实例
样例代码来自于io.netty.example.telnet.TelnetClient，完整样例请参考NettyExample工程。
客户端和服务端比较相似，所以本篇会在一定程度上略去重复的部分，以减少篇幅。
public void run() throws Exception {
EventLoopGroup group = new NioEventLoopGroup();
try {
Bootstrap b = new Bootstrap();
b.group(group)
.channel(NioSocketChannel.class)
.handler(new TelnetClientInitializer());
// Start the connection attempt.
Channel ch = b.connect(host, port).sync().channel();
// Read commands from the stdin.
ChannelFuture lastWriteFuture = null;
BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
for (;;) {
String line = in.readLine();
if (line == null) {
break;
}
// Sends the received line to the server.
lastWriteFuture = ch.writeAndFlush(line + "\r\n");
// If user typed the 'bye' command, wait until the server closes
// the connection.
if ("bye".equals(line.toLowerCase())) {
ch.closeFuture().sync();
break;
}
}
// Wait until all messages are flushed before closing the channel.
if (lastWriteFuture != null) {
lastWriteFuture.sync();
}
} finally {
group.shutdownGracefully();
}
}
客户端启动
Bootstrap b = new Bootstrap(); //tag0
b.group(group) //tag1
.channel(NioSocketChannel.class) //tag2
.handler(new TelnetClientInitializer());//tag3
tag0代码主要初始化了父类的 options和attrs 属性；代码略。
tag1设置了group属性
@SuppressWarnings("unchecked")
public B group(EventLoopGroup group) {
if (group == null) {
throw new NullPointerException("group");
}
if (this.group != null) {
throw new IllegalStateException("group set already");
}
this.group = group;
return (B) this;
}
tag2设置了channelFactory属性
public Bootstrap channel(Class<? extends Channel> channelClass) {
if (channelClass == null) {
throw new NullPointerException("channelClass");
}
return channelFactory(new BootstrapChannelFactory<Channel>(channelClass));
}
tag3设置了handler属性
public B handler(ChannelHandler handler) { if (handler == null) { throw new NullPointerException("handler"); } this.handler = handler; return (B) this; }
下面开始第二段代码分析，依次执行下面的方法。
Channel ch = b.connect(host, port) //tag4
.sync().channel(); //tag5
public ChannelFuture connect(String inetHost, int inetPort) {
return connect(new InetSocketAddress(inetHost, inetPort));
}
public ChannelFuture connect(SocketAddress remoteAddress) {
if (remoteAddress == null) {
throw new NullPointerException("remoteAddress");
}
validate();
return doConnect(remoteAddress, localAddress());
}
private ChannelFuture doConnect(final SocketAddress remoteAddress, final SocketAddress localAddress) {
final ChannelFuture regFuture = initAndRegister();//tag4.1
final Channel channel = regFuture.channel();
if (regFuture.cause() != null) {
return regFuture;
}
final ChannelPromise promise = channel.newPromise();
if (regFuture.isDone()) {
doConnect0(regFuture, channel, remoteAddress, localAddress, promise);//tag4.2
} else {
regFuture.addListener(new ChannelFutureListener() {
@Override
public void operationComplete(ChannelFuture future) throws Exception {
doConnect0(regFuture, channel, remoteAddress, localAddress, promise);
}
});
}
return promise;
}
分析tag4.1代码，细心的读者注意到，这些和服务端的代码执行过程是一样的。运用模板模式，子类定义独特的实现。
final ChannelFuture AbstractBootstrap.initAndRegister() {
Channel channel;
try {
channel = createChannel();//tag4.1.1
} catch (Throwable t) {
return VoidChannel.INSTANCE.newFailedFuture(t);
}
try {
init(channel);//tag4.1.2
} catch (Throwable t) {
channel.unsafe().closeForcibly();
return channel.newFailedFuture(t);
}
ChannelPromise regFuture = channel.newPromise();
channel.unsafe().register(regFuture);//tag4.1.3
if (regFuture.cause() != null) {
if (channel.isRegistered()) {
channel.close();
} else {
channel.unsafe().closeForcibly();
}
}
分析 tag4.1.1，里面通过反射来实例化NioSocketChannel
@Override
Channel createChannel() {
EventLoop eventLoop = group().next();
return channelFactory().newChannel(eventLoop);//tag4.1.1.1
}
public NioSocketChannel(EventLoop eventLoop) {
this(eventLoop, newSocket());//调用下面的newSocket()方法
}
private static SocketChannel newSocket() {
try {
return SocketChannel.open();
} catch (IOException e) {
throw new ChannelException("Failed to open a socket.", e);
}
}
public NioSocketChannel(EventLoop eventLoop, SocketChannel socket) {
this(null, eventLoop, socket);
}
protected AbstractNioByteChannel(Channel parent, EventLoop eventLoop, SelectableChannel ch) {
super(parent, eventLoop, ch, SelectionKey.OP_READ);//调用父类方法
}
