前面的一些章節，我們分析了Netty的三大元件 —— Channel 、EventLoop、Pipeline ，對Netty的工作原理有了深入的瞭解。在此基礎上，我們來分析一下當Netty服務端啟動後，Netty是如何處理新連線接入的。
本文內容主要分為以下四部分：
- 新連線檢測
- NioSocketChannel建立
- NioSocketChannel初始化與註冊
- NioSocketChannel註冊READ興趣集
新連線檢測
前面，我們在講 EventLoop的啟動過程原始碼分析 時，解讀過下面這段程式碼：
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) {
// 讀取read事件
unsafe.read();
}
} catch (CancelledKeyException ignored) {
unsafe.close(unsafe.voidPromise());
}
...
}
...
}
複製程式碼我們還是以服務端 NioServerSocketChannel 為例，它繫結的unsafe例項為 NioMessageUnsafe 。上面的 unsafe.read() 介面，會向下呼叫到 NioMessageUnsafe.read() 介面，如下：
public abstract class AbstractNioMessageChannel extends AbstractNioChannel {
...
private final class NioMessageUnsafe extends AbstractNioUnsafe {
// 用於儲存新建立的 NioSocketChannel 的集合
private final List<Object> readBuf = new ArrayList<Object>();
@Override
public void read() {
// 確保在當前執行緒與EventLoop中的一致
assert eventLoop().inEventLoop();
// 獲取 NioServerSocketChannel config配置
final ChannelConfig config = config();
// 獲取 NioServerSocketChannel 繫結的 pipeline
final ChannelPipeline pipeline = pipeline();
// 獲取RecvByteBuf 分配器 Handle
// 當channel在接收資料時，allocHandle 會用於分配ByteBuf來儲存資料
// 關於allocHandle後面再去做詳細介紹
final RecvByteBufAllocator.Handle allocHandle = unsafe().recvBufAllocHandle();
// 重置已累積的所有計數器，併為下一個讀取迴圈讀取多少訊息/位元組資料提供建議
allocHandle.reset(config);
boolean closed = false;
Throwable exception = null;
try {
try {
do {
// 呼叫後面的 doReadMessages 介面，讀取到message則返回1
int localRead = doReadMessages(readBuf);
if (localRead == 0) {
break;
}
if (localRead < 0) {
closed = true;
break;
}
// 對當前read迴圈所讀取到的message數量計數+1
allocHandle.incMessagesRead(localRead);
// 判斷是否繼續讀取message
} while (allocHandle.continueReading());
} catch (Throwable t) {
exception = t;
}
int size = readBuf.size();
for (int i = 0; i < size; i ++) {
readPending = false;
// 呼叫pipeline傳播ChannelRead事件
pipeline.fireChannelRead(readBuf.get(i));
}
// 清空readBuf
readBuf.clear();
allocHandle.readComplete();
// 呼叫pipeline傳播 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();
}
}
}
}
...
}
複製程式碼對於 doReadMessages(...) 的分析：
public class NioServerSocketChannel extends AbstractNioMessageChannel implements io.netty.channel.socket.ServerSocketChannel {
...
// 讀取訊息
@Override
protected int doReadMessages(List<Object> buf) throws Exception {
// 獲取 SocketChannel
SocketChannel ch = SocketUtils.accept(javaChannel());
try {
if (ch != null) {
// 使用SocketChannel建立NioSocketChannel，將其存入buf list中
// 關於NioSocketChannel的建立請看後面的分析
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;
}
...
}
複製程式碼對於 continueReading() 介面的分析，至於結果為什麼返回false，後面會單獨分析：
public abstract class DefaultMaxMessagesRecvByteBufAllocator implements MaxMessagesRecvByteBufAllocator {
private volatile int maxMessagesPerRead;
private volatile boolean respectMaybeMoreData = true;
...
public abstract class MaxMessageHandle implements ExtendedHandle {
private ChannelConfig config;
// 每次讀取最大的訊息數
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;
}
};
...
// 判斷是否繼續讀取message
@Override
public boolean continueReading() {
return continueReading(defaultMaybeMoreSupplier);
}
// 判斷是否繼續讀取message
@Override
public boolean continueReading(UncheckedBooleanSupplier maybeMoreDataSupplier) {
// 預設情況下 config.isAutoRead() 為true
// respectMaybeMoreData 預設為 true
// maybeMoreDataSupplier.get() 為false
// totalMessages第一次迴圈則為1
// maxMessagePerRead為16
// 結果返回false
return config.isAutoRead() &&
(!respectMaybeMoreData || maybeMoreDataSupplier.get()) &&
totalMessages < maxMessagePerRead &&
totalBytesRead > 0;
}
...
