上文我们从netty-example的Discard服务器端示例分析了netty的组件，今天我们从另一个简单的示例Echo客户端分析一下上个示例中没有出现的netty组件。
1. 服务端的连接处理，读写处理
echo客户端代码：
/** * Sends one message when a connection is open and echoes back any received * data to the server. Simply put, the echo client initiates the ping-pong * traffic between the echo client and server by sending the first message to * the server. */ public final class EchoClient { static final boolean SSL = System.getProperty("ssl") != null; static final String HOST = System.getProperty("host", "127.0.0.1"); static final int PORT = Integer.parseInt(System.getProperty("port", "8007")); static final int SIZE = Integer.parseInt(System.getProperty("size", "256")); public static void main(String[] args) throws Exception { // Configure SSL.git final SslContext sslCtx; if (SSL) { sslCtx = SslContextBuilder.forClient() .trustManager(InsecureTrustManagerFactory.INSTANCE).build(); } else { sslCtx = null; } // Configure the client. EventLoopGroup group = new NioEventLoopGroup(); try { Bootstrap b = new Bootstrap(); b.group(group) .channel(NioSocketChannel.class) .option(ChannelOption.TCP_NODELAY, true) .handler(new ChannelInitializer<SocketChannel>() { @Override public void initChannel(SocketChannel ch) throws Exception { ChannelPipeline p = ch.pipeline(); if (sslCtx != null) { p.addLast(sslCtx.newHandler(ch.alloc(), HOST, PORT)); } //p.addLast(new LoggingHandler(LogLevel.INFO)); p.addLast(new EchoClientHandler()); } }); // Start the client. ChannelFuture f = b.connect(HOST, PORT).sync(); // Wait until the connection is closed. f.channel().closeFuture().sync(); } finally { // Shut down the event loop to terminate all threads. group.shutdownGracefully(); } } }
从上面的代码可以看出，discard的服务端代码和echo的客户端代码基本相似，不同的是一个使用ServerBootStrap,另一个使用BootStrap而已。先看一下连接过程
NioEventLoop处理key的过程，
private static void processSelectedKey(SelectionKey k, AbstractNioChannel ch) {
final AbstractNioChannel.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 ignored) {
unsafe.close(unsafe.voidPromise());
}
}
2.1 连接流程
调用AbstractNioByteChannel的finishConnect()方法
@Override
public final 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();
try {
boolean wasActive = isActive();
doFinishConnect();
fulfillConnectPromise(connectPromise, wasActive);
} catch (Throwable t) {
fulfillConnectPromise(connectPromise, annotateConnectException(t, requestedRemoteAddress));
} 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;
}
}
触发channelActive操作：
private void fulfillConnectPromise(ChannelPromise promise, boolean wasActive) {
if (promise == null) {
// Closed via cancellation and the promise has been notified already.
return;
}
// 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();
}
// If a user cancelled the connection attempt, close the channel, which is followed by channelInactive().
if (!promiseSet) {
close(voidPromise());
}
}
2.2 读操作流程
调用AbstractNioByteChannel的read()方法，
典型的autoRead流程如下：
1. 当socket建立连接时，Netty触发一个inbound事件channelActive，然后提交一个read()请求给本身(参考DefaultChannelPipeline.fireChannelActive())
2. 接收到read()请求后，Netty从socket读取消息。
3. 当读取到消息时，Netty触发channelRead()。
4. 当读取不到消息后，Netty触发ChannelReadCompleted().
5. Netty提交另外一个read()请求来继续从socket中读取消息。
@Override
public final void read() {
final ChannelConfig config = config();
if (!config.isAutoRead() && !isReadPending()) {
// ChannelConfig.setAutoRead(false) was called in the meantime
removeReadOp();
return;
}
final ChannelPipeline pipeline = pipeline();
final ByteBufAllocator allocator = config.getAllocator();
final RecvByteBufAllocator.Handle allocHandle = recvBufAllocHandle();
allocHandle.reset(config);
ByteBuf byteBuf = null;
try {
boolean needReadPendingReset = true;
do {
byteBuf = allocHandle.allocate(allocator);
allocHandle.lastBytesRead(doReadBytes(byteBuf));
if (allocHandle.lastBytesRead() <= 0) {
// nothing was read. release the buffer.