protected AbstractNioChannel(Channel parent, EventLoop eventLoop, SelectableChannel ch, int readInterestOp) {
super(parent, eventLoop);//调用父类方法,tag4.1.1.1
this.ch = ch;
this.readInterestOp = readInterestOp;
try {
ch.configureBlocking(false);//tag4.1.1.2
} catch (IOException e) {
try {
ch.close();
} catch (IOException e2) {
if (logger.isWarnEnabled()) {
logger.warn(
"Failed to close a partially initialized socket.", e2);
}
}
throw new ChannelException("Failed to enter non-blocking mode.", e);
}
}
protected AbstractChannel(Channel parent, EventLoop eventLoop) {
this.parent = parent;
this.eventLoop = validate(eventLoop);
unsafe = newUnsafe();
pipeline = new DefaultChannelPipeline(this);//tag4.1.1.1.1
}
分析tag4.1.1.1.1，里面调用DefaultChannelPipeline构造器，和服务端的逻辑一样，故不作分析。
此时系统返回到tag4.1.1.2 继续执行ch.configureBlocking(false);，此时完成tag4.1.1 方法执行，开始执行tag4.1.2方法
@Override
@SuppressWarnings("unchecked")
void init(Channel channel) throws Exception {
ChannelPipeline p = channel.pipeline();
p.addLast(handler());//tag4.1.2.1
final Map<ChannelOption<?>, Object> options = options();
synchronized (options) {
for (Entry<ChannelOption<?>, Object> e: options.entrySet()) {
try {
if (!channel.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: " + channel, t);
}
}
}
final Map<AttributeKey<?>, Object> attrs = attrs();
synchronized (attrs) {
for (Entry<AttributeKey<?>, Object> e: attrs.entrySet()) {
channel.attr((AttributeKey<Object>) e.getKey()).set(e.getValue());
}
}
}
分析tag4.1.2.1，里面将这个TelnetClientInitializer Handler加入到pipeline中，此时handler链是HeadHandler,TelnetClientInitializer,TailHandler,共计3个。
此时程序返回到tag4.1.3继续执行，
@Override
public final void AbstractChannel.register(final ChannelPromise promise) {
if (eventLoop.inEventLoop()) {
register0(promise);
} else {
try {
eventLoop.execute(new Runnable() {
@Override
public void run() {
register0(promise);//tag4.1.3.1
}
});
} 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 AbstractChannel.AbstractUnsafe.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;
}
doRegister();//tag4.1.3.1.1
registered = true;
promise.setSuccess();
pipeline.fireChannelRegistered();//tag4.1.3.1.2
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);
}
}
}
tag4.1.3.1.1 代码如下
@Override
protected void AbstractNioChannel.doRegister() throws Exception {
boolean selected = false;
for (;;) {
try {
selectionKey = javaChannel().register(eventLoop().selector, 0, this);//tag4.1.3.1.2.1
return;
} catch (CancelledKeyException e) {
if (!selected) {
// Force the Selector to select now as the "canceled" SelectionKey may still be
// cached and not removed because no Select.select(..) operation was called yet.
eventLoop().selectNow();
selected = true;
} else {
// We forced a select operation on the selector before but the SelectionKey is still cached
// for whatever reason. JDK bug ?
throw e;
}
}
}
}
tag4.1.3.1.2.1 把selector注册到javaChannel上；然后程序继续执行tag4.1.3.1.2代码。
@Override
public ChannelPipeline fireChannelRegistered() {
head.fireChannelRegistered();
return this;
}
@Override
public ChannelHandlerContext fireChannelRegistered() {
DefaultChannelHandlerContext next = findContextInbound(MASK_CHANNEL_REGISTERED);
next.invoker.invokeChannelRegistered(next);
return this;
}
@Override
@SuppressWarnings("unchecked")
public final void ChannelInitializer.channelRegistered(ChannelHandlerContext ctx) throws Exception {
ChannelPipeline pipeline = ctx.pipeline();
boolean success = false;
try {
initChannel((C) ctx.channel());//tag4.1.3.1.2.1
pipeline.remove(this);//tag4.1.3.1.2.2
ctx.fireChannelRegistered();//tag4.1.3.1.2.3
success = true;
} catch (Throwable t) {
logger.warn("Failed to initialize a channel. Closing: " + ctx.channel(), t);
} finally {
if (pipeline.context(this) != null) {
pipeline.remove(this);
}
if (!success) {
ctx.close();
}
}
}
后面的逻辑和服务端类似，此时执行的handler是TelnetClientInitializer，并执行ChannelInitializer的channelRegistered方法，channelRegistered方法里面接着调用了initChannel。
标记 tag4.1.3.1.2.1 代码如下
@Override
public void TelnetClientInitializer.initChannel(SocketChannel ch) throws Exception {
ChannelPipeline pipeline = ch.pipeline();
// Add the text line codec combination first,
pipeline.addLast("framer", new DelimiterBasedFrameDecoder(
8192, Delimiters.lineDelimiter()));
pipeline.addLast("decoder", DECODER);
pipeline.addLast("encoder", ENCODER);
// and then business logic.
