在上一篇博客【Netty源码解析】入门示例中我们介绍了一个Netty入门的示例代码，接下来的博客我们会分析一下整个demo工程运行过程的运行机制。 无论在Netty应用的客户端还是服务端都首先会初始化EventLoopGroup，其封装了线程的创建，以及loop和线程的关联等内容。
EventLoopGroup group = new NioEventLoopGroup();
接下来我们一步一步看NioEventLoopGroup在初始化过程中都做了什么事情，其源码如下：
/**
* {@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, (Executor) 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());
}
public NioEventLoopGroup(int nThreads, Executor executor) {
this(nThreads, executor, 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) {
this(nThreads, threadFactory, selectorProvider, DefaultSelectStrategyFactory.INSTANCE);
}
public NioEventLoopGroup(int nThreads, ThreadFactory threadFactory,
final SelectorProvider selectorProvider, final SelectStrategyFactory selectStrategyFactory) {
super(nThreads, threadFactory, selectorProvider, selectStrategyFactory, RejectedExecutionHandlers.reject());
}
public NioEventLoopGroup(
int nThreads, Executor executor, final SelectorProvider selectorProvider) {
this(nThreads, executor, selectorProvider, DefaultSelectStrategyFactory.INSTANCE);
}
public NioEventLoopGroup(int nThreads, Executor executor, final SelectorProvider selectorProvider,
final SelectStrategyFactory selectStrategyFactory) {
super(nThreads, executor, selectorProvider, selectStrategyFactory, RejectedExecutionHandlers.reject());
}
public NioEventLoopGroup(int nThreads, Executor executor, EventExecutorChooserFactory chooserFactory,
final SelectorProvider selectorProvider,
final SelectStrategyFactory selectStrategyFactory) {
super(nThreads, executor, chooserFactory, selectorProvider, selectStrategyFactory,
RejectedExecutionHandlers.reject());
}
public NioEventLoopGroup(int nThreads, Executor executor, EventExecutorChooserFactory chooserFactory,
final SelectorProvider selectorProvider,
final SelectStrategyFactory selectStrategyFactory,
final RejectedExecutionHandler rejectedExecutionHandler) {
super(nThreads, executor, chooserFactory, selectorProvider, selectStrategyFactory, rejectedExecutionHandler);
}
/**
* Sets the percentage of the desired amount of time spent for I/O in the child event loops. The default value is
* {@code 50}, which means the event loop will try to spend the same amount of time for I/O as for non-I/O tasks.
*/
public void setIoRatio(int ioRatio) {
for (EventExecutor e: this) {
((NioEventLoop) e).setIoRatio(ioRatio);
}
}
/**
* Replaces the current {@link Selector}s of the child event loops with newly created {@link Selector}s to work
* around the infamous epoll 100% CPU bug.
*/
public void rebuildSelectors() {
for (EventExecutor e: this) {
((NioEventLoop) e).rebuildSelector();
}
}
@Override
protected EventLoop newChild(Executor executor, Object... args) throws Exception {
return new NioEventLoop(this, executor, (SelectorProvider) args[0],
((SelectStrategyFactory) args[1]).newSelectStrategy(), (RejectedExecutionHandler) args[2]);
}
}
由上面的源码我们了解到NioEventLoopGroup构造函数初始化的结果是调用父类MultithreadEventLoopGroup的构造函数，接下来我们看一下MultithreadEventLoopGroup的构造函数做了什么事情。
public abstract class MultithreadEventExecutorGroup extends AbstractEventExecutorGroup {
private final EventExecutor[] children;
private final Set<EventExecutor> readonlyChildren;
private final AtomicInteger terminatedChildren = new AtomicInteger();
private final Promise<?> terminationFuture = new DefaultPromise(GlobalEventExecutor.INSTANCE);
private final EventExecutorChooserFactory.EventExecutorChooser chooser;
protected MultithreadEventExecutorGroup(int nThreads, ThreadFactory threadFactory, Object... args) {
this(nThreads, threadFactory == null ? null : new ThreadPerTaskExecutor(threadFactory), args);
}
protected MultithreadEventExecutorGroup(int nThreads, Executor executor, Object... args) {
this(nThreads, executor, DefaultEventExecutorChooserFactory.INSTANCE, args);
}
/**
* Create a new instance.
