Netty 学习之旅 ------ 图说 Netty 线程模型
1、图说 Netty 线程模型
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提到 Netty 的线程模型，我们不得不再重复提到主从 Reactor 线程模型，Netty 线程模型基本上基于主从 Reactor 模型的实现方式，Netty 线程模型将从如下两个图进行展开：
Reactor?主从线程模型
Netty 服务端示例代码。
关于 Reactor 主从多线程模型的讲解，请重点关注Netty 线程模型前置篇，Reactor 主从模式源码实现，从 Netty 服务端示例代码可以看出，Netty 的线程模型，必然与 EventLoopGroup 脱离不了干系。不错，上文中的 bossGroup 就是相当与 Reactor 模式中的 Main?Reactor,而 workGroup,则相当于 SubReactor。
Netty 线程模型类层次结构：
Netty 线程模型类继承图：
从如上图可以知道，Netty 的线程模型，中有 4 个基础接口，它们分别是 EventLoopGroup、EventLoop、EventExecuteGroup、EventExecute。其中 EventExecute 扩展自 java.util.concurrent.ScheduledExecutorService 接口，类似与线程池（执行）的职责，而 EventLoop 首先继承自 EventExecute，并主要扩展了 register 方法,就是将通道 Channel 注册到 Selector 的方法。
NioEventLoop,就是基于 Nio 的实现。在这个类中有一个亮点，就是规避了 JDK nio 的一个 bug,Selector select 方法的空轮询,核心思想是，如果连续多少次（默认为 512）在没有超时的情况就返回，并且已经准备就绪的键的数量为 0，则认为发生了空轮询，如果发生了空轮询，就新建一个新的 Selector,并重新将通道，关心的事件注册到新的 Selector,并关闭旧的 Selector,其源码实现如下：
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 interrupt
ed 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);
}
// Harmless exception - log anyway
}
}
/**
Replaces the current {@link Selector} of this event loop with newly created {@link Selector}s to work
around the infamous epoll 100% CPU bug.
*/
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 (;;) {
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