netty4.0.x源码分析—channel
备注：本文的分析基于netty 4.0.9final版本，仅对Nio进行分析，因为本人对Socket编程比较感兴趣。
1、channel总体机构图
nio channel的总体结构图如下：
2、关键类和接口分析
2.1 基于NioServerSocketChannel进行分析
1）Channel
Channel是顶层接口，继承了AttributeMap, ChannelOutboundInvoker, ChannelPropertyAccess, Comparable<Channel>，它作为一个具体IO能力的组件提供给开发者，包括read, write, connect, and bind等操作。另外还提供了Channel配置的功能，以及获取Channel所在的eventloop的功能。
2）AbstractChannel
AbstractChannel实现Channel接口，关键代码如下：
private final Channel parent;
private final long hashCode = ThreadLocalRandom.current().nextLong();
private final Unsafe unsafe;
private final DefaultChannelPipeline pipeline;
private final ChannelFuture succeededFuture = new SucceededChannelFuture(this, null);
private final VoidChannelPromise voidPromise = new VoidChannelPromise(this, true);
private final VoidChannelPromise unsafeVoidPromise = new VoidChannelPromise(this, false);
private final CloseFuture closeFuture = new CloseFuture(this);
private volatile SocketAddress localAddress;
private volatile SocketAddress remoteAddress;
private volatile EventLoop eventLoop;
private volatile boolean registered;
/** Cache for the string representation of this channel */
private boolean strValActive;
private String strVal;<pre name="code" class="java"> /**
* Creates a new instance.
*
* @param parent
* the parent of this channel. {@code null} if there's no parent.
*/
protected AbstractChannel(Channel parent) {
this.parent = parent;
unsafe = newUnsafe();
pipeline = new DefaultChannelPipeline(this);
}
比较重要的对象是pipeline和unsafe，它们提供对read，write，bind等操作的具体实现。
3）AbstractNioChannel
AbstractNioChannel继承AbstractChannel，从这个类开始涉及到JDK的socket，参考如下关键代码：
private final SelectableChannel ch;
protected final int readInterestOp;
private volatile SelectionKey selectionKey;
private volatile boolean inputShutdown;
<pre name="code" class="java"> @Override
protected void doRegister() throws Exception {
boolean selected = false;
for (;;) {
try {
selectionKey = javaChannel().register(eventLoop().selector, 0, this);
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;
}
}
}
}
/** * Create a new instance * * @param parent the parent {@link Channel} by which this instance was created. May be {@code null} * @param ch the underlying {@link SelectableChannel} on which it operates * @param readInterestOp the ops to set to receive data from the {@link SelectableChannel} */ protected AbstractNioChannel(Channel parent, SelectableChannel ch, int readInterestOp) { super(parent); this.ch = ch; this.readInterestOp = readInterestOp; try { ch.configureBlocking(false); } 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); } } 从上面的代码可以看出，这里定义真正的Socket Channel(SelectableChannel)，关心的事件，注册后的key。将Socket设置为非阻塞，这是所有异步IO的关键，也就是说不管多么好的框架，底层基础还是不会变，可见学好基础的重要性啊，^_^。这里重点要关注一下register函数，这个函数是将Channel和事件循环进行关联的关键。每个事件循环都有一个自己的selector，channel实际上是注册到了相应eventloop的selector中，这也是Nio Socket编程的基础。
从这个类中已经可以看到netty的channel是如何和socket 的nio channel关联的了，以及channel是如何和eventloop关联的了。
4）AbstractNioMessageChannel
这个类继承AbstractNioChannel，主要是提供了一个newUnsafe方法返回NioMessageUnsafe对象的实例（实现read方法）。另外还定义doReadMessages和doWriteMessage两个抽象方法。
5）ServerSocketChannel和ServerChannel
这两个接口主要是定义了一个config方法，以及获取网络地址的方法。
6）NioServerSocketChannel
NioServerSocketChannel继承AbstractNioMessageChannel，实现ServerSocketChannel，它是一个具体类，提供给开发者使用。
/**
* A {@link io.netty.channel.socket.ServerSocketChannel} implementation which uses
* NIO selector based implementation to accept new connections.
