netty4.0.x源码分析—executor和loop
之前在第一篇文章里面对event的总体结构进行了简单的分析，没有结合代码理清流程，所以对于excutor和loop的具体执行机制还是很模糊，这篇文章针对代码从上至下进行分析，相信对理解excutor和loop到底是怎么关联的有所帮助。
我们基于服务端常用的代码，EventLoopGroup bossGroup = new NioEventLoopGroup();为主线进行分析，这个函数就封装了线程的创建，以及loop和线程的关联等内容。
先看NioEventLoopGroup的源代码：
/**
* {@link MultithreadEventLoopGroup} implementations which is used for NIO {@link Selector} based {@link Channel}s.
*/
public class NioEventLoopGroup extends MultithreadEventLoopGroup {
/**
* Create a new instance using the default number of threads, the default {@link ThreadFactory} and
* the {@link SelectorProvider} which is returned by {@link SelectorProvider#provider()}.
*/
public NioEventLoopGroup() {
this(0);
}
/**
* Create a new instance using the specified number of threads, {@link ThreadFactory} and the
* {@link SelectorProvider} which is returned by {@link SelectorProvider#provider()}.
*/
public NioEventLoopGroup(int nThreads) {
this(nThreads, null);
}
/**
* Create a new instance using the specified number of threads, the given {@link ThreadFactory} and the
* {@link SelectorProvider} which is returned by {@link SelectorProvider#provider()}.
*/
public NioEventLoopGroup(int nThreads, ThreadFactory threadFactory) {
this(nThreads, threadFactory, SelectorProvider.provider());
}
/**
* Create a new instance using the specified number of threads, the given {@link ThreadFactory} and the given
* {@link SelectorProvider}.
*/
public NioEventLoopGroup(
int nThreads, ThreadFactory threadFactory, final SelectorProvider selectorProvider) {
super(nThreads, threadFactory, selectorProvider);
}
@Override
protected EventExecutor newChild(
ThreadFactory threadFactory, Object... args) throws Exception {
return new NioEventLoop(this, threadFactory, (SelectorProvider) args[0]);
}
}从这个构造函数可以看出，实际上NioEventLoopGroup的构造就是调用了父类的构造函数完成的，那么就看父类的构造函数做了些什么，它的父类是MultithreadEventLoopGroup，下面是它的关键代码：
/**
* Abstract base class for {@link EventLoopGroup} implementations that handles their tasks with multiple threads at
* the same time.
*/
public abstract class MultithreadEventLoopGroup extends MultithreadEventExecutorGroup implements EventLoopGroup {
private static final InternalLogger logger = InternalLoggerFactory.getInstance(MultithreadEventLoopGroup.class);
private static final int DEFAULT_EVENT_LOOP_THREADS;
static {
DEFAULT_EVENT_LOOP_THREADS = Math.max(1, SystemPropertyUtil.getInt(
"io.netty.eventLoopThreads", Runtime.getRuntime().availableProcessors() * 2));
if (logger.isDebugEnabled()) {
logger.debug("-Dio.netty.eventLoopThreads: {}", DEFAULT_EVENT_LOOP_THREADS);
}
}
/**
* @see {@link MultithreadEventExecutorGroup#MultithreadEventExecutorGroup(int, ThreadFactory, Object...)}
*/
protected MultithreadEventLoopGroup(int nThreads, ThreadFactory threadFactory, Object... args) {
super(nThreads == 0? DEFAULT_EVENT_LOOP_THREADS : nThreads, threadFactory, args);
}
@Override
protected ThreadFactory newDefaultThreadFactory() {
return new DefaultThreadFactory(getClass(), Thread.MAX_PRIORITY);
}
@Override
public EventLoop next() {
return (EventLoop) super.next();
}
@Override
public ChannelFuture register(Channel channel) {
return next().register(channel);
}
@Override
public ChannelFuture register(Channel channel, ChannelPromise promise) {
return next().register(channel, promise);
}
}从上面的代码可以看出，若给定的线程数为0，则将默认的事件循环线程数作为参数继续调用父类的构造函数，继续看它的父类MultithreadEventExecutorGroup的定义，关键代码如下：
/**
* Abstract base class for {@link EventExecutorGroup} implementations that handles their tasks with multiple threads at
* the same time.
*/
public abstract class MultithreadEventExecutorGroup extends AbstractEventExecutorGroup {
private final EventExecutor[] children;
private final AtomicInteger childIndex = new AtomicInteger();
private final AtomicInteger terminatedChildren = new AtomicInteger();
private final Promise<?> terminationFuture = new DefaultPromise(GlobalEventExecutor.INSTANCE);
/**
* Create a new instance.
*
* @param nThreads the number of threads that will be used by this instance.
* @param threadFactory the ThreadFactory to use, or {@code null} if the default should be used.
