众所周知，netty是一款性能非常出色的nio框架，作为dubbo等众多优秀项目底层的数据传输框架，研究吃透它，对于我们今后的开发是绝对有益无害的，所以从今天开始我们就研究netty。本次分析基于netty4，请诸位看官自行下载jar包及源码。好了，我们今天说一下netty的线程池。
我们经常会看到netty的代码中有下面这一句。
EventLoopGroup workerGroup = new NioEventLoopGroup();
简单的new了一个事件的处理组（也没看官方怎么解释这个概念的，自己定义了一下吧，勿喷）。但他里面所作的事情却远不止看到的这么简单。这也是我们阅读源码的一个准则，不要忽略每一个你认为的不起眼的代码，也许他的作用是举足轻重的。他的具体实现
### io.netty.channel.nio.NioEventLoopGroup#NioEventLoopGroup()
/**
* 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);
}
### io.netty.channel.MultithreadEventLoopGroup#MultithreadEventLoopGroup(int, java.util.concurrent.Executor, java.lang.Object...)
/**
* @see MultithreadEventExecutorGroup#MultithreadEventExecutorGroup(int, Executor, Object...)
*/
protected MultithreadEventLoopGroup(int nThreads, Executor executor, Object... args) {
super(nThreads == 0 ? DEFAULT_EVENT_LOOP_THREADS : nThreads, executor, args);
}
### MultithreadEventLoopGroup.java:39
static {
DEFAULT_EVENT_LOOP_THREADS = Math.max(1, SystemPropertyUtil.getInt(
"io.netty.eventLoopThreads", NettyRuntime.availableProcessors() * 2));
if (logger.isDebugEnabled()) {
logger.debug("-Dio.netty.eventLoopThreads: {}", DEFAULT_EVENT_LOOP_THREADS);
}
}
### 表示代码出自的类及方法名，因为他们的相关性很大，所以我就放到同一个代码块中，防止思维跳跃太大，大家跟不上节奏。虽然现在初始化的时候设置了线程数为0，但是并不是最后的结果，经过了诸多的构造函数的调用及父类构造函数的引用,在这里做了一个转化，当为0时，会取DEFAULT_EVENT_LOOP_THREADS的值，而他的值,他取的是，如果设置io.netty.eventLoopThreads的值就取这个值，没有设置的话，会取默认值可用核数的两倍，同1比较去一个最大的进行赋值。最后我们到达了最好的构造函数
### io.netty.util.concurrent.MultithreadEventExecutorGroup#MultithreadEventExecutorGroup(int, java.util.concurrent.Executor, io.netty.util.concurrent.EventExecutorChooserFactory, java.lang.Object...)
/**
* 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) { //如何executor为空，那么设置默认执行器为ThreadPerTaskExecutor
executor = new ThreadPerTaskExecutor(newDefaultThreadFactory());
}
children = new EventExecutor[nThreads]; //MultithreadEventExecutorGroup是一个总的管理的类，具体和线程相关的都交给他的children进行处理，是一个EventExecutor的数组
for (int i = 0; i < nThreads; i ++) {
boolean success = false;
try {
children[i] = newChild(executor, args); //初始化每一个EventExecutor的实例，下面会有详细解释
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); //创建线程的选择器，选择是有哪个线程来处理
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); //将这些子处理器设置为只读，不能添加
}
在上面的代码中都有关键步骤的注释，总的来说就是最后的脏活累活都不是这个Group干的，都交给自己内部的children来干，都交给EventExecutor来干，我们看一下这个EventExecutor是如何实例化的
### io.netty.channel.nio.NioEventLoopGroup#newChild
@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]);
}
### io.netty.channel.nio.NioEventLoop#NioEventLoop
NioEventLoop(NioEventLoopGroup parent, Executor executor, SelectorProvider selectorProvider,
SelectStrategy strategy, RejectedExecutionHandler rejectedExecutionHandler) {
super(parent, executor, false, DEFAULT_MAX_PENDING_TASKS, rejectedExecutionHandler);
if (selectorProvider == null) {
throw new NullPointerException("selectorProvider");
}
if (strategy == null) {
throw new NullPointerException("selectStrategy");
}
provider = selectorProvider;
final SelectorTuple selectorTuple = openSelector();
selector = selectorTuple.selector;
unwrappedSelector = selectorTuple.unwrappedSelector;
selectStrategy = strategy;
}
EventExecutor是使用NioEventLoop进行初始化的，这个NioEventLoop是SingleThreadEventLoop的子类，所以super调用的是SingleThreadEventLoop的构造方法
/**
* Create a new instance
*
* @param parent the {@link EventExecutorGroup} which is the parent of this instance and belongs to it
* @param executor the {@link Executor} which will be used for executing
* @param addTaskWakesUp {@code true} if and only if invocation of {@link #addTask(Runnable)} will wake up the
* executor thread
* @param maxPendingTasks the maximum number of pending tasks before new tasks will be rejected.
* @param rejectedHandler the {@link RejectedExecutionHandler} to use.
*/
protected SingleThreadEventExecutor(EventExecutorGroup parent, Executor executor,
boolean addTaskWakesUp, int maxPendingTasks,
RejectedExecutionHandler rejectedHandler) {
super(parent);
this.addTaskWakesUp = addTaskWakesUp;
this.maxPendingTasks = Math.max(16, maxPendingTasks);
this.executor = ObjectUtil.checkNotNull(executor, "executor");
taskQueue = newTaskQueue(this.maxPendingTasks); //设置任务的处理队列，后续任务会添加到这里面
rejectedExecutionHandler = ObjectUtil.checkNotNull(rejectedHandler, "rejectedHandler");
}
OK,每一个处理子事件处理器都会有一个任务的队列。目前为止线程池的初始化就告一段落了，感觉没过瘾，咱们就下一篇见。
