继承关系： 
NioEventLoop初始化
NioEventLoop(NioEventLoopGroup parent, ThreadFactory threadFactory, SelectorProvider selectorProvider) {
super(parent, threadFactory, false);
if (selectorProvider == null) {
throw new NullPointerException("selectorProvider");
}
provider = selectorProvider;
selector = openSelector();
}1、调用父类方法构造一个taskQueue，它是一个LinkedBlockingQueue
2、openSelector(): Netty是基于Nio实现的，所以也离不开selector。
3、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.
4、主要优化在哪: Netty通过反射将selectedKeySet与sun.nio.ch.SelectorImpl中的两个field selectedKeys和publicSelectedKeys绑定,大家知道SelectorImpl原来的selectedKeys和publicSelectedKeys数据结构是HashSet,而HashSet的数据结构是数组+链表，新的数据结构是由2个数组A、B组成，初始大小是1024，避免了HashSet扩容带来的性能问题。除了扩容外，遍历效率也是一个原因，对于需要遍历selectedKeys的全部元素, 数组效率无疑是最高的。
private Selector openSelector() {
final Selector selector;
try {
selector = provider.openSelector();
} catch (IOException e) {
throw new ChannelException("failed to open a new selector", e);
}
if (DISABLE_KEYSET_OPTIMIZATION) {
return selector;
}
try {
SelectedSelectionKeySet selectedKeySet = new SelectedSelectionKeySet();
Class<?> selectorImplClass =
Class.forName("sun.nio.ch.SelectorImpl", false, PlatformDependent.getSystemClassLoader());
// Ensure the current selector implementation is what we can instrument.
if (!selectorImplClass.isAssignableFrom(selector.getClass())) {
return selector;
}
Field selectedKeysField = selectorImplClass.getDeclaredField("selectedKeys");
Field publicSelectedKeysField = selectorImplClass.getDeclaredField("publicSelectedKeys");
selectedKeysField.setAccessible(true);
publicSelectedKeysField.setAccessible(true);
selectedKeysField.set(selector, selectedKeySet);
publicSelectedKeysField.set(selector, selectedKeySet);
selectedKeys = selectedKeySet;
logger.trace("Instrumented an optimized java.util.Set into: {}", selector);
} catch (Throwable t) {
selectedKeys = null;
logger.trace("Failed to instrument an optimized java.util.Set into: {}", selector, t);
}
return selector;
}NioEventLoop的启动
在上一遍讲过NioEventLoop中维护了一个线程，线程启动时会调用NioEventLoop的run方法，loop会不断循环一个过程：select -> processSelectedKeys(IO任务) -> runAllTasks(非IO任务)
I/O任务： 即selectionKey中ready的事件，如accept、connect、read、write等
非IO任务： 添加到taskQueue中的任务，如bind、channelActive等
@Override
protected void run() {
for (;;) {
boolean oldWakenUp = wakenUp.getAndSet(false);
try {
// 判断是否有非IO任务,如果有立刻返回
if (hasTasks()) {
selectNow();
} else {
select(oldWakenUp);
if (wakenUp.get()) {
selector.wakeup();
}
}
cancelledKeys = 0;
needsToSelectAgain = false;
final int ioRatio = this.ioRatio;
if (ioRatio == 100) {
// IO任务
processSelectedKeys();
// 非IO任务
runAllTasks();
} else {
// 用以控制IO任务与非IO任务的运行时间比
final long ioStartTime = System.nanoTime();
// IO任务
processSelectedKeys();
final long ioTime = System.nanoTime() - ioStartTime;
// 非IO任务
runAllTasks(ioTime * (100 - ioRatio) / ioRatio);
}
if (isShuttingDown()) {
closeAll();
if (confirmShutdown()) {
break;
}
}
} catch (Throwable t) {
// Prevent possible consecutive immediate failures that lead to
// excessive CPU consumption.
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
// Ignore.
}
}
}
}1、wakenUp： 用来决定是否调用selector.wakeup()，只有当wakenUp未true时才会调用，目的是为了减少wake-up的负载，因为Selector.wakeup()是一个昂贵的操作。
2、hasTask(): 判断是否有非IO任务,如果有的话,选择调用非阻塞的selectNow()让select立即返回, 否则以阻塞的方式调用select.timeoutMillis是阻塞时间
3、ioRatio： 控制两种任务的执行时间,你可以通过它来限制非IO任务的执行时间, 默认值是50, 表示允许非IO任务获得和IO任务相同的执行时间,这个值根据自己的具体场景来设置.
