrappedSelector, selectedKeySet));
    }

这段先使用provider的openSelector方法，即WindowsSelectorProvider的openSelector方法，创建了WindowsSelectorImpl对象，再判断DISABLE_KEYSET_OPTIMIZATION，初始值为false，
DISABLE_KEYSET_OPTIMIZATION： 判断是否需要对sun.nio.ch.SelectorImpl中的selectedKeys进行优化, 不做配置的话默认需要优化，主要优化在哪?

private static final boolean DISABLE_KEYSET_OPTIMIZATION =
            SystemPropertyUtil.getBoolean("io.netty.noKeySetOptimization", false);

如果true了就直接创建一个SelectorTuple对象返回

private static final class SelectorTuple {
        final Selector unwrappedSelector;
        final Selector selector;

        SelectorTuple(Selector unwrappedSelector) {
            this.unwrappedSelector = unwrappedSelector;
            this.selector = unwrappedSelector;
        }

        SelectorTuple(Selector unwrappedSelector, Selector selector) {
            this.unwrappedSelector = unwrappedSelector;
            this.selector = selector;
        }

这里将unwrappedSelector和selector封装了。
openSelector结束后，就可以分别对包装过的Selector和未包装过的Selector (selector和unwrappedSelector）成员赋值,再由selectStrategy保存刚才产生的选择策略，用于Selector的轮询。

来源：CSDN

作者：究极机器

链接：https://blog.csdn.net/weixin_43257196/article/details/103544952