byteBuf.release();
byteBuf = null;
break;
}
allocHandle.incMessagesRead(1);
if (needReadPendingReset) {
needReadPendingReset = false;
setReadPending(false);
}
pipeline.fireChannelRead(byteBuf);
byteBuf = null;
} while (allocHandle.continueReading());
allocHandle.readComplete();
pipeline.fireChannelReadComplete();
if (allocHandle.lastBytesRead() < 0) {
closeOnRead(pipeline);
}
} catch (Throwable t) {
handleReadException(pipeline, byteBuf, t, allocHandle.lastBytesRead() < 0, allocHandle);
} 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 (!config.isAutoRead() && !isReadPending()) {
removeReadOp();
}
}
}
}
触发读操作
@Override
public ChannelHandlerContext fireChannelRead(Object msg) {
AbstractChannelHandlerContext next = findContextInbound();
next.invoker().invokeChannelRead(next, pipeline.touch(msg, next));
return this;
}
读完触发完成事件
@Override
public ChannelPipeline fireChannelReadComplete() {
head.fireChannelReadComplete();
if (channel.config().isAutoRead()) {
read();
}
return this;
}
@Override
public ChannelHandlerContext fireChannelReadComplete() {
AbstractChannelHandlerContext next = findContextInbound();
next.invoker().invokeChannelReadComplete(next);
return this;
}
2.3 写操作流程
写操作
@SuppressWarnings("deprecation")
protected void flush0() {
if (inFlush0) {
// Avoid re-entrance
return;
}
final ChannelOutboundBuffer outboundBuffer = this.outboundBuffer;
if (outboundBuffer == null || outboundBuffer.isEmpty()) {
return;
}
inFlush0 = true;
// Mark all pending write requests as failure if the channel is inactive.
if (!isActive()) {
try {
if (isOpen()) {
outboundBuffer.failFlushed(NOT_YET_CONNECTED_EXCEPTION, true);
} else {
// Do not trigger channelWritabilityChanged because the channel is closed already.
outboundBuffer.failFlushed(CLOSED_CHANNEL_EXCEPTION, false);
}
} finally {
inFlush0 = false;
}
return;
}
try {
doWrite(outboundBuffer);
} catch (Throwable t) {
if (t instanceof IOException && config().isAutoClose()) {
/**
* Just call {@link #close(ChannelPromise, Throwable, boolean)} here which will take care of
* failing all flushed messages and also ensure the actual close of the underlying transport
* will happen before the promises are notified.
*
* This is needed as otherwise {@link #isActive()} , {@link #isOpen()} and {@link #isWritable()}
* may still return {@code true} even if the channel should be closed as result of the exception.
*/
close(voidPromise(), t, false);
} else {
outboundBuffer.failFlushed(t, true);
}
} finally {
inFlush0 = false;
}
}
写操作具体实现(以NioSocketChannel为例)：
@Override
protected void doWrite(ChannelOutboundBuffer in) throws Exception {
for (;;) {
int size = in.size();
if (size == 0) {
// All written so clear OP_WRITE
clearOpWrite();
break;
}
long writtenBytes = 0;
boolean done = false;
boolean setOpWrite = false;
// Ensure the pending writes are made of ByteBufs only.
ByteBuffer[] nioBuffers = in.nioBuffers();
int nioBufferCnt = in.nioBufferCount();
long expectedWrittenBytes = in.nioBufferSize();
SocketChannel ch = javaChannel();
// Always us nioBuffers() to workaround data-corruption.
// See https://github.com/netty/netty/issues/2761
switch (nioBufferCnt) {
case 0:
// We have something else beside ByteBuffers to write so fallback to normal writes.
super.doWrite(in);
return;
case 1:
// Only one ByteBuf so use non-gathering write
ByteBuffer nioBuffer = nioBuffers[0];
for (int i = config().getWriteSpinCount() - 1; i >= 0; i --) {
final int localWrittenBytes = ch.write(nioBuffer);
if (localWrittenBytes == 0) {
setOpWrite = true;
break;
}
expectedWrittenBytes -= localWrittenBytes;
writtenBytes += localWrittenBytes;
if (expectedWrittenBytes == 0) {
done = true;
break;
}
}
break;
default:
for (int i = config().getWriteSpinCount() - 1; i >= 0; i --) {
final long localWrittenBytes = ch.write(nioBuffers, 0, nioBufferCnt);
if (localWrittenBytes == 0) {
setOpWrite = true;
break;
}
expectedWrittenBytes -= localWrittenBytes;
writtenBytes += localWrittenBytes;
if (expectedWrittenBytes == 0) {
done = true;
break;
}
}
break;
}
// Release the fully written buffers, and update the indexes of the partially written buffer.
in.removeBytes(writtenBytes);
if (!done) {
// Did not write all buffers completely.
