之前的文章Netty Server端启动之源码分析里过了一遍整个Netty Server端的启动流程，但是其中有几个地方没有细说，若都详细介绍的话会导致单篇文章过长，从而失了主题，以后我会以单章的方式结合源码详细的介绍Netty中我认为比较重要的部分，这样大家看文章也比较好容易理解和深入
NioEventLoop在整个Netty中的作用
用一句话来描述就是：负责注册Channel到Selector和事件循环
接下来的部分我们就围绕这两个功能，结合源码看看NioEventLoop到底是实现这两个功能的
1. 注册Channel
Netty中的Channel注册一开始都是通过EventLoopGroup去开始注册的，因为发生注册之前，我需要知道到底用Group组中的NioEventLoop来负责这次的注册工作（利用EventExecutorChooser选择一个）
选择到了具体的NioEventLoop之后，我们就可以执行具体的注册工作了，记住一个NioEventLoop 即是一个SingleThreadEventLoop
这里直接看1SingleThreadEventLoop抽象类实现的注册方法
@Override
public ChannelFuture register(Channel channel) {
return register(new DefaultChannelPromise(channel, this));
}
将Channel包装成了一个DefaultChannelPromise对象，该对象包含了需要注册的Channel以及负责注册的NioEventLoop实例
继续往下看
@Override
public ChannelFuture register(final ChannelPromise promise) {
ObjectUtil.checkNotNull(promise, "promise");
promise.channel().unsafe().register(this, promise);
return promise;
}
promise.channel()，就是包装之前的NioServerSocketChannel实例，每个AbstractChannel都持有一个Unsafe类型的unsafe属性
下面先介绍一个这个Unsafe接口
Unsafe接口是Netty框架中Channel接口的内部定义接口，并且规定用于执行实际的I/O操作，且必须在I/O线程中执行。该接口不应该被用户代码直接使用，仅在netty内部使用。
因此通过.unsafe()方法我们可以获取到NioServerSocketChannel实例持有的Unsafe实例 为NioMessageUnsafe实例，
NioMessageUnsafe.png
而NioMessageUnsafe类没有重写接口Unsafe的register方法，我们直接看接口Unsafe默认实现AbstractUnsafe抽象类中的方法定义
public final void register(EventLoop eventLoop, final ChannelPromise promise) {
if (eventLoop == null) {
throw new NullPointerException("eventLoop");
}
if (isRegistered()) {
promise.setFailure(new IllegalStateException("registered to an event loop already"));
return;
}
if (!isCompatible(eventLoop)) {
promise.setFailure(
new IllegalStateException("incompatible event loop type: " + eventLoop.getClass().getName()));
return;
}
AbstractChannel.this.eventLoop = eventLoop;
if (eventLoop.inEventLoop()) {
register0(promise);
} else {
try {
eventLoop.execute(new Runnable() {
@Override
public void run() {
register0(promise);
}
});
} catch (Throwable t) {
logger.warn(
"Force-closing a channel whose registration task was not accepted by an event loop: {}",
AbstractChannel.this, t);
closeForcibly();
closeFuture.setClosed();
safeSetFailure(promise, t);
}
}
}
可以看到这里if判断当前执行线程是否是NioEventLoop自身的事件循环线程，不是的话就以提交任务的方式执行register0(promise)来注册，从而保证注册事件始终由NioEventLoop自身的事件循环线程完成
再往下看register0做了什么事情
private void register0(ChannelPromise promise) {
try {
// check if the channel is still open as it could be closed in the mean time when the register
// call was outside of the eventLoop
if (!promise.setUncancellable() || !ensureOpen(promise)) {
return;
}
boolean firstRegistration = neverRegistered;
// 1.
doRegister();
neverRegistered = false;
registered = true;
// Ensure we call handlerAdded(...) before we actually notify the promise. This is needed as the
// user may already fire events through the pipeline in the ChannelFutureListener.
// 2.
pipeline.invokeHandlerAddedIfNeeded();
safeSetSuccess(promise);
// 3.
pipeline.fireChannelRegistered();
// Only fire a channelActive if the channel has never been registered. This prevents firing
// multiple channel actives if the channel is deregistered and re-registered.
if (isActive()) {
if (firstRegistration) {
pipeline.fireChannelActive();
} else if (config().isAutoRead()) {
// This channel was registered before and autoRead() is set. This means we need to begin read
// again so that we process inbound data.
