原文:https://wangwei.one/posts/netty-new-connection-and-niosocketchannel-analyse.html
前面的一些章节，我们分析了Netty的三大组件 —— Channel 、EventLoop、Pipeline ，对Netty的工作原理有了深入的了解。在此基础上，我们来分析一下当Netty服务端启动后，Netty是如何处理新连接接入的。
本文内容主要分为以下四部分：
- 新连接检测
- NioSocketChannel创建
- NioSocketChannel初始化与注册
- NioSocketChannel注册READ兴趣集
新连接检测
前面，我们在讲 EventLoop的启动过程源码分析 时，解读过下面这段代码：
public final class NioEventLoop extends SingleThreadEventLoop {
...
private void processSelectedKey(SelectionKey k, AbstractNioChannel ch) {
...
try {
...
if ((readyOps & (SelectionKey.OP_READ | SelectionKey.OP_ACCEPT)) != 0 || readyOps == 0) {
// 读取read事件
unsafe.read();
}
} catch (CancelledKeyException ignored) {
unsafe.close(unsafe.voidPromise());
}
...
}
...
}
我们还是以服务端 NioServerSocketChannel 为例，它绑定的unsafe实例为 NioMessageUnsafe 。上面的 unsafe.read() 接口，会向下调用到 NioMessageUnsafe.read() 接口，如下：
public abstract class AbstractNioMessageChannel extends AbstractNioChannel {
...
private final class NioMessageUnsafe extends AbstractNioUnsafe {
// 用于保存新建立的 NioSocketChannel 的集合
private final List<Object> readBuf = new ArrayList<Object>();
@Override
public void read() {
// 确保在当前线程与EventLoop中的一致
assert eventLoop().inEventLoop();
// 获取 NioServerSocketChannel config配置
final ChannelConfig config = config();
// 获取 NioServerSocketChannel 绑定的 pipeline
final ChannelPipeline pipeline = pipeline();
// 获取RecvByteBuf 分配器 Handle
// 当channel在接收数据时，allocHandle 会用于分配ByteBuf来保存数据
// 关于allocHandle后面再去做详细介绍
final RecvByteBufAllocator.Handle allocHandle = unsafe().recvBufAllocHandle();
// 重置已累积的所有计数器，并为下一个读取循环读取多少消息/字节数据提供建议
allocHandle.reset(config);
boolean closed = false;
Throwable exception = null;
try {
try {
do {
// 调用后面的 doReadMessages 接口，读取到message则返回1
int localRead = doReadMessages(readBuf);
if (localRead == 0) {
break;
}
if (localRead < 0) {
closed = true;
break;
}
// 对当前read循环所读取到的message数量计数+1
allocHandle.incMessagesRead(localRead);
// 判断是否继续读取message
} while (allocHandle.continueReading());
} catch (Throwable t) {
exception = t;
}
int size = readBuf.size();
for (int i = 0; i < size; i ++) {
readPending = false;
// 调用pipeline传播ChannelRead事件
pipeline.fireChannelRead(readBuf.get(i));
}
// 清空readBuf
readBuf.clear();
allocHandle.readComplete();
// 调用pipeline传播 ChannelReadComplete 事件
pipeline.fireChannelReadComplete();
if (exception != null) {
closed = closeOnReadError(exception);
pipeline.fireExceptionCaught(exception);
}
if (closed) {
inputShutdown = true;
if (isOpen()) {
close(voidPromise());
}
}
} finally {
// Check if there is a readPending which was not processed yet.
// This could be for two reasons:
// * The user called Channel.read() or ChannelHandlerContext.read() in channelRead(...) method
// * The user called Channel.read() or ChannelHandlerContext.read() in channelReadComplete(...) method
//
// See https://github.com/netty/netty/issues/2254
if (!readPending && !config.isAutoRead()) {
removeReadOp();
}
}
}
}
...
}
对于 doReadMessages(...) 的分析：
public class NioServerSocketChannel extends AbstractNioMessageChannel implements io.netty.channel.socket.ServerSocketChannel {
...