}
...
}
複製程式碼NioSocketChannel建立
上面分析新連線接入，提到了 NioSocketChannel 的建立，我們這裡來詳細分析一下，NioSocketChannel的建立過程與此前我們分析 NioServerSocketChannel建立 大體類似。
構造器
先來看看 NioSocketChannel 的建構函式：
public class NioSocketChannel extends AbstractNioByteChannel implements io.netty.channel.socket.SocketChannel {
...
public NioSocketChannel(Channel parent, SocketChannel socket) {
// 呼叫父類構造器
super(parent, socket);
// 建立NioSocketChannelConfig
config = new NioSocketChannelConfig(this, socket.socket());
}
...
}
複製程式碼父類 AbstractNioByteChannel 構造器：
public abstract class AbstractNioByteChannel extends AbstractNioChannel {
...
protected AbstractNioByteChannel(Channel parent, SelectableChannel ch) {
// 呼叫父類構造器，並設定興趣集為SelectionKey.OP_READ，對read事件感興趣
super(parent, ch, SelectionKey.OP_READ);
}
...
}
複製程式碼父類 AbstractNioChannel 構造器：
public abstract class AbstractNioChannel extends AbstractChannel {
...
protected AbstractNioChannel(Channel parent, SelectableChannel ch, int readInterestOp) {
// 呼叫父類構造器
super(parent);
// 設定channel
this.ch = ch;
// 設定興趣集
this.readInterestOp = readInterestOp;
try {
// 設定為非阻塞
ch.configureBlocking(false);
} catch (IOException e) {
...
}
}
}
複製程式碼父類 AbstractChannel 構造器：
public abstract class AbstractChannel extends DefaultAttributeMap implements Channel {
...
protected AbstractChannel(Channel parent) {
// 設定parent
this.parent = parent;
// 建立channelId
id = newId();
// 建立unsafe
unsafe = newUnsafe();
// 建立pipeline
pipeline = newChannelPipeline();
}
...
}
複製程式碼ChannelConfig建立
接著我們看看 NioSocketChannelConfig 的建立邏輯：
public class NioSocketChannel extends AbstractNioByteChannel implements io.netty.channel.socket.SocketChannel {
...
private NioSocketChannelConfig(NioSocketChannel channel, Socket javaSocket) {
// 呼叫父類構造器
super(channel, javaSocket);
calculateMaxBytesPerGatheringWrite();
}
...
}
複製程式碼父類 DefaultSocketChannelConfig 構造器：
public class DefaultSocketChannelConfig extends DefaultChannelConfig implements SocketChannelConfig {
...
public DefaultSocketChannelConfig(SocketChannel channel, Socket javaSocket) {
// 呼叫父類構造器，繫結socketchannel
super(channel);
if (javaSocket == null) {
throw new NullPointerException("javaSocket");
}
// 繫結java socket
this.javaSocket = javaSocket;
// Enable TCP_NODELAY by default if possible.
// netty一般執行在伺服器上，不在Android上，canEnableTcpNoDelayByDefault返回true
if (PlatformDependent.canEnableTcpNoDelayByDefault()) {
try {
// 開啟 TCP_NODELAY ，開啟TCP的nagle演算法
// 儘量不要等待，只要傳送緩衝區中有資料，並且傳送視窗是開啟的，就儘量把資料傳送到網路上去。
setTcpNoDelay(true);
} catch (Exception e) {
// Ignore.
}
}
}
...
}
複製程式碼NioSocketChannel初始化與註冊
上面小節分析了NioSocketChannel的建立邏輯，建立完成之後，我們來分析一下NioSocketChannel是如何註冊到NioEventLoop上去的。
在前面小節分析新連線檢測的有如下小段程式碼：
private final class NioMessageUnsafe extends AbstractNioUnsafe {
...
int size = readBuf.size();
for (int i = 0; i < size; i ++) {
readPending = false;
// 呼叫pipeline傳播ChannelRead事件
pipeline.fireChannelRead(readBuf.get(i));
}
...