4、processSelectedKeys()： 处理IO事件
5、runAllTasks()： 处理非IO任务
6、isShuttingDown()： 检查state是否被标记为ST_SHUTTING_DOWN
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;
}
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 selector returned prematurely many times in a row.
// Rebuild the selector to work around the problem.
logger.warn(
"Selector.select() returned prematurely {} times in a row; rebuilding selector.",
selectCnt);
rebuildSelector();
selector = this.selector;
// Select again to populate selectedKeys.
selector.selectNow();
selectCnt = 1;
break;
}
currentTimeNanos = time;
}
if (selectCnt > MIN_PREMATURE_SELECTOR_RETURNS) {
if (logger.isDebugEnabled()) {
logger.debug("Selector.select() returned prematurely {} times in a row.", selectCnt - 1);
}
}
} catch (CancelledKeyException e) {
if (logger.isDebugEnabled()) {
logger.debug(CancelledKeyException.class.getSimpleName() + " raised by a Selector - JDK bug?", e);
}
}
}protected long delayNanos(long currentTimeNanos) {
ScheduledFutureTask<?> scheduledTask = peekScheduledTask();
if (scheduledTask == null) {
return SCHEDULE_PURGE_INTERVAL;
}
return scheduledTask.delayNanos(currentTimeNanos);
}
public long delayNanos(long currentTimeNanos) {
return Math.max(0, deadlineNanos() - (currentTimeNanos - START_TIME));
}
public long deadlineNanos() {
return deadlineNanos;
}
1、delayNanos(currentTimeNanos)： 在父类SingleThreadEventExecutor中有一个延迟执行任务的队列，delayNanos就是去这个延迟队列里看是否有非IO任务未执行
* 如果没有则返回1秒钟。
* 如果延迟队列里有任务并且最终的计算出来的时间(selectDeadLineNanos - currentTimeNanos)小于500000L纳秒，就调用selectNow()直接返回，反之执行阻塞的select
2、select如果遇到以下几种情况会立即返回
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;
}- Selected something 如果select到了就绪连接(selectedKeys > 0)
- waken up by user 被用户唤醒了
- the task queue has a pending task.任务队列来了一个新任务
- a scheduled task is ready for processing 延迟队列里面有个预约任务需要到期执行
3、selectCnt： 记录select空转的次数(selectCnt)，该方法解决了Nio中臭名昭著selector的select方法导致cpu100%的BUG，当空转的次数超过了512(定义一个阀值，这个阀值默认是512，可以在应用层通过设置系统属性io.netty.selectorAutoRebuildThreshold传入)，Netty会重新构建新的Selector，将老Selector上注册的Channel转移到新建的Selector上，关闭老Selector，用新的Selector代替老Selector。详细看下面rebuildSelector()方法
4、rebuildSelector(): 就是上面说过得。
public void rebuildSelector() {
if (!inEventLoop()) {
execute(new Runnable() {
@Override
public void run() {
rebuildSelector();
}
});
return;
}
final Selector oldSelector = selector;
final Selector newSelector;
if (oldSelector == null) {
return;
}
try {
newSelector = openSelector();
} catch (Exception e) {
logger.warn("Failed to create a new Selector.", e);
return;
}
// Register all channels to the new Selector.
int nChannels = 0;
for (;;) {
try {
for (SelectionKey key: oldSelector.keys()) {
Object a = key.attachment();
try {
if (!key.isValid() || key.channel().keyFor(newSelector) != null) {
continue;
}
int interestOps = key.interestOps();
key.cancel();
SelectionKey newKey = key.channel().register(newSelector, interestOps, a);
if (a instanceof AbstractNioChannel) {
// Update SelectionKey
((AbstractNioChannel) a).selectionKey = newKey;
}
nChannels ++;
} catch (Exception e) {
logger.warn("Failed to re-register a Channel to the new Selector.", e);
if (a instanceof AbstractNioChannel) {
AbstractNioChannel ch = (AbstractNioChannel) a;
ch.unsafe().close(ch.unsafe().voidPromise());
} else {
@SuppressWarnings("unchecked")
NioTask<SelectableChannel> task = (NioTask<SelectableChannel>) a;
invokeChannelUnregistered(task, key, e);
}
}
}
} catch (ConcurrentModificationException e) {
// Probably due to concurrent modification of the key set.
continue;
}
break;
}
selector = newSelector;
try {
// time to close the old selector as everything else is registered to the new one
oldSelector.close();
} catch (Throwable t) {
if (logger.isWarnEnabled()) {
logger.warn("Failed to close the old Selector.", t);
}
}
logger.info("Migrated " + nChannels + " channel(s) to the new Selector.");
}作者：小毛驴，一个游戏人
梦想：世界和平
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