// 读取消息
@Override
protected int doReadMessages(List<Object> buf) throws Exception {
// 获取 SocketChannel
SocketChannel ch = SocketUtils.accept(javaChannel());
try {
if (ch != null) {
// 使用SocketChannel创建NioSocketChannel，将其存入buf list中
// 关于NioSocketChannel的创建请看后面的分析
buf.add(new NioSocketChannel(this, 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;
}
...
}
对于 continueReading() 接口的分析，至于结果为什么返回false，后面会单独分析：
public abstract class DefaultMaxMessagesRecvByteBufAllocator implements MaxMessagesRecvByteBufAllocator {
private volatile int maxMessagesPerRead;
private volatile boolean respectMaybeMoreData = true;
...
public abstract class MaxMessageHandle implements ExtendedHandle {
private ChannelConfig config;
// 每次读取最大的消息数
private int maxMessagePerRead;
private int totalMessages;
private int totalBytesRead;
private int attemptedBytesRead;
private int lastBytesRead;
private final boolean respectMaybeMoreData = DefaultMaxMessagesRecvByteBufAllocator.this.respectMaybeMoreData;
private final UncheckedBooleanSupplier defaultMaybeMoreSupplier = new UncheckedBooleanSupplier() {
@Override
public boolean get() {
return attemptedBytesRead == lastBytesRead;
}
};
...
// 判断是否继续读取message
@Override
public boolean continueReading() {
return continueReading(defaultMaybeMoreSupplier);
}
// 判断是否继续读取message
@Override
public boolean continueReading(UncheckedBooleanSupplier maybeMoreDataSupplier) {
// 默认情况下 config.isAutoRead() 为true
// respectMaybeMoreData 默认为 true
// maybeMoreDataSupplier.get() 为false
// totalMessages第一次循环则为1
// maxMessagePerRead为16
// 结果返回false
return config.isAutoRead() &&
(!respectMaybeMoreData || maybeMoreDataSupplier.get()) &&
totalMessages < maxMessagePerRead &&
totalBytesRead > 0;
}
...
}
...
}
NioSocketChannel创建
上面分析新连接接入，提到了 NioSocketChannel 的创建，我们这里来详细分析一下，NioSocketChannel的创建过程与此前我们分析 NioServerSocketChannel创建 大体类似。
构造器
先来看看 NioSocketChannel 的构造函数：
public class NioSocketChannel extends AbstractNioByteChannel implements io.netty.channel.socket.SocketChannel {
...
public NioSocketChannel(Channel parent, SocketChannel socket) {
// 调用父类构造器
super(parent, socket);
// 创建NioSocketChannelConfig
config = new NioSocketChannelConfig(this, socket.socket());
}
...
}
父类 AbstractNioByteChannel 构造器：
public abstract class AbstractNioByteChannel extends AbstractNioChannel {
...
protected AbstractNioByteChannel(Channel parent, SelectableChannel ch) {
// 调用父类构造器，并设置兴趣集为SelectionKey.OP_READ，对read事件感兴趣
super(parent, ch, SelectionKey.OP_READ);
}
...
}
父类 AbstractNioChannel 构造器：
public abstract class AbstractNioChannel extends AbstractChannel {
...
protected AbstractNioChannel(Channel parent, SelectableChannel ch, int readInterestOp) {
// 调用父类构造器
super(parent);
// 设置channel
this.ch = ch;
// 设置兴趣集
this.readInterestOp = readInterestOp;
try {
// 设置为非阻塞
ch.configureBlocking(false);
} catch (IOException e) {
...
}
}
}
父类 AbstractChannel 构造器：
public abstract class AbstractChannel extends DefaultAttributeMap implements Channel {
...
protected AbstractChannel(Channel parent) {
// 设置parent
this.parent = parent;
// 创建channelId
id = newId();
// 创建unsafe
unsafe = newUnsafe();
// 创建pipeline
pipeline = newChannelPipeline();
}
...
}
ChannelConfig创建
接着我们看看 NioSocketChannelConfig 的创建逻辑：
public class NioSocketChannel extends AbstractNioByteChannel implements io.netty.channel.socket.SocketChannel {
...
private NioSocketChannelConfig(NioSocketChannel channel, Socket javaSocket) {
// 调用父类构造器
super(channel, javaSocket);
calculateMaxBytesPerGatheringWrite();
}
...