}
複製程式碼呼叫pipeline傳播ChannelRead事件，這裡的Pipeline是服務端Channel，也就是NioServerSocketChannel所繫結的Pipeline，此時的Pipeline的內部結構是怎麼樣子的呢？
那這個 ServerBootstrapAcceptor 是從哪裡來的呢？
在此前，我們分析 NioServerSocketChannel初始化 時，有過下面這段程式碼：
public class ServerBootstrap extends AbstractBootstrap<ServerBootstrap, ServerChannel> {
...
// NioServerSocketChannel初始化
void init(Channel channel) throws Exception {
// 獲取啟動器 啟動時配置的option引數，主要是TCP的一些屬性
final Map<ChannelOption<?>, Object> options = options0();
// 將獲得到 options 配置到 ChannelConfig 中去
synchronized (options) {
setChannelOptions(channel, options, logger);
}
// 獲取 ServerBootstrap 啟動時配置的 attr 引數
final Map<AttributeKey<?>, Object> attrs = attrs0();
// 配置 Channel attr，主要是設定使用者自定義的一些引數
synchronized (attrs) {
for (Entry<AttributeKey<?>, Object> e: attrs.entrySet()) {
@SuppressWarnings("unchecked")
AttributeKey<Object> key = (AttributeKey<Object>) e.getKey();
channel.attr(key).set(e.getValue());
}
}
// 獲取channel中的 pipeline，這個pipeline使我們前面在channel建立過程中設定的 pipeline
ChannelPipeline p = channel.pipeline();
// 將啟動器中配置的 childGroup 儲存到區域性變數 currentChildGroup
final EventLoopGroup currentChildGroup = childGroup;
// 將啟動器中配置的 childHandler 儲存到區域性變數 currentChildHandler
final ChannelHandler currentChildHandler = childHandler;
final Entry<ChannelOption<?>, Object>[] currentChildOptions;
final Entry<AttributeKey<?>, Object>[] currentChildAttrs;
// 儲存使用者設定的 childOptions 到區域性變數 currentChildOptions
synchronized (childOptions) {
currentChildOptions = childOptions.entrySet().toArray(newOptionArray(childOptions.size()));
}
// 儲存使用者設定的 childAttrs 到區域性變數 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();
// 獲取啟動器上配置的handler
ChannelHandler handler = config.handler();
if (handler != null) {
// 新增 handler 到 pipeline 中
pipeline.addLast(handler);
}
ch.eventLoop().execute(new Runnable() {
@Override
public void run() {
// 用child相關的引數建立出一個新連線接入器ServerBootstrapAcceptor
// 通過 ServerBootstrapAcceptor 可以將一個新連線繫結到一個執行緒上去
// 每次有新的連線進來 ServerBootstrapAcceptor 都會用child相關的屬性對它們進行配置，並註冊到ChaildGroup上去
pipeline.addLast(new ServerBootstrapAcceptor(
ch, currentChildGroup, currentChildHandler, currentChildOptions, currentChildAttrs));
}
});
}
});
}
...
}
複製程式碼ServerBootstrapAcceptor
NioServerSocketChannel初始化時，向NioServerSocketChannel所繫結的Pipeline新增了一個InboundHandler節點 —— ServerBootstrapAcceptor ，其程式碼如下：
public class ServerBootstrap extends AbstractBootstrap<ServerBootstrap, ServerChannel> {
...
private static class ServerBootstrapAcceptor extends ChannelInboundHandlerAdapter {
// 子EventLoopGroup，即為workGroup
private final EventLoopGroup childGroup;
// ServerBootstrap啟動時配置的 childHandler
private final ChannelHandler childHandler;
// ServerBootstrap啟動時配置的 childOptions
private final Entry<ChannelOption<?>, Object>[] childOptions;
// ServerBootstrap啟動時配置的 childAttrs
private final Entry<AttributeKey<?>, Object>[] childAttrs;
private final Runnable enableAutoReadTask;
// 建構函式
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);
}
};
}
// 處理Pipeline所傳播的channelRead事件
// 也就是前面新連線檢測時看到的那段程式碼
// pipeline.fireChannelRead(readBuf.get(i));
// ServerBootstrapAcceptor的channelRead介面將會被呼叫，用於處理channelRead事件
@Override
@SuppressWarnings("unchecked")
public void channelRead(ChannelHandlerContext ctx, Object msg) {
// 獲取傳播事件的物件資料，即為前面的readBuf.get(i)
// readBuf.get(i)取出的物件為 NioSocketChannel
final Channel child = (Channel) msg;
// 向 NioSocketChannel 新增childHandler，也就是我們常看到的
// ServerBootstrap在啟動時配置的程式碼：
// ServerBootstrap.childHandler(new ChannelInitializer<SocketChannel>() {...} ）
// 最終的結果就是向NioSocketChannel的Pipeline新增使用者自定義的ChannelHandler
// 用於處理客戶端的channel連線
child.pipeline().addLast(childHandler);
// 配置 NioSocketChannel的TCP屬性
setChannelOptions(child, childOptions, logger);
// 配置 NioSocketChannel 一些使用者自定義資料
for (Entry<AttributeKey<?>, Object> e: childAttrs) {
child.attr((AttributeKey<Object>) e.getKey()).set(e.getValue());
}
// 將NioSocketChannel註冊到childGroup，也就是Netty的WorkerGroup當中去
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);
}
}
...