* @param args arguments which will passed to each {@link #newChild(ThreadFactory, Object...)} call
*/
protected MultithreadEventExecutorGroup(int nThreads, ThreadFactory threadFactory, Object... args) {
if (nThreads <= 0) {
throw new IllegalArgumentException(String.format("nThreads: %d (expected: > 0)", nThreads));
}
if (threadFactory == null) {
threadFactory = newDefaultThreadFactory();
}
children = new SingleThreadEventExecutor[nThreads];
for (int i = 0; i < nThreads; i ++) {
boolean success = false;
try {
children[i] = newChild(threadFactory, args);
success = true;
} catch (Exception e) {
// TODO: Think about if this is a good exception type
throw new IllegalStateException("failed to create a child event loop", e);
} finally {
if (!success) {
for (int j = 0; j < i; j ++) {
children[j].shutdownGracefully();
}
for (int j = 0; j < i; j ++) {
EventExecutor e = children[j];
try {
while (!e.isTerminated()) {
e.awaitTermination(Integer.MAX_VALUE, TimeUnit.SECONDS);
}
} catch (InterruptedException interrupted) {
Thread.currentThread().interrupt();
break;
}
}
}
}
}
final FutureListener<Object> terminationListener = new FutureListener<Object>() {
@Override
public void operationComplete(Future<Object> future) throws Exception {
if (terminatedChildren.incrementAndGet() == children.length) {
terminationFuture.setSuccess(null);
}
}
};
for (EventExecutor e: children) {
e.terminationFuture().addListener(terminationListener);
}
}
protected ThreadFactory newDefaultThreadFactory() {
return new DefaultThreadFactory(getClass());
}
@Override
public EventExecutor next() {
return children[Math.abs(childIndex.getAndIncrement() % children.length)];
}
@Override
public Iterator<EventExecutor> iterator() {
return children().iterator();
}
/**
* Return the number of {@link EventExecutor} this implementation uses. This number is the maps
* 1:1 to the threads it use.
*/
public final int executorCount() {
return children.length;
}
/**
* Return a safe-copy of all of the children of this group.
*/
protected Set<EventExecutor> children() {
Set<EventExecutor> children = Collections.newSetFromMap(new LinkedHashMap<EventExecutor, Boolean>());
Collections.addAll(children, this.children);
return children;
}
/**
* Create a new EventExecutor which will later then accessible via the {@link #next()} method. This method will be
* called for each thread that will serve this {@link MultithreadEventExecutorGroup}.
*
*/
protected abstract EventExecutor newChild(
ThreadFactory threadFactory, Object... args) throws Exception;
@Override
public Future<?> shutdownGracefully(long quietPeriod, long timeout, TimeUnit unit) {
for (EventExecutor l: children) {
l.shutdownGracefully(quietPeriod, timeout, unit);
}
return terminationFuture();
}
}重点是它的构造函数，我们看出它里面实际上还是由多个SingleThreadEventExecutor构造的，调用了newChild函数为每个EventExecutor赋值，继续看newChild的实现，我们是基于NioEventLoopGroup进行分析的所以，我们直接看它的newChild实现：
@Override
protected EventExecutor newChild(
ThreadFactory threadFactory, Object... args) throws Exception {
return new NioEventLoop(this, threadFactory, (SelectorProvider) args[0]);
}上面的代码进而调用了new NioEventLoop函数构造EventExecutor，下面是NioEventLoop构造函数
NioEventLoop(NioEventLoopGroup parent, ThreadFactory threadFactory, SelectorProvider selectorProvider) {
super(parent, threadFactory, false);
if (selectorProvider == null) {
throw new NullPointerException("selectorProvider");
}
provider = selectorProvider;
selector = openSelector();
}从这个代码，我们可以发现EventExcutor->EventLoop->Selector，三种的对应关系。继续看它的父类SingleThreadEventLoop的关键代码
/**
* Abstract base class for {@link EventLoop}'s that execute all its submitted tasks in a single thread.
*
*/
public abstract class SingleThreadEventLoop extends SingleThreadEventExecutor implements EventLoop {
/**
* @see {@link SingleThreadEventExecutor#SingleThreadEventExecutor(EventExecutorGroup, ThreadFactory, boolean)}
*/
protected SingleThreadEventLoop(EventLoopGroup parent, ThreadFactory threadFactory, boolean addTaskWakesUp) {
super(parent, threadFactory, addTaskWakesUp);
}
@Override
public EventLoopGroup parent() {
return (EventLoopGroup) super.parent();
}
@Override
public EventLoop next() {
return (EventLoop) super.next();
}
@Override
public ChannelFuture register(Channel channel) {
return register(channel, channel.newPromise());
}
@Override
public ChannelFuture register(final Channel channel, final ChannelPromise promise) {
if (channel == null) {
throw new NullPointerException("channel");
}
if (promise == null) {
throw new NullPointerException("promise");
}
channel.unsafe().register(this, promise);
return promise;
}
}它的构造函数还是调用了父类的构造函数，继续看它的父类SingleThreadEventExecutor的关键代码，
/**
* Create a new instance
*
* @param parent the {@link EventExecutorGroup} which is the parent of this instance and belongs to it
* @param threadFactory the {@link ThreadFactory} which will be used for the used {@link Thread}
* @param addTaskWakesUp {@code true} if and only if invocation of {@link #addTask(Runnable)} will wake up the
* executor thread
*/
protected SingleThreadEventExecutor(
EventExecutorGroup parent, ThreadFactory threadFactory, boolean addTaskWakesUp) {
if (threadFactory == null) {
throw new NullPointerException("threadFactory");
}
this.parent = parent;
this.addTaskWakesUp = addTaskWakesUp;
thread = threadFactory.newThread(new Runnable() {
@Override
public void run() {
boolean success = false;
updateLastExecutionTime();
try {
SingleThreadEventExecutor.this.run();
success = true;
} catch (Throwable t) {
logger.warn("Unexpected exception from an event executor: ", t);
} finally {
if (state < ST_SHUTTING_DOWN) {
state = ST_SHUTTING_DOWN;
}
// Check if confirmShutdown() was called at the end of the loop.