}
父类 DefaultSocketChannelConfig 构造器：
public class DefaultSocketChannelConfig extends DefaultChannelConfig implements SocketChannelConfig {
...
public DefaultSocketChannelConfig(SocketChannel channel, Socket javaSocket) {
// 调用父类构造器，绑定socketchannel
super(channel);
if (javaSocket == null) {
throw new NullPointerException("javaSocket");
}
// 绑定java socket
this.javaSocket = javaSocket;
// Enable TCP_NODELAY by default if possible.
// netty一般运行在服务器上，不在Android上，canEnableTcpNoDelayByDefault返回true
if (PlatformDependent.canEnableTcpNoDelayByDefault()) {
try {
// 开启 TCP_NODELAY ，开启TCP的nagle算法
// 尽量不要等待，只要发送缓冲区中有数据，并且发送窗口是打开的，就尽量把数据发送到网络上去。
setTcpNoDelay(true);
} catch (Exception e) {
// Ignore.
}
}
}
...
}
NioSocketChannel初始化与注册
上面小节分析了NioSocketChannel的创建逻辑，创建完成之后，我们来分析一下NioSocketChannel是如何注册到NioEventLoop上去的。
在前面小节分析新连接检测的有如下小段代码：
private final class NioMessageUnsafe extends AbstractNioUnsafe {
...
int size = readBuf.size();
for (int i = 0; i < size; i ++) {
readPending = false;
// 调用pipeline传播ChannelRead事件
pipeline.fireChannelRead(readBuf.get(i));
}
...
}
调用pipeline传播ChannelRead事件，这里的Pipeline是服务端Channel，也就是NioServerSocketChannel所绑定的Pipeline，此时的Pipeline的内部结构是怎么样子的呢？
![Pipeline-ServerBootstrapAcceptor]( "Pipeline-ServerBootstrapAcceptor")
那这个 ServerBootstrapAcceptor 是从哪里来的呢？
在此前，我们分析 NioServerSocketChannel初始化 时，有过下面这段代码：
public class ServerBootstrap extends AbstractBootstrap<ServerBootstrap, ServerChannel> {
...
// NioServerSocketChannel初始化
void init(Channel channel) throws Exception {
// 获取启动器 启动时配置的option参数，主要是TCP的一些属性
final Map<ChannelOption<?>, Object> options = options0();
// 将获得到 options 配置到 ChannelConfig 中去
synchronized (options) {
setChannelOptions(channel, options, logger);
}
// 获取 ServerBootstrap 启动时配置的 attr 参数
final Map<AttributeKey<?>, Object> attrs = attrs0();
// 配置 Channel attr，主要是设置用户自定义的一些参数
synchronized (attrs) {
for (Entry<AttributeKey<?>, Object> e: attrs.entrySet()) {
@SuppressWarnings("unchecked")
AttributeKey<Object> key = (AttributeKey<Object>) e.getKey();
channel.attr(key).set(e.getValue());
}
}
// 获取channel中的 pipeline，这个pipeline使我们前面在channel创建过程中设置的 pipeline
ChannelPipeline p = channel.pipeline();
// 将启动器中配置的 childGroup 保存到局部变量 currentChildGroup
final EventLoopGroup currentChildGroup = childGroup;
// 将启动器中配置的 childHandler 保存到局部变量 currentChildHandler
final ChannelHandler currentChildHandler = childHandler;
final Entry<ChannelOption<?>, Object>[] currentChildOptions;
final Entry<AttributeKey<?>, Object>[] currentChildAttrs;
// 保存用户设置的 childOptions 到局部变量 currentChildOptions
synchronized (childOptions) {
currentChildOptions = childOptions.entrySet().toArray(newOptionArray(childOptions.size()));
}
// 保存用户设置的 childAttrs 到局部变量 currentChildAttrs
synchronized (childAttrs) {
currentChildAttrs = childAttrs.entrySet().toArray(newAttrArray(childAttrs.size()));
}
p.addLast(new ChannelInitializer<Channel>() {
@Override
public void initChannel(final Channel ch) throws Exception {
final ChannelPipeline pipeline = ch.pipeline();
// 获取启动器上配置的handler
ChannelHandler handler = config.handler();
if (handler != null) {
// 添加 handler 到 pipeline 中
pipeline.addLast(handler);
}
ch.eventLoop().execute(new Runnable() {
@Override
public void run() {
// 用child相关的参数创建出一个新连接接入器ServerBootstrapAcceptor
// 通过 ServerBootstrapAcceptor 可以将一个新连接绑定到一个线程上去
// 每次有新的连接进来 ServerBootstrapAcceptor 都会用child相关的属性对它们进行配置，并注册到ChaildGroup上去
pipeline.addLast(new ServerBootstrapAcceptor(
ch, currentChildGroup, currentChildHandler, currentChildOptions, currentChildAttrs));
}
});
}
});
}
...