*
* @param nThreads the number of threads that will be used by this instance.
* @param executor the Executor to use, or {@code null} if the default should be used.
* @param chooserFactory the {@link EventExecutorChooserFactory} to use.
* @param args arguments which will passed to each {@link #newChild(Executor, Object...)} call
*/
protected MultithreadEventExecutorGroup(int nThreads, Executor executor,
EventExecutorChooserFactory chooserFactory, Object... args) {
if (nThreads <= 0) {
throw new IllegalArgumentException(String.format("nThreads: %d (expected: > 0)", nThreads));
}
if (executor == null) {
executor = new ThreadPerTaskExecutor(newDefaultThreadFactory());
}
children = new EventExecutor[nThreads];
for (int i = 0; i < nThreads; i ++) {
boolean success = false;
try {
children[i] = newChild(executor, 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) {
// Let the caller handle the interruption.
Thread.currentThread().interrupt();
break;
}
}
}
}
}
chooser = chooserFactory.newChooser(children);
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);
}
Set<EventExecutor> childrenSet = new LinkedHashSet<EventExecutor>(children.length);
Collections.addAll(childrenSet, children);
readonlyChildren = Collections.unmodifiableSet(childrenSet);
}
protected ThreadFactory newDefaultThreadFactory() {
return new DefaultThreadFactory(getClass());
}
@Override
public EventExecutor next() {
return chooser.next();
}
@Override
public Iterator<EventExecutor> iterator() {
return readonlyChildren.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;
}
/**
* 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(Executor executor, 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();
}
@Override
public Future<?> terminationFuture() {
return terminationFuture;
}
@Override
@Deprecated
public void shutdown() {
for (EventExecutor l: children) {
l.shutdown();
}
}
@Override
public boolean isShuttingDown() {
for (EventExecutor l: children) {
if (!l.isShuttingDown()) {
return false;
}
}
return true;
}
@Override
public boolean isShutdown() {
for (EventExecutor l: children) {
if (!l.isShutdown()) {
return false;
}
}
return true;
}
@Override
public boolean isTerminated() {
for (EventExecutor l: children) {
if (!l.isTerminated()) {
return false;
}
}
return true;
}
@Override
public boolean awaitTermination(long timeout, TimeUnit unit)
throws InterruptedException {
long deadline = System.nanoTime() + unit.toNanos(timeout);
loop: for (EventExecutor l: children) {
for (;;) {
long timeLeft = deadline - System.nanoTime();
if (timeLeft <= 0) {
break loop;
}
if (l.awaitTermination(timeLeft, TimeUnit.NANOSECONDS)) {
break;
}
}
}
return isTerminated();
}
}
从上面代码的构造函数中我们发现了真正处理功能
（1）executor = new ThreadPerTaskExecutor(newDefaultThreadFactory()); 首先创建一个单线程执行的线程池
（2）children = new EventExecutor[nThreads]; 创建处理器两倍数目的处理线程数组
（3）children[i] = newChild(executor, args) 实例化线程数组，具体实现在NioEventLoopGroup
@Override
protected EventLoop newChild(Executor executor, Object... args) throws Exception {
return new NioEventLoop(this, executor, (SelectorProvider) args[0],
((SelectStrategyFactory) args[1]).newSelectStrategy(), (RejectedExecutionHandler) args[2]);
}
这样EventLoopGroup的主要功能就完成了，其主要机制就是创建了一定数目的NioEventLoop。
EventLoopGroup类图结构：
