NIO Select 知识
select 示例代码 :
//创建 channel 并设置为非阻塞
ServerSocketChannel serverChannel = ServerSocketChannel.open();
serverChannel.configureBlocking(false);
serverChannel.socket().bind(new InetSocketAddress(port));
//打开 selector
Selector selector = Selector.open();
//注册到 selector
serverChannel.register(selector, SelectionKey.OP_ACCEPT);
//还是要循环遍历，主要的就是通过 select()方法找到 SelectionKey ,利用监听的事件类型处理响应的事情
//其中需要注意那两个注册方法
while(true){
int n = selector.select();
if (n == 0) continue;
Iterator ite = this.selector.selectedKeys().iterator();
while(ite.hasNext()){
SelectionKey key = (SelectionKey)ite.next();
if (key.isAcceptable()){
SocketChannel clntChan = ((ServerSocketChannel) key.channel()).accept();
clntChan.configureBlocking(false);
//将选择器注册到连接到的客户端信道，
//并指定该信道key值的属性为OP_READ，
//同时为该信道指定关联的附件
clntChan.register(key.selector(), SelectionKey.OP_READ, ByteBuffer.allocate(bufSize));
}
if (key.isReadable()){
handleRead(key);
}
if (key.isWritable() && key.isValid()){
handleWrite(key);
}
if (key.isConnectable()){
System.out.println("isConnectable = true");
}
ite.remove();
}
}
源码阅读
通过上一篇文章我们知道了，netty 实际是由两个 Reactor 组成，前者维护一个 Acceptor 绑定接口，处理客户端的连接，然后再将读写，解码编码工作交给另外一个 Reactor ，我们先来看一下这样一个过程，明白总体的过程后再了解系统逻辑实现的细节。
两个类的组成 ：
AbstractEventExecutorGroup
- MultithreadEventExecuteGroup
- MultithreadEventLoopGroup
- NioEventLoopGroup
- MultithreadEventLoopGroup
AbstractChannel
- AbstractNioChannel
- AbstractNioByteChannel
- NioSocketChannel
- AbstractNioByteChannel
重要类 ：
- NioServerSocketChannel : 服务端连接端口，连接的 channel
- ChannelPipeline : 存放Handler的链
- NioEventLoop : SingleThreadEventLoop implementation which register the Channel's to a Selector and so does the multi-plexing of these in the event loop 负责网络读取，连接和客户端请求接入的 Reactor线程就是 NioEventLoop
我们直接看一下 bind 方法,AbstractBootstrap 类
/**
* Create a new {@link Channel} and bind it.
*/
public ChannelFuture bind(String inetHost, int inetPort) {
return bind(new InetSocketAddress(inetHost, inetPort));
}
/**
* Create a new {@link Channel} and bind it.
*/
public ChannelFuture bind(SocketAddress localAddress) {
validate();
if (localAddress == null) {
throw new NullPointerException("localAddress");
}
return doBind(localAddress);
}
private ChannelFuture doBind(final SocketAddress localAddress) {
final ChannelFuture regFuture = initAndRegister();
final Channel channel = regFuture.channel();
if (regFuture.cause() != null) {
return regFuture;
}
final ChannelPromise promise;
if (regFuture.isDone()) {
promise = channel.newPromise();
doBind0(regFuture, channel, localAddress, promise);
} else {
// Registration future is almost always fulfilled already, but just in case it's not.
promise = new DefaultChannelPromise(channel, GlobalEventExecutor.INSTANCE);
regFuture.addListener(new ChannelFutureListener() {
@Override
public void operationComplete(ChannelFuture future) throws Exception {
doBind0(regFuture, channel, localAddress, promise);
}
});
}
return promise;
}
/**
*
*
*
*/
final ChannelFuture initAndRegister() {
Channel channel;
try {
// NO.1 Acceptor 线程绑定监听端口，等待来自客户端的连接
channel = createChannel();
} catch (Throwable t) {
return VoidChannel.INSTANCE.newFailedFuture(t);
}
try {
// No.2 附加属性，添加 Handler
init(channel);
} catch (Throwable t) {
channel.unsafe().closeForcibly();
return channel.newFailedFuture(t);
}
ChannelPromise regFuture = channel.newPromise();
// No.3 registed 注册连接事件
channel.unsafe().register(regFuture);
if (regFuture.cause() != null) {
if (channel.isRegistered()) {
channel.close();
} else {
channel.unsafe().closeForcibly();
}
}
// If we are here and the promise is not failed, it's one of the following cases:
// 1) If we attempted registration from the event loop, the registration has been completed at this point.