}
ServerBootstrapAcceptor
NioServerSocketChannel初始化时，向NioServerSocketChannel所绑定的Pipeline添加了一个InboundHandler节点 —— ServerBootstrapAcceptor ，其代码如下：
public class ServerBootstrap extends AbstractBootstrap<ServerBootstrap, ServerChannel> {
...
private static class ServerBootstrapAcceptor extends ChannelInboundHandlerAdapter {
// 子EventLoopGroup，即为workGroup
private final EventLoopGroup childGroup;
// ServerBootstrap启动时配置的 childHandler
private final ChannelHandler childHandler;
// ServerBootstrap启动时配置的 childOptions
private final Entry<ChannelOption<?>, Object>[] childOptions;
// ServerBootstrap启动时配置的 childAttrs
private final Entry<AttributeKey<?>, Object>[] childAttrs;
private final Runnable enableAutoReadTask;
// 构造函数
ServerBootstrapAcceptor(
final Channel channel, EventLoopGroup childGroup, ChannelHandler childHandler,
Entry<ChannelOption<?>, Object>[] childOptions, Entry<AttributeKey<?>, Object>[] childAttrs) {
this.childGroup = childGroup;
this.childHandler = childHandler;
this.childOptions = childOptions;
this.childAttrs = childAttrs;
// Task which is scheduled to re-enable auto-read.
// It's important to create this Runnable before we try to submit it as otherwise the URLClassLoader may
// not be able to load the class because of the file limit it already reached.
//
// See https://github.com/netty/netty/issues/1328
enableAutoReadTask = new Runnable() {
@Override
public void run() {
channel.config().setAutoRead(true);
}
};
}
// 处理Pipeline所传播的channelRead事件
// 也就是前面新连接检测时看到的那段代码
// pipeline.fireChannelRead(readBuf.get(i));
// ServerBootstrapAcceptor的channelRead接口将会被调用，用于处理channelRead事件
@Override
@SuppressWarnings("unchecked")
public void channelRead(ChannelHandlerContext ctx, Object msg) {
// 获取传播事件的对象数据，即为前面的readBuf.get(i)
// readBuf.get(i)取出的对象为 NioSocketChannel
final Channel child = (Channel) msg;
// 向 NioSocketChannel 添加childHandler，也就是我们常看到的
// ServerBootstrap在启动时配置的代码：
// ServerBootstrap.childHandler(new ChannelInitializer<SocketChannel>() {...} ）
// 最终的结果就是向NioSocketChannel的Pipeline添加用户自定义的ChannelHandler
// 用于处理客户端的channel连接
child.pipeline().addLast(childHandler);
// 配置 NioSocketChannel的TCP属性
setChannelOptions(child, childOptions, logger);
// 配置 NioSocketChannel 一些用户自定义数据
for (Entry<AttributeKey<?>, Object> e: childAttrs) {
child.attr((AttributeKey<Object>) e.getKey()).set(e.getValue());
}
// 将NioSocketChannel注册到childGroup，也就是Netty的WorkerGroup当中去
try {
childGroup.register(child).addListener(new ChannelFutureListener() {
@Override
public void operationComplete(ChannelFuture future) throws Exception {
if (!future.isSuccess()) {
forceClose(child, future.cause());
}
}
});
} catch (Throwable t) {
forceClose(child, t);
}
}
...