//
// See https://github.com/netty/netty/issues/4805
beginRead();
}
}
} catch (Throwable t) {
// Close the channel directly to avoid FD leak.
closeForcibly();
closeFuture.setClosed();
safeSetFailure(promise, t);
}
}
我们挑选主要的点来说明
- doRegister()
doRegister方法是AbstractChannel抽象类定义的一个模板方法，由子类自己实现注册的真实细节，这里我们直接看其子类AbstractNioChannel的实现
@Override
protected void doRegister() throws Exception {
boolean selected = false;
for (;;) {
try {
selectionKey = javaChannel().register(eventLoop().unwrappedSelector(), 0, this);
return;
} catch (CancelledKeyException e) {
if (!selected) {
// Force the Selector to select now as the "canceled" SelectionKey may still be
// cached and not removed because no Select.select(..) operation was called yet.
eventLoop().selectNow();
selected = true;
} else {
// We forced a select operation on the selector before but the SelectionKey is still cached
// for whatever reason. JDK bug ?
throw e;
}
}
}
}
到了这里我们看到其实就是利用了JDK提供的Selector注册Channel，并返回SelectionKey标识这次注册，如果JDK的Selector还不熟悉的可以移步JDK IO多路复用基础
2. 事件循环
NioEventLoop的事件循环线程的启动是由首次提交任务触发的
直接看其父类SingleThreadEventExecutor覆写的execute方法
@Override
public void execute(Runnable task) {
if (task == null) {
throw new NullPointerException("task");
}
boolean inEventLoop = inEventLoop();
addTask(task);
if (!inEventLoop) {
startThread();
if (isShutdown()) {
boolean reject = false;
try {
if (removeTask(task)) {
reject = true;
}
} catch (UnsupportedOperationException e) {
// The task queue does not support removal so the best thing we can do is to just move on and
// hope we will be able to pick-up the task before its completely terminated.
// In worst case we will log on termination.
}
if (reject) {
reject();
}
}
}
if (!addTaskWakesUp && wakesUpForTask(task)) {
wakeup(inEventLoop);
}
}
1：将提交的任务（如注册任务），加入队列中
protected void addTask(Runnable task) {
if (task == null) {
throw new NullPointerException("task");
}
if (!offerTask(task)) {
reject(task);
}
}
这里就是将任务加入到LinkedBlockingQueue阻塞队列中，这个队列的默认容量是Integer的最大值
final boolean offerTask(Runnable task) {
if (isShutdown()) {
reject();
}
return taskQueue.offer(task);
}
注意当加入队列失败会执行reject方法，也就是拒绝任务的决策
- 如果当前执行线程不是该NioEventLoop自身事件循环线程的话，则执行startThread()方法
private void startThread() {
if (state == ST_NOT_STARTED) {
if (STATE_UPDATER.compareAndSet(this, ST_NOT_STARTED, ST_STARTED)) {
try {
doStartThread();
} catch (Throwable cause) {
STATE_UPDATER.set(this, ST_NOT_STARTED);
PlatformDependent.throwException(cause);
}
}
}
}
该方法只有在state == ST_NOT_STARTED时才会进行CAS修改state属性值为ST_STARTED，当CAS成功，则执行doStartThread()方法
private void doStartThread() {
assert thread == null;
executor.execute(new Runnable() {
@Override
public void run() {
thread = Thread.currentThread();
if (interrupted) {
thread.interrupt();
}
boolean success = false;
updateLastExecutionTime();
try {
SingleThreadEventExecutor.this.run();
success = true;
} catch (Throwable t) {
logger.warn("Unexpected exception from an event executor: ", t);
} finally {
for (;;) {
int oldState = state;
if (oldState >= ST_SHUTTING_DOWN || STATE_UPDATER.compareAndSet(
SingleThreadEventExecutor.this, oldState, ST_SHUTTING_DOWN)) {
break;
}
}
// Check if confirmShutdown() was called at the end of the loop.
if (success && gracefulShutdownStartTime == 0) {
if (logger.isErrorEnabled()) {
logger.error("Buggy " + EventExecutor.class.getSimpleName() + " implementation; " +
SingleThreadEventExecutor.class.getSimpleName() + ".confirmShutdown() must " +
"be called before run() implementation terminates.");
}
}
try {
// Run all remaining tasks and shutdown hooks.
for (;;) {
if (confirmShutdown()) {
break;
}
}
} finally {
try {
cleanup();
} finally {
// Lets remove all FastThreadLocals for the Thread as we are about to terminate and notify
// the future. The user may block on the future and once it unblocks the JVM may terminate
// and start unloading classes.