}
...
}
複製程式碼關於 ChannelInitializer 的講解，可以看此前 Pipeline原始碼分析 文章。
後面的register邏輯，就與我們前面講解 NioServerSocketChannel註冊 大體類似了，這裡簡單介紹一下。
public abstract class AbstractChannel extends DefaultAttributeMap implements Channel {
...
// 註冊NioSocketChannel
// eventLoop為childGroup
@Override
public final void register(EventLoop eventLoop, final ChannelPromise promise) {
...
// 繫結eventLoop到NioSocketChannel上
AbstractChannel.this.eventLoop = eventLoop;
// 現在分析的邏輯是在服務端的執行緒上，eventLoop與主執行緒不同，返回false
if (eventLoop.inEventLoop()) {
register0(promise);
} else {
try {
eventLoop.execute(new Runnable() {
@Override
public void run() {
// 這裡來呼叫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);
}
}
}
// 註冊
private void register0(ChannelPromise promise) {
try {
...
boolean firstRegistration = neverRegistered;
// 呼叫 doRegister()
doRegister();
neverRegistered = false;
registered = true;
pipeline.invokeHandlerAddedIfNeeded();
safeSetSuccess(promise);
pipeline.fireChannelRegistered();
// 服務端的NioServerSocketChannel已經與客戶端的NioSocketChannel建立了連線
// 所以，NioSocketChannel是處於啟用狀態，isActive()返回ture
if (isActive()) {
// 對於新連線，是第一次註冊
if (firstRegistration) {
// 傳播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);
}
}
...
}
複製程式碼呼叫到NioSocketChannel中的doRegister()方法：
public abstract class AbstractNioChannel extends AbstractChannel {
...
@Override
protected void doRegister() throws Exception {
boolean selected = false;
for (;;) {
try {
// 將selector註冊到底層JDK channel上，並附加了NioSocketChannel物件
// 興趣集設定為0，表示不關心任何事件
selectionKey = javaChannel().register(eventLoop().unwrappedSelector(), 0, this);
return;
} catch (CancelledKeyException e) {
...
}
}
}
...
}
複製程式碼NioSocketChannel 註冊OP_READ興趣集
緊接著上面的分析，傳播ChannelActive事件之後的邏輯，主要就是向客戶端的NioSocketChannel註冊一個Read興趣集
if (isActive()) {
// 對於新連線，是第一次註冊
if (firstRegistration) {
// 傳播ChannelActive事件
pipeline.fireChannelActive();
} else if (config().isAutoRead()) {
beginRead();
}
}
複製程式碼通過 Pipeline的傳播機制 ，最終會呼叫到doBeginRead()介面，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
// 儲存selectionKey到區域性變數
final SelectionKey selectionKey = this.selectionKey;
// 判斷有效性
if (!selectionKey.isValid()) {
return;
}
readPending = true;
// 獲取selectionKey的興趣集
// 前面小結分析doRegister()介面提到，selectionKey的興趣集設定為0
final int interestOps = selectionKey.interestOps();
// 這裡的 readInterestOp 是前面講NioSocketChannel建立時設定的值
// 為 SelectionKey.OP_READ，也就是1
if ((interestOps & readInterestOp) == 0) {
// 這樣，selectionKey最終設定的興趣集為SelectionKey.OP_READ
// 表示對讀事件感興趣
selectionKey.interestOps(interestOps | readInterestOp);
}
}
...
}
...
}
複製程式碼小結
- Netty是在哪裡檢測有新連線接入的？
- 新連線是怎樣註冊到NioEventLoop執行緒上的？