*/
public class NioServerSocketChannel extends AbstractNioMessageChannel
implements io.netty.channel.socket.ServerSocketChannel {
private static final ChannelMetadata METADATA = new ChannelMetadata(false);
private static final InternalLogger logger = InternalLoggerFactory.getInstance(NioServerSocketChannel.class);
private static ServerSocketChannel newSocket() {
try {
return ServerSocketChannel.open();
} catch (IOException e) {
throw new ChannelException(
"Failed to open a server socket.", e);
}
}
private final ServerSocketChannelConfig config;
/**
* Create a new instance
*/
public NioServerSocketChannel() {
super(null, newSocket(), SelectionKey.OP_ACCEPT);
config = new DefaultServerSocketChannelConfig(this, javaChannel().socket());
}
@Override
protected ServerSocketChannel javaChannel() {
return (ServerSocketChannel) super.javaChannel();
}
@Override
protected void doBind(SocketAddress localAddress) throws Exception {
javaChannel().socket().bind(localAddress, config.getBacklog());
}
@Override
protected void doClose() throws Exception {
javaChannel().close();
}
@Override
protected int doReadMessages(List<Object> buf) throws Exception {
SocketChannel ch = javaChannel().accept();
try {
if (ch != null) {
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;
}
// Unnecessary stuff
@Override
protected boolean doConnect(
SocketAddress remoteAddress, SocketAddress localAddress) throws Exception {
throw new UnsupportedOperationException();
}
@Override
protected void doFinishConnect() throws Exception {
throw new UnsupportedOperationException();
}
@Override
protected SocketAddress remoteAddress0() {
return null;
}
@Override
protected void doDisconnect() throws Exception {
throw new UnsupportedOperationException();
}
@Override
protected boolean doWriteMessage(Object msg, ChannelOutboundBuffer in) throws Exception {
throw new UnsupportedOperationException();
}
}
从这个具体类中，我们可以看到，调用JDK函数ServerSocketChannel.open();生成了底层ServerSocketChannel对象，将NioServerSocketChannel和ServerSocketChannel相关联，并且传递了感兴趣的事件OP_ACCEPT给父类。实现了doReadMessage函数，实际上就是accept一个SocketChanel。
2.2 基于NioSocketChannel进行分析
在NioServerSocketChannel中介绍过的类和接口，这里不再介绍。其实和NioServerSocketChannel差不多，只是它是基于Byte的。
1）AbstractNioByteChannel
这个类继承AbstractNioChannel，主要也是提供了一个newUnsafe方法返回NioByteUnsafe对象的实例（实现read方法）。另外还定义doReadBytes和doWriteBytes两个抽象方法。
2）SocketChannel
这个接口继承了Channel接口，定义了多个shutdown方法，以及一个parent方法，返回该SocketChannel相应的ServerSocketChannel。
3）NioSocketChannel
这个类继承AbstractNioByteChannel，并且实现SocketChannel接口，是一个具体类，提供给开发者使用。
/**
* {@link io.netty.channel.socket.SocketChannel} which uses NIO selector based implementation.
*/
public class NioSocketChannel extends AbstractNioByteChannel implements io.netty.channel.socket.SocketChannel {
private static final ChannelMetadata METADATA = new ChannelMetadata(false);
private static SocketChannel newSocket() {
try {
return SocketChannel.open();
} catch (IOException e) {
throw new ChannelException("Failed to open a socket.", e);
}
}
private final SocketChannelConfig config;
/**
* Create a new instance
*/
public NioSocketChannel() {
this(newSocket());
}
/**
* Create a new instance using the given {@link SocketChannel}.
*/
public NioSocketChannel(SocketChannel socket) {
this(null, socket);
}
/**
* Create a new instance
*