// See https://github.com/netty/netty/issues/6596.
FastThreadLocal.removeAll();
STATE_UPDATER.set(SingleThreadEventExecutor.this, ST_TERMINATED);
threadLock.release();
if (!taskQueue.isEmpty()) {
if (logger.isWarnEnabled()) {
logger.warn("An event executor terminated with " +
"non-empty task queue (" + taskQueue.size() + ')');
}
}
terminationFuture.setSuccess(null);
}
}
}
}
});
}
可以看到这里调用了SingleThreadEventExecutor.this.run();
该方法是一个模板方法
protected abstract void run();
由子类NioEventLoop实现，此时开始执行真正事件循环的方法
@Override
protected void run() {
for (;;) {
try {
try {
switch (selectStrategy.calculateStrategy(selectNowSupplier, hasTasks())) {
case SelectStrategy.CONTINUE:
continue;
case SelectStrategy.BUSY_WAIT:
// fall-through to SELECT since the busy-wait is not supported with NIO
case SelectStrategy.SELECT:
select(wakenUp.getAndSet(false));
// 'wakenUp.compareAndSet(false, true)' is always evaluated
// before calling 'selector.wakeup()' to reduce the wake-up
// overhead. (Selector.wakeup() is an expensive operation.)
//
// However, there is a race condition in this approach.
// The race condition is triggered when 'wakenUp' is set to
// true too early.
//
// 'wakenUp' is set to true too early if:
// 1) Selector is waken up between 'wakenUp.set(false)' and
// 'selector.select(...)'. (BAD)
// 2) Selector is waken up between 'selector.select(...)' and
// 'if (wakenUp.get()) { ... }'. (OK)
//
// In the first case, 'wakenUp' is set to true and the
// following 'selector.select(...)' will wake up immediately.
// Until 'wakenUp' is set to false again in the next round,
// 'wakenUp.compareAndSet(false, true)' will fail, and therefore
// any attempt to wake up the Selector will fail, too, causing
// the following 'selector.select(...)' call to block
// unnecessarily.
//
// To fix this problem, we wake up the selector again if wakenUp
// is true immediately after selector.select(...).
// It is inefficient in that it wakes up the selector for both
// the first case (BAD - wake-up required) and the second case
// (OK - no wake-up required).
if (wakenUp.get()) {
selector.wakeup();
}
// fall through
default:
}
} catch (IOException e) {
// If we receive an IOException here its because the Selector is messed up. Let's rebuild
// the selector and retry. https://github.com/netty/netty/issues/8566
rebuildSelector0();
handleLoopException(e);
continue;
}
cancelledKeys = 0;
needsToSelectAgain = false;
final int ioRatio = this.ioRatio;
if (ioRatio == 100) {
try {
processSelectedKeys();
} finally {
// Ensure we always run tasks.
runAllTasks();
}
} else {
final long ioStartTime = System.nanoTime();
try {
processSelectedKeys();
} finally {
// Ensure we always run tasks.
final long ioTime = System.nanoTime() - ioStartTime;
runAllTasks(ioTime * (100 - ioRatio) / ioRatio);
}
}
} catch (Throwable t) {
handleLoopException(t);
}
// Always handle shutdown even if the loop processing threw an exception.
try {
if (isShuttingDown()) {
closeAll();
if (confirmShutdown()) {
return;
}
}
} catch (Throwable t) {
handleLoopException(t);
}
}
}
整个一个for(;;)循环，根据switch判断后执行不同的事件循环策略
@Override
public int calculateStrategy(IntSupplier selectSupplier, boolean hasTasks) throws Exception {
return hasTasks ? selectSupplier.get() : SelectStrategy.SELECT;
}
-
当此时taskQueue队列有任务时，执行selectNow()非阻塞调用
-
否则执行
select(wakenUp.getAndSet(false))阻塞调用
继续看select方法做了什么
private void select(boolean oldWakenUp) throws IOException {
Selector selector = this.selector;
try {
int selectCnt = 0;
long currentTimeNanos = System.nanoTime();
long selectDeadLineNanos = currentTimeNanos + delayNanos(currentTimeNanos);
for (;;) {
long timeoutMillis = (selectDeadLineNanos - currentTimeNanos + 500000L) / 1000000L;
if (timeoutMillis <= 0) {
if (selectCnt == 0) {
selector.selectNow();
selectCnt = 1;
}
break;
}
// If a task was submitted when wakenUp value was true, the task didn't get a chance to call
// Selector#wakeup. So we need to check task queue again before executing select operation.