incompleteWrite(setOpWrite);
break;
}
}
}
2. ChannelInboundHandler和ChannelInboundHandler
Echo的handler代码如下：
/** * Handler implementation for the echo client. It initiates the ping-pong * traffic between the echo client and server by sending the first message to * the server. */ public class EchoClientHandler extends ChannelInboundHandlerAdapter { private final ByteBuf firstMessage; /** * Creates a client-side handler. */ public EchoClientHandler() { firstMessage = Unpooled.buffer(EchoClient.SIZE); for (int i = 0; i < firstMessage.capacity(); i ++) { firstMessage.writeByte((byte) i); } } @Override public void channelActive(ChannelHandlerContext ctx) { ctx.writeAndFlush(firstMessage); } @Override public void channelRead(ChannelHandlerContext ctx, Object msg) { ctx.write(msg); } @Override public void channelReadComplete(ChannelHandlerContext ctx) { ctx.flush(); } @Override public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) { // Close the connection when an exception is raised. cause.printStackTrace(); ctx.close(); }
上面的代码出现了两个重要的netty组件：ChannelInboundHandlerAdapter和ByteBuf。其中ByteBuf在另一篇文章已经讲到。我们这次重点分析一下 ChannelInboundHandlerAdapter及其相关类。
ChannelInboundHandlerAdapter继承了ChannelInboundHandler，它的作用是将operation转到ChannelPipeline中的下一个ChannelHandler。子类可以重写一个方法的实现来改变。注意：在方法#channelRead(ChannelHandlerContext, Object)自动返回前，message不会释放。若需要一个可以自动释放接收消息的ChannelInboundHandler实现时，请考虑SimpleChannelInboundHandler。
ChannelOutboundHandlerAdapter继承了ChannelOutboundHandler，它仅通过调用ChannelHandlerContext跳转到每个方法。
ChannelInboundHandler处理输入的事件，事件由外部事件源产生，例如从一个socket接收到数据。
ChannelOutboundHandler解析你自己应用提交的操作。
2.1 ChannelInboundHandler.channelActive()
从源码角度看一下，Netty触发一个inbound事件channelActive(以LoggingHandler为例)：
@Override public void channelActive(ChannelHandlerContext ctx) throws Exception { if (logger.isEnabled(internalLevel)) { logger.log(internalLevel, format(ctx, "ACTIVE")); } ctx.fireChannelActive(); }
触发操作如下：
@Override public ChannelHandlerContext fireChannelActive() { AbstractChannelHandlerContext next = findContextInbound(); next.invoker().invokeChannelActive(next); return this; } private AbstractChannelHandlerContext findContextInbound() { AbstractChannelHandlerContext ctx = this; do { ctx = ctx.next; } while (!ctx.inbound); return ctx; }
invokeChannelActive方法实现：
@Override public void invokeChannelActive(final ChannelHandlerContext ctx) { if (executor.inEventLoop()) { invokeChannelActiveNow(ctx); } else { executor.execute(new OneTimeTask() { @Override public void run() { invokeChannelActiveNow(ctx); } }); } } public static void invokeChannelActiveNow(final ChannelHandlerContext ctx) { try { ((ChannelInboundHandler) ctx.handler()).channelActive(ctx); } catch (Throwable t) { notifyHandlerException(ctx, t); } }
2.2 ChannelOutboundHandler.Read()
读的流程：
@Override public ChannelHandlerContext read() { AbstractChannelHandlerContext next = findContextOutbound(); next.invoker().invokeRead(next); return this; }
查找outbound的过程：
private AbstractChannelHandlerContext findContextOutbound() { AbstractChannelHandlerContext ctx = this; do { ctx = ctx.prev; } while (!ctx.outbound); return ctx; }
触发读操作：
@Override public void invokeRead(final ChannelHandlerContext ctx) { if (executor.inEventLoop()) { invokeReadNow(ctx); } else { AbstractChannelHandlerContext dctx = (AbstractChannelHandlerContext) ctx; Runnable task = dctx.invokeReadTask; if (task == null) { dctx.invokeReadTask = task = new Runnable() { @Override public void run() { invokeReadNow(ctx); } }; } executor.execute(task); } }
2.3 ChannelOutboundHandler.write()
以实现类LoggingHandler为例：
@Override public void write(ChannelHandlerContext ctx, Object msg, ChannelPromise promise) throws Exception { if (logger.isEnabled(internalLevel)) { logger.log(internalLevel, format(ctx, "WRITE", msg)); } ctx.write(msg, promise); }
具体实现：
@Override public ChannelFuture write(Object msg, ChannelPromise promise) { AbstractChannelHandlerContext next = findContextOutbound(); next.invoker().invokeWrite(next, pipeline.touch(msg, next), promise); return promise; }
写操作的触发
@Override public void invokeWrite(ChannelHandlerContext ctx, Object msg, ChannelPromise promise) { if (msg == null) { throw new NullPointerException("msg"); } if (!validatePromise(ctx, promise, true)) { // promise cancelled ReferenceCountUtil.release(msg); return; } if (executor.inEventLoop()) { invokeWriteNow(ctx, msg, promise); } else { safeExecuteOutbound(WriteTask.newInstance(ctx, msg, promise), promise, msg); } }
立刻触发
public static void invokeWriteNow(ChannelHandlerContext ctx, Object msg, ChannelPromise promise) { try { ((ChannelOutboundHandler) ctx.handler()).write(ctx, msg, promise); } catch (Throwable t) { notifyOutboundHandlerException(t, promise); } }
小结：
Netty中，可以注册多个handler。ChannelInboundHandler按照注册的先后顺序执行；ChannelOutboundHandler按照注册的先后顺序逆序执行，如下图所示，按照注册的先后顺序对Handler进行排序，request进入Netty后的执行顺序为：
参考文献
【1】http://blog.csdn.net/u013252773/article/details/21195593
【2】http://stackoverflow.com/questions/22354135/in-netty4-why-read-and-write-both-in-outboundhandler
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