NioEventLoop的轮训事件
直接看代码：
//事件循环
@Override
protected void run() {
for (;;) {
try {
try {
//hasTasks() 若taskQueue or tailTasks任务队列中有任务 返回false 没有则返回true
// //有任务返回selectnow的返回值 没任务返回-1
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:
//首先轮询注册到reactor线程对应的selector上的所有的channel的IO事件
//wakenUp 表示是否应该唤醒正在阻塞的select操作，netty在每次进行新的loop之前，都会将wakeUp 被设置成false，标志新的一轮loop的开始
select(wakenUp.getAndSet(false));
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 {
//2.处理产生网络IO事件的channel
processSelectedKeys();
} finally {
// Ensure we always run tasks.
final long ioTime = System.nanoTime() - ioStartTime;
//3.处理任务队列
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);
}
}
}这段代码主要做四件事情
1.select(wakenUp.getAndSet(false));轮训select
2.processSelectedKeys();处理io事件
3.runAllTasks(ioTime * (100 - ioRatio) / ioRatio);处理任务
4.closeAll();
今天主要分析select(wakenUp.getAndSet(false))
private void select(boolean oldWakenUp) throws IOException {
Selector selector = this.selector;
try {
int selectCnt = 0;
long currentTimeNanos = System.nanoTime();
//当scheduledTaskQueue为空时 selectDeadLineNanos=当前时间加一秒
long selectDeadLineNanos = currentTimeNanos + delayNanos(currentTimeNanos);
for (;;) {
//1.定时任务截止事时间快到了，中断本次轮询
long timeoutMillis = (selectDeadLineNanos - currentTimeNanos + 500000L) / 1000000L;
//当前的定时任务队列中有任务的截止事件快到了(<=0.5ms)，就跳出循环。
if (timeoutMillis <= 0) {
//如果到目前还没有进行过select操作 调用selectNow()
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.
// 2.轮询过程中发现有任务加入，中断本次轮询 netty为了保证任务队列能够及时执行，在进行阻塞select操作之前会判断任务队列是否为空，如果不为空，就执行一次非阻塞select操作，跳出循环
//hasTasks() && wakenUp.compareAndSet(false, true) 如果队列中有任务 则设置wakenUp为true 并返回true
if (hasTasks() && wakenUp.compareAndSet(false, true)) {
selector.selectNow();
selectCnt = 1;
break;
}
//阻塞式select操作
//执行到这一步，说明netty任务队列里面队列为空，并且所有定时任务延迟时间还未到(大于0.5ms)，
//于是，在这里进行一次阻塞select操作，截止到第一个定时任务的截止时间
int selectedKeys = selector.select(timeoutMillis);
selectCnt ++;
if (selectedKeys != 0 || oldWakenUp || wakenUp.get() || hasTasks() || hasScheduledTasks()) {
// - 轮询到io事件
// - oldWakenUp 参数为true
// - 用户主动唤醒
// - 任务队列里面有任务
// - 第一个定时任务即将要被执行
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();
//现在的时间-select阻塞的时间=>运行之前的时间
if (time - TimeUnit.MILLISECONDS.toNanos(timeoutMillis) >= currentTimeNanos) {
// timeoutMillis在没有选择任何内容的情况下运行。
selectCnt = 1;
} else if (SELECTOR_AUTO_REBUILD_THRESHOLD > 0 &&
selectCnt >= SELECTOR_AUTO_REBUILD_THRESHOLD) {
//如果selectCnt>=512就重新创建新的selector并替换
//创建新的selector
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
}
}下面我们看下重建一个selectRebuildSelector
private Selector selectRebuildSelector(int selectCnt) throws IOException {
// 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, selector);
rebuildSelector();
Selector selector = this.selector;
//再次选择以填充selectedKeys。
selector.selectNow();
return selector;
}private void rebuildSelector0() {
final Selector oldSelector = selector;
final SelectorTuple newSelectorTuple;
if (oldSelector == null) {
return;
}
try {
newSelectorTuple = openSelector();
} catch (Exception e) {
logger.warn("Failed to create a new Selector.", e);
return;
}
// 将所有通道注册到新的选择器。
/**
* 拿到有效的key
* 取消该key在旧的selector上的事件注册
* 将该key对应的channel注册到新的selector上
* 重新绑定channel和新的key的关系
* 如果附加属性是AbstractNioChannel则修改selectionKey属性为最新key
*/
int nChannels = 0;
for (SelectionKey key: oldSelector.keys()) {
Object a = key.attachment();
try {
if (!key.isValid() || key.channel().keyFor(newSelectorTuple.unwrappedSelector) != null) {
continue;
}
int interestOps = key.interestOps();
key.cancel();
SelectionKey newKey = key.channel().register(newSelectorTuple.unwrappedSelector, 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 task = (NioTask) a;
invokeChannelUnregistered(task, key, e);
}
}
}
//赋值最新selector
selector = newSelectorTuple.selector;
unwrappedSelector = newSelectorTuple.unwrappedSelector;
try {
// 是时候关闭旧的选择器了，因为其他所有内容都已注册到新选择器
oldSelector.close();
} catch (Throwable t) {
if (logger.isWarnEnabled()) {
logger.warn("Failed to close the old Selector.", t);
}
}
if (logger.isInfoEnabled()) {
logger.info("Migrated " + nChannels + " channel(s) to the new Selector.");
}
}