// i.e. It's safe to attempt bind() or connect() now beause the channel has been registered.
// 2) If we attempted registration from the other thread, the registration request has been successfully
// added to the event loop's task queue for later execution.
// i.e. It's safe to attempt bind() or connect() now:
// because bind() or connect() will be executed *after* the scheduled registration task is executed
// because register(), bind(), and connect() are all bound to the same thread.
return regFuture;
}
init 方法
@Override
void init(Channel channel) throws Exception {
final Map<ChannelOption<?>, Object> options = options();
synchronized (options) {
channel.config().setOptions(options);
}
final Map<AttributeKey<?>, Object> attrs = attrs();
synchronized (attrs) {
for (Entry<AttributeKey<?>, Object> e: attrs.entrySet()) {
@SuppressWarnings("unchecked")
AttributeKey<Object> key = (AttributeKey<Object>) e.getKey();
channel.attr(key).set(e.getValue());
}
}
//下面的ChannelPipeline 存放着一个链表，链表内部放的都是 handler ，p 是父类的 ChannelPipeline
// child下面的ChannelPipeline 是 child Reactor 的，这里会判断有没有 handler 有就加进去（我们开头例子中的 handle（）方法），没有的话
//这个方法的尾部会加多一个 ServerBootstrapAcceptor 作为的默认的链表节点
//那 childHandler 的handler 在哪里加入呢？在 ServerBootstrapAcceptor 中，我们后续会介绍
ChannelPipeline p = channel.pipeline();
if (handler() != null) {
p.addLast(handler());
}
final ChannelHandler currentChildHandler = childHandler;
final Entry<ChannelOption<?>, Object>[] currentChildOptions;
final Entry<AttributeKey<?>, Object>[] currentChildAttrs;
synchronized (childOptions) {
currentChildOptions = childOptions.entrySet().toArray(newOptionArray(childOptions.size()));
}
synchronized (childAttrs) {
currentChildAttrs = childAttrs.entrySet().toArray(newAttrArray(childAttrs.size()));
}
//注意这里： ServerBootstrapAcceptor 放到了pipleline 中，
//后续当接受客户端请求的时候会执行pipleline 中的方法，具体的是在
//NioMessageUnsafe 的read 方法
p.addLast(new ChannelInitializer<Channel>() {
@Override
public void initChannel(Channel ch) throws Exception {
ch.pipeline().addLast(new ServerBootstrapAcceptor(currentChildHandler, currentChildOptions,
currentChildAttrs));
}
});
}
register 方法
//先会判断是不是NioEventLoop 自身发起的操作，如果是，不存在并发问题，直接使用 Channel注册
//如果是由其他线程发起，则封装成一个 Task 放入到消息队列中异步执行。此处由于是由 ServerBootstrap 所在
//线程执行的注册操作，所有应该会封装成 Task 投递到NioEventLoop中执行。
@Override
public final void register(final ChannelPromise promise) {
if (eventLoop.inEventLoop()) {
register0(promise);
} else {
try {
eventLoop.execute(new Runnable() {
@Override
public void run() {
register0(promise);
}
});
} catch (Throwable t) {
logger.warn(
"Force-closing a channel whose registration task was not accepted by an event loop: {}",
AbstractChannel.this, t);
closeForcibly();
closeFuture.setClosed();
promise.setFailure(t);
}
}
}
private void register0(ChannelPromise promise) {
try {
// check if the channel is still open as it could be closed in the mean time when the register
// call was outside of the eventLoop
if (!ensureOpen(promise)) {
return;
}
// No.1 注册逻辑
doRegister();
registered = true;
promise.setSuccess();
// No.2 注册成功后，在channelPiple 的 HeaderHandler 和 TailHandler 中流转 (注意 ：内部 SelectKey 注册为 0 ，不是 OP_ACCEPT,会在后面修改)
pipeline.fireChannelRegistered();
// No.3 在channelPiple 的 HeaderHandler 和 TailHandler 中流转 ，HeaderHandler的 read 方法会调用
// SelectKey 修改为 OP_ACCEPT
if (isActive()) {
pipeline.fireChannelActive();
}
} catch (Throwable t) {
// Close the channel directly to avoid FD leak.