if (success && gracefulShutdownStartTime == 0) {
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 {
synchronized (stateLock) {
state = ST_TERMINATED;
}
threadLock.release();
if (!taskQueue.isEmpty()) {
logger.warn(
"An event executor terminated with " +
"non-empty task queue (" + taskQueue.size() + ')');
}
terminationFuture.setSuccess(null);
}
}
}
}
});
taskQueue = newTaskQueue();
}<pre name="code" class="java"> @Override
public void execute(Runnable task) {
if (task == null) {
throw new NullPointerException("task");
}
boolean inEventLoop = inEventLoop();
if (inEventLoop) {
addTask(task);
} else {
startThread();
addTask(task);
if (isShutdown() && removeTask(task)) {
reject();
}
}
if (!addTaskWakesUp) {
wakeup(inEventLoop);
}
}
这个类非常重要，有时间需要再仔细看看，这里我们可以清晰的看到它生成了一个线程，并且构造了一个任务队列。并且它还重写了excute方法，将线程任务都添加到任务队列，run方法则不断的从任务队列里面取任务执行，从子类的run方法可以清晰的看到这点。
@Override
protected void run() {
for (;;) {
oldWakenUp = wakenUp.getAndSet(false);
try {
if (hasTasks()) {
selectNow();
} else {
select();
// '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();
}
}
cancelledKeys = 0;
final long ioStartTime = System.nanoTime();
needsToSelectAgain = false;
if (selectedKeys != null) {
processSelectedKeysOptimized(selectedKeys.flip());
} else {
processSelectedKeysPlain(selector.selectedKeys());
}
final long ioTime = System.nanoTime() - ioStartTime;
final int ioRatio = this.ioRatio;
runAllTasks(ioTime * (100 - ioRatio) / ioRatio);
if (isShuttingDown()) {
closeAll();
if (confirmShutdown()) {
break;
}
}
} catch (Throwable t) {
logger.warn("Unexpected exception in the selector loop.", t);
// Prevent possible consecutive immediate failures that lead to
// excessive CPU consumption.
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
// Ignore.
}
}
}
}上面的代码又调用了父类定义的runAllTasks函数，下面是SingleThreadEventExecutor中的定义：
/**
* Poll all tasks from the task queue and run them via {@link Runnable#run()} method. This method stops running
* the tasks in the task queue and returns if it ran longer than {@code timeoutNanos}.
*/
protected boolean runAllTasks(long timeoutNanos) {
fetchFromDelayedQueue();
Runnable task = pollTask();
if (task == null) {
return false;
}
final long deadline = ScheduledFutureTask.nanoTime() + timeoutNanos;
long runTasks = 0;
long lastExecutionTime;
for (;;) {
try {
task.run();
} catch (Throwable t) {
logger.warn("A task raised an exception.", t);
}
runTasks ++;
// Check timeout every 64 tasks because nanoTime() is relatively expensive.
// XXX: Hard-coded value - will make it configurable if it is really a problem.
if ((runTasks & 0x3F) == 0) {
lastExecutionTime = ScheduledFutureTask.nanoTime();
if (lastExecutionTime >= deadline) {
break;
}
}
task = pollTask();
if (task == null) {
lastExecutionTime = ScheduledFutureTask.nanoTime();
break;
}
}
this.lastExecutionTime = lastExecutionTime;
return true;
}从这里，我们可以看到任务队列是如何取任务执行了。
总结：
通过上面的分析，我们基本上了解了eventloop和excutor，以及线程之间的关系了，其实netty就是实现了自己的一个线程池，来执行线程任务。大概的流程就是在eventloop的selector中注册channel，然后channel的事件处理都以线程任务的形式执行，并且先放入任务队列中（因为它重写了executor方法，在SingleThreadEventExecutor中我们可以看到重写的executor函数的实现），然后线程不断的从任务队列里面取任务执行。