pipeline.addLast("handler", CLIENTHANDLER);
}
在完成tag4.1.3.1.2.2的 pipeline.remove(this);后，此时handler链如下：HeadHandler，DelimiterBasedFrameDecoder，StringDecoder，StringEncoder，TelnetClientHandler， TailHandler。
接着程序又开始执行下一个handler，最终找到TailHandler的channelRegistered方法。TailHandler的channelRegistered方法是空方法。
此时 tag4.1 的代码执行结束，开始执行 tag4.2的代码
private static void doConnect0( final ChannelFuture regFuture, final Channel channel, final SocketAddress remoteAddress, final SocketAddress localAddress, final ChannelPromise promise) {
// This method is invoked before channelRegistered() is triggered. Give user handlers a chance to set up
// the pipeline in its channelRegistered() implementation.
channel.eventLoop().execute(new Runnable() {
@Override
public void run() {
if (regFuture.isSuccess()) {
if (localAddress == null) {
channel.connect(remoteAddress, promise);//tag4.2.1
} else {
channel.connect(remoteAddress, localAddress, promise);
}
promise.addListener(ChannelFutureListener.CLOSE_ON_FAILURE);
} else {
promise.setFailure(regFuture.cause());
}
}
});
}
@Override
public ChannelFuture AbstractChannel.connect(SocketAddress remoteAddress, ChannelPromise promise) {
return pipeline.connect(remoteAddress, promise);
}
经过一番计算，找到HeadHandler，执行unsafe的方法。
@Override
public void HeadHandler.connect(
ChannelHandlerContext ctx,
SocketAddress remoteAddress, SocketAddress localAddress,
ChannelPromise promise) throws Exception {
unsafe.connect(remoteAddress, localAddress, promise);
}
AbstractNioChannel.AbstractNioUnsafe的 connect方法如下：
@Override
public void connect(
final SocketAddress remoteAddress, final SocketAddress localAddress, final ChannelPromise promise) {
if (!ensureOpen(promise)) {
return;
}
try {
if (connectPromise != null) {
throw new IllegalStateException("connection attempt already made");
}
boolean wasActive = isActive();
if (doConnect(remoteAddress, localAddress)) {//tag4.2.1.1
fulfillConnectPromise(promise, wasActive);//tag4.2.1.2
} else {
connectPromise = promise;
requestedRemoteAddress = remoteAddress;
// Schedule connect timeout.
int connectTimeoutMillis = config().getConnectTimeoutMillis();
if (connectTimeoutMillis > 0) {
connectTimeoutFuture = eventLoop().schedule(new Runnable() {
@Override
public void run() {//tag4.2.1.3
ChannelPromise connectPromise = AbstractNioChannel.this.connectPromise;
ConnectTimeoutException cause =
new ConnectTimeoutException("connection timed out: " + remoteAddress);
if (connectPromise != null && connectPromise.tryFailure(cause)) {
close(voidPromise());
}
}
}, connectTimeoutMillis, TimeUnit.MILLISECONDS);
}
promise.addListener(new ChannelFutureListener() {
@Override
public void operationComplete(ChannelFuture future) throws Exception {
if (future.isCancelled()) {
if (connectTimeoutFuture != null) {
connectTimeoutFuture.cancel(false);
}
connectPromise = null;
close(voidPromise());
}
}
});
}
} catch (Throwable t) {
if (t instanceof ConnectException) {
Throwable newT = new ConnectException(t.getMessage() + ": " + remoteAddress);
newT.setStackTrace(t.getStackTrace());
t = newT;
}
promise.tryFailure(t);
closeIfClosed();
}
}
tag4.2.1.1 代码如下，进行了bind本地端口和connect远程服务器的操作。
@Override
protected boolean doConnect(SocketAddress remoteAddress, SocketAddress localAddress) throws Exception {
if (localAddress != null) {
javaChannel().socket().bind(localAddress);
}
boolean success = false;
try {
boolean connected = javaChannel().connect(remoteAddress);//tag4.2.1.1.1