closeForcibly();
closeFuture.setClosed();
if (!promise.tryFailure(t)) {
logger.warn(
"Tried to fail the registration promise, but it is complete already. " +
"Swallowing the cause of the registration failure:", t);
}
}
}
initAndRegisted 方法中，我们从方法明就能知道该方法的主要的操作是初始化和注册，是上面的 No.1 2 3 是关于 channel 的操作。 ServerBootstrap 这个类的 createChannel方法
@Override
Channel createChannel() {
// next 方法会获取一个线程来做连接工作
EventLoop eventLoop = group().next();
return channelFactory().newChannel(eventLoop, childGroup);
}
MultithreadEventExecuteGroup 类中的next 方法 ，group() 返回的就是 bossGroup，它的 next 方法用于从线程组中获取可用线程
@Override
public EventExecutor next() {
return children[Math.abs(childIndex.getAndIncrement() % children.length)];
}
NioServerSocketChannel 在 createChannel()方法中通过反射被创建，同时注册了连接的事件
/**
* Create a new instance
*/
public NioServerSocketChannel(EventLoop eventLoop, EventLoopGroup childGroup) {
super(null, eventLoop, childGroup, newSocket(), SelectionKey.OP_ACCEPT);
config = new DefaultServerSocketChannelConfig(this, javaChannel().socket());
}
加入服务端已经做好了与客户端的连接操作，那么下一步应该到了IO操作应该会到达 workGroup 的 Reactor 中进行读写处理（创建一个新的线程进行处理），服务端处理在 NioEventLoop 类中的run 方法进行的，
NioEventLoop 的run 方法
@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.
}
}
}
}
// 无论是 processSelectedKeysOptimized 方法 还是 processSelectedKeysPlain 都有经过下面这个方法
// 可以看到一下就是就是 select 方法监听到的读写操作。
private static void processSelectedKey(SelectionKey k, AbstractNioChannel ch) {
final NioUnsafe unsafe = ch.unsafe();
if (!k.isValid()) {
// close the channel if the key is not valid anymore
unsafe.close(unsafe.voidPromise());
return;
}
try {
int readyOps = k.readyOps();
// Also check for readOps of 0 to workaround possible JDK bug which may otherwise lead
// to a spin loop
if ((readyOps & (SelectionKey.OP_READ | SelectionKey.OP_ACCEPT)) != 0 || readyOps == 0) {
unsafe.read();
if (!ch.isOpen()) {
// Connection already closed - no need to handle write.