// If we don't, the task might be pended until select operation was timed out.
// It might be pended until idle timeout if IdleStateHandler existed in pipeline.
if (hasTasks() && wakenUp.compareAndSet(false, true)) {
selector.selectNow();
selectCnt = 1;
break;
}
int selectedKeys = selector.select(timeoutMillis);
selectCnt ++;
if (selectedKeys != 0 || oldWakenUp || wakenUp.get() || hasTasks() || hasScheduledTasks()) {
// - Selected something,
// - waken up by user, or
// - the task queue has a pending task.
// - a scheduled task is ready for processing
break;
}
if (Thread.interrupted()) {
// Thread was interrupted so reset selected keys and break so we not run into a busy loop.
// As this is most likely a bug in the handler of the user or it's client library we will
// also log it.
//
// See https://github.com/netty/netty/issues/2426
if (logger.isDebugEnabled()) {
logger.debug("Selector.select() returned prematurely because " +
"Thread.currentThread().interrupt() was called. Use " +
"NioEventLoop.shutdownGracefully() to shutdown the NioEventLoop.");
}
selectCnt = 1;
break;
}
long time = System.nanoTime();
if (time - TimeUnit.MILLISECONDS.toNanos(timeoutMillis) >= currentTimeNanos) {
// timeoutMillis elapsed without anything selected.
selectCnt = 1;
} else if (SELECTOR_AUTO_REBUILD_THRESHOLD > 0 &&
selectCnt >= SELECTOR_AUTO_REBUILD_THRESHOLD) {
// The code exists in an extra method to ensure the method is not too big to inline as this
// branch is not very likely to get hit very frequently.
selector = selectRebuildSelector(selectCnt);
selectCnt = 1;
break;
}
currentTimeNanos = time;
}
if (selectCnt > MIN_PREMATURE_SELECTOR_RETURNS) {
if (logger.isDebugEnabled()) {
logger.debug("Selector.select() returned prematurely {} times in a row for Selector {}.",
selectCnt - 1, selector);
}
}
} catch (CancelledKeyException e) {
if (logger.isDebugEnabled()) {
logger.debug(CancelledKeyException.class.getSimpleName() + " raised by a Selector {} - JDK bug?",
selector, e);
}
// Harmless exception - log anyway
}
}
-
获取到当前NioEventLoop持有的JDK Selector
-
计算此次selectDeadLineNanos阻塞截止时间
计算规则：
-
如果有定时任务需要执行，则计算
当前时间到最近一个定时任务开始执行的时间差作为此次select的阻塞时间，而若有定时任务的话其间隔为1秒，所以阻塞时间总是小于1秒的 -
若没有定时任务需要执行，则默认
阻塞1秒钟
-
当select方法返回后，会执行run方法的后半部分，根据ioRatio比例执行processSelectedKeys方法和runAllTasks方法
-
主要看处理SelectedKeys方法的实现
private void processSelectedKeys() {
if (selectedKeys != null) {
processSelectedKeysOptimized();
} else {
processSelectedKeysPlain(selector.selectedKeys());
}
}
看processSelectedKeysOptimized方法
private void processSelectedKeysOptimized() {
for (int i = 0; i < selectedKeys.size; ++i) {
final SelectionKey k = selectedKeys.keys[i];
// null out entry in the array to allow to have it GC'ed once the Channel close
// See https://github.com/netty/netty/issues/2363
selectedKeys.keys[i] = null;
final Object a = k.attachment();
if (a instanceof AbstractNioChannel) {
processSelectedKey(k, (AbstractNioChannel) a);
} else {
@SuppressWarnings("unchecked")
NioTask<SelectableChannel> task = (NioTask<SelectableChannel>) a;
processSelectedKey(k, task);
}
if (needsToSelectAgain) {
// null out entries in the array to allow to have it GC'ed once the Channel close
// See https://github.com/netty/netty/issues/2363
selectedKeys.reset(i + 1);
selectAgain();
i = -1;
}
}
}
遍历所有已选择的selectedKeys，获取到其attachment，我们知道该值就是我们附加进去的NioServerSocketChannel实例，这里会进入到processSelectedKey（k， (AbstractNioChannel) a）内部
// ... 省略其他代码
try {
int readyOps = k.readyOps();
// We first need to call finishConnect() before try to trigger a read(...) or write(...) as otherwise
// the NIO JDK channel implementation may throw a NotYetConnectedException.