}
...
}
关于 ChannelInitializer 的讲解，可以看此前 Pipeline源码分析 文章。
后面的register逻辑，就与我们前面讲解 NioServerSocketChannel注册 大体类似了，这里简单介绍一下。
public abstract class AbstractChannel extends DefaultAttributeMap implements Channel {
...
// 注册NioSocketChannel
// eventLoop为childGroup
@Override
public final void register(EventLoop eventLoop, final ChannelPromise promise) {
...
// 绑定eventLoop到NioSocketChannel上
AbstractChannel.this.eventLoop = eventLoop;
// 现在分析的逻辑是在服务端的线程上，eventLoop与主线程不同，返回false
if (eventLoop.inEventLoop()) {
register0(promise);
} else {
try {
eventLoop.execute(new Runnable() {
@Override
public void run() {
// 这里来调用register0方法
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();
safeSetFailure(promise, t);
}
}
}
// 注册
private void register0(ChannelPromise promise) {
try {
...
boolean firstRegistration = neverRegistered;
// 调用 doRegister()
doRegister();
neverRegistered = false;
registered = true;
pipeline.invokeHandlerAddedIfNeeded();
safeSetSuccess(promise);
pipeline.fireChannelRegistered();
// 服务端的NioServerSocketChannel已经与客户端的NioSocketChannel建立了连接
// 所以，NioSocketChannel是处于激活状态，isActive()返回ture
if (isActive()) {
// 对于新连接，是第一次注册
if (firstRegistration) {
// 传播ChannelActive事件
pipeline.fireChannelActive();
} else if (config().isAutoRead()) {
beginRead();
}
}
...
} catch (Throwable t) {
// Close the channel directly to avoid FD leak.
closeForcibly();
closeFuture.setClosed();
safeSetFailure(promise, t);
}
}
...
}
调用到NioSocketChannel中的doRegister()方法：
public abstract class AbstractNioChannel extends AbstractChannel {
...
@Override
protected void doRegister() throws Exception {
boolean selected = false;
for (;;) {
try {
// 将selector注册到底层JDK channel上，并附加了NioSocketChannel对象
// 兴趣集设置为0，表示不关心任何事件
selectionKey = javaChannel().register(eventLoop().unwrappedSelector(), 0, this);
return;
} catch (CancelledKeyException e) {
...
}
}
}
...
}
NioSocketChannel 注册OP_READ兴趣集
紧接着上面的分析，传播ChannelActive事件之后的逻辑，主要就是向客户端的NioSocketChannel注册一个Read兴趣集
if (isActive()) {
// 对于新连接，是第一次注册
if (firstRegistration) {
// 传播ChannelActive事件
pipeline.fireChannelActive();
} else if (config().isAutoRead()) {
beginRead();
}
}
通过 Pipeline的传播机制 ，最终会调用到doBeginRead()接口，如下：
public abstract class AbstractNioChannel extends AbstractChannel {
...
protected abstract class AbstractNioUnsafe extends AbstractUnsafe implements NioUnsafe {
...
@Override
protected void doBeginRead() throws Exception {
// Channel.read() or ChannelHandlerContext.read() was called
// 保存selectionKey到局部变量
final SelectionKey selectionKey = this.selectionKey;
// 判断有效性
if (!selectionKey.isValid()) {
return;
}
readPending = true;
// 获取selectionKey的兴趣集
// 前面小结分析doRegister()接口提到，selectionKey的兴趣集设置为0
final int interestOps = selectionKey.interestOps();
// 这里的 readInterestOp 是前面讲NioSocketChannel创建时设置的值
// 为 SelectionKey.OP_READ，也就是1
if ((interestOps & readInterestOp) == 0) {
// 这样，selectionKey最终设置的兴趣集为SelectionKey.OP_READ
// 表示对读事件感兴趣
selectionKey.interestOps(interestOps | readInterestOp);
}
}
...
}
...
}
小结
- Netty是在哪里检测有新连接接入的？
- 新连接是怎样注册到NioEventLoop线程上的？
- NioSocketChannel是怎样初始化及注册的？
参考资料