if (!connected) {
selectionKey().interestOps(SelectionKey.OP_CONNECT);//tag4.2.1.1.2
}
success = true;
return connected;
} finally {
if (!success) {
doClose();
}
}
}
tag4.2.1.1.2 里面执行了connect远程服务器的操作，，我的机器上该方法返回false（返回值详见connect方法说明）。然后会触发执行selectionKey().interestOps(SelectionKey.OP_CONNECT);
需要额外说明的是，此时服务器会触发channelActivi事件。在本例的服务端代码里，会在客户端连接时，发送消息给客户端。不过先暂时忽略服务端和客户端的数据交互，下文分析。
然后tag4.2.1.1 执行结束，由于此时的返回值是false，所以不会执行tag4.2.1.2的 fulfillConnectPromise(promise, wasActive);
然后程序继续执行tag4.2.1.3 代码，进行连接超时处理：如果设置了超时时间，那么等待指定的超时时间后，再看看是否已经连接上。如果连不上，则设置失败状态。
接着开始下一个事件循环，由于在tag4.2.1.1.2执行了selectionKey().interestOps(SelectionKey.OP_CONNECT)操作，会进入到下面的代码。这里我们重点关注tag4.3的代码。
private static void NioEventLoop.processSelectedKey(SelectionKey k, AbstractNioChannel ch) {
//略XXXX
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();//tag4.3
//略XXXX
}
@Override
public void finishConnect() {
// Note this method is invoked by the event loop only if the connection attempt was
// neither cancelled nor timed out.
assert eventLoop().inEventLoop();
assert connectPromise != null;
try {
boolean wasActive = isActive();
doFinishConnect();//tag4.3.1
fulfillConnectPromise(connectPromise, wasActive);//tag4.3.2
} catch (Throwable t) {
if (t instanceof ConnectException) {
Throwable newT = new ConnectException(t.getMessage() + ": " + requestedRemoteAddress);
newT.setStackTrace(t.getStackTrace());
t = newT;
}
// Use tryFailure() instead of setFailure() to avoid the race against cancel().
connectPromise.tryFailure(t);
closeIfClosed();
} finally {
// Check for null as the connectTimeoutFuture is only created if a connectTimeoutMillis > 0 is used
// See https://github.com/netty/netty/issues/1770
if (connectTimeoutFuture != null) {
connectTimeoutFuture.cancel(false);
}
connectPromise = null;
}
}
@Override
protected void NioSocketChannel.doFinishConnect() throws Exception {
if (!javaChannel().finishConnect()) {
throw new Error();
}
}
在执行完下面的boolean promiseSet = promise.trySuccess(); 方法后，实例代码中的 Channel ch = b.connect(host, port).sync().channel();就执行完毕了，然后主线程就阻塞在实例代码中的 String line = in.readLine();这句代码里了。
private void AbstractNioChannel.AbstractNioUnsafe.fulfillConnectPromise(ChannelPromise promise, boolean wasActive) {
// trySuccess() will return false if a user cancelled the connection attempt.
boolean promiseSet = promise.trySuccess();
// Regardless if the connection attempt was cancelled, channelActive() event should be triggered,
// because what happened is what happened.
if (!wasActive && isActive()) {
pipeline().fireChannelActive();//tag4.3.2.1
}
// If a user cancelled the connection attempt, close the channel, which is followed by channelInactive().
if (!promiseSet) {
close(voidPromise());
}
}
@Override
public ChannelPipeline fireChannelActive() {
head.fireChannelActive();//tag4.3.2.1.1
if (channel.config().isAutoRead()) {
channel.read();//tag4.3.2.1.2
}
return this;
}
此时，继续执行tag4.3.2.1的代码，进而执行tag4.3.2.1.1的代码，最终执行TailHandler.channelActive方法。由于TailHandler类内部的方法基本都是空实现，所以不再贴代码了。然后再执行tag4.3.2.1.2的channel.read();代码，最终执行了AbstractNioChannel.doBeginRead()方法的selectionKey.interestOps(interestOps | readInterestOp);，等同于执行了selectionKey.interestOps(SelectionKey.OP_READ);。
此时方法返回，在NioEventLoop.run()经过了一些简单的数据清理后，然后有机会对服务端的channelActive时发送的数据进行处理了(在tag4.2.1.1.2曾经提过)。客户端和服务端交互过程详见下篇。
来源：oschina
链接：https://my.oschina.net/u/1409288/blog/193305