if ((readyOps & SelectionKey.OP_CONNECT) != 0) {
// remove OP_CONNECT as otherwise Selector.select(..) will always return without blocking
// See https://github.com/netty/netty/issues/924
int ops = k.interestOps();
ops &= ~SelectionKey.OP_CONNECT;
k.interestOps(ops);
unsafe.finishConnect();
}
// Process OP_WRITE first as we may be able to write some queued buffers and so free memory.
if ((readyOps & SelectionKey.OP_WRITE) != 0) {
// Call forceFlush which will also take care of clear the OP_WRITE once there is nothing left to write
ch.unsafe().forceFlush();
}
// Also check for readOps of 0 to workaround possible JDK bug which may otherwise lead
// to a spin loop
if ((readyOps & (SelectionKey.OP_READ | SelectionKey.OP_ACCEPT)) != 0 || readyOps == 0) {
unsafe.read();
}
} catch (CancelledKeyException ignored) {
unsafe.close(unsafe.voidPromise());
}
看到这里是不是很熟悉了？根据每个SelectionKey的readyOps不同，分别调用其Channel对应的unsafe的不同的方法，拿NioServerSocketChannel的OP_ACCEPT为例，这里会调用其unsafe.read()方法
跳转到AbstractNioMessageChannel类中内部类NioMessageUnsafe的read方法，看看当NioServerSocketChannel有Accept事件到来时，会做那些工作
@Override
public void read() {
assert eventLoop().inEventLoop();
final ChannelConfig config = config();
final ChannelPipeline pipeline = pipeline();
final RecvByteBufAllocator.Handle allocHandle = unsafe().recvBufAllocHandle();
allocHandle.reset(config);
boolean closed = false;
Throwable exception = null;
try {
try {
do {
// 1.
int localRead = doReadMessages(readBuf);
if (localRead == 0) {
break;
}
if (localRead < 0) {
closed = true;
break;
}
allocHandle.incMessagesRead(localRead);
} while (allocHandle.continueReading());
} catch (Throwable t) {
exception = t;
}
int size = readBuf.size();
for (int i = 0; i < size; i ++) {
readPending = false;
// 2. 激活channelRead事件
pipeline.fireChannelRead(readBuf.get(i));
}
readBuf.clear();
allocHandle.readComplete();
// 3. 激活读完成事件
pipeline.fireChannelReadComplete();
if (exception != null) {
closed = closeOnReadError(exception);
// 4. 激活异常
pipeline.fireExceptionCaught(exception);
}
if (closed) {
inputShutdown = true;
if (isOpen()) {
close(voidPromise());
}
}
} finally {
// Check if there is a readPending which was not processed yet.
// This could be for two reasons:
// * The user called Channel.read() or ChannelHandlerContext.read() in channelRead(...) method
// * The user called Channel.read() or ChannelHandlerContext.read() in channelReadComplete(...) method
//
// See https://github.com/netty/netty/issues/2254
if (!readPending && !config.isAutoRead()) {
removeReadOp();
}
}
}
}
- 看下它的doReadMessages做了什么
@Override
protected int doReadMessages(List<Object> buf) throws Exception {
SocketChannel ch = SocketUtils.accept(javaChannel());
try {
if (ch != null) {
buf.add(new NioSocketChannel(this, ch));
return 1;
}
} catch (Throwable t) {
logger.warn("Failed to create a new channel from an accepted socket.", t);
try {
ch.close();
} catch (Throwable t2) {
logger.warn("Failed to close a socket.", t2);
}
}
return 0;
}
这里可以看出，NioServerSocketChannel的ACCEPT事件到来时，就是去实例化一个NioSocketChannel，实例化以后，剩下的事情（包括NioSocketChannel的Netty相关init，register等）都是交给NioServerSocketChannel的pipeline去传递，并通过相应的Handler做的，这个在Netty Server端启动之源码分析已分析过
总结
NioEventLoop.png
重要属性.png
- SingleThreadEventExecutor
SingleThreadEventExecutor.png
- AbstractScheduledEventExecutor
AbstractScheduledEventExecutor.png
基于以上，我们知道了NioEventLoop实现了Executor接口，有了执行器的能力，同时通过继承了SingleThreadEventExecutor有了实现单线程事件循环执行的能力，同时自身包含有JDK相关的Selector属性，具有了注册Channel相关的能力，同时基于强大的Netty Pipeline实现整个事件传递机制
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