return;
}
}
if ((readyOps & SelectionKey.OP_WRITE) != 0) {
// Call forceFlush which will also take care of clear the OP_WRITE once there is nothing left to write
ch.unsafe().forceFlush();
}
if ((readyOps & SelectionKey.OP_CONNECT) != 0) {
// remove OP_CONNECT as otherwise Selector.select(..) will always return without blocking
// See https://github.com/netty/netty/issues/924
int ops = k.interestOps();
ops &= ~SelectionKey.OP_CONNECT;
k.interestOps(ops);
unsafe.finishConnect();
}
} catch (CancelledKeyException e) {
unsafe.close(unsafe.voidPromise());
}
}
假如是 read 事件，那么调用 unsafe.read() 方法,会走到 AbstractNioMessageChannel 的内部类 NioMessageUnsafe 的 read 方法
public abstract class AbstractNioMessageChannel extends AbstractNioChannel {
protected AbstractNioMessageChannel(
Channel parent, EventLoop eventLoop, SelectableChannel ch, int readInterestOp) {
super(parent, eventLoop, ch, readInterestOp);
}
@Override
protected AbstractNioUnsafe newUnsafe() {
return new NioMessageUnsafe();
}
private final class NioMessageUnsafe extends AbstractNioUnsafe {
private final List<Object> readBuf = new ArrayList<Object>();
private void removeReadOp() {
SelectionKey key = selectionKey();
int interestOps = key.interestOps();
if ((interestOps & readInterestOp) != 0) {
// only remove readInterestOp if needed
key.interestOps(interestOps & ~readInterestOp);
}
}
@Override
public void read() {
assert eventLoop().inEventLoop();
if (!config().isAutoRead()) {
removeReadOp();
}
final ChannelConfig config = config();
final int maxMessagesPerRead = config.getMaxMessagesPerRead();
final boolean autoRead = config.isAutoRead();
final ChannelPipeline pipeline = pipeline();
boolean closed = false;
Throwable exception = null;
// 这里有个循环
try {
for (;;) {
// No.1
int localRead = doReadMessages(readBuf);
if (localRead == 0) {
break;
}
if (localRead < 0) {
closed = true;
break;
}
if (readBuf.size() >= maxMessagesPerRead | !autoRead) {
break;
}
}
} catch (Throwable t) {
exception = t;
}
int size = readBuf.size();
for (int i = 0; i < size; i ++) {
// No.2 调用 pipleline里的方法
pipeline.fireChannelRead(readBuf.get(i));
}
readBuf.clear();
pipeline.fireChannelReadComplete();
if (exception != null) {
if (exception instanceof IOException) {
// ServerChannel should not be closed even on IOException because it can often continue
// accepting incoming connections. (e.g. too many open files)
closed = !(AbstractNioMessageChannel.this instanceof ServerChannel);
}
pipeline.fireExceptionCaught(exception);
}
if (closed) {
if (isOpen()) {
close(voidPromise());
}
}
}
}
...
...
最终到了 NioServerSocketChannel 的 doReadMessages 方法 。
@Override
protected int doReadMessages(List<Object> buf) throws Exception {
SocketChannel ch = javaChannel().accept();
try {
if (ch != null) {
//childGroup reactor 中开启一个线程让执行读写操作，并将 NioSocketChannel 放入传入的数组中
buf.add(new NioSocketChannel(this, childEventLoopGroup().next(), ch));
return 1;
}
} catch (Throwable t) {
logger.warn("Failed to create a new channel from an accepted socket.", t);
try {
ch.close();
} catch (Throwable t2) {
logger.warn("Failed to close a socket.", t2);
}
}
return 0;
}
doReadMessages 执行结束后我们会继续继续执行 No.2 处的代码，执行 pipleline 中放入对象的的方法，又上一篇可以知道将会执行
ServerBootstrapAcceptor的 channnelRead 方法
@Override
@SuppressWarnings("unchecked")
public void channelRead(ChannelHandlerContext ctx, Object msg) {
Channel child = (Channel) msg;
// No.1 将启动时的childHandler 加入到客户端的SocketChannel 的 ChannelPiple中
// 回头看一下我们开始的例子，这就是 childHandler 加入的地方
child.pipeline().addLast(childHandler);
// No.2 设置客户端SocketChannel的TCP参数
for (Entry<ChannelOption<?>, Object> e: childOptions) {
try {
if (!child.config().setOption((ChannelOption<Object>) e.getKey(), e.getValue())) {
logger.warn("Unknown channel option: " + e);
}
} catch (Throwable t) {
logger.warn("Failed to set a channel option: " + child, t);
}
}
for (Entry<AttributeKey<?>, Object> e: childAttrs) {
child.attr((AttributeKey<Object>) e.getKey()).set(e.getValue());
}
// No.3
child.unsafe().register(child.newPromise());
}
参考资料
- http://www.blogjava.net/DLevin/archive/2015/09/02/427045.html （Reator 模型 ）
- https://www.jianshu.com/p/052035037297
- https://www.jianshu.com/p/0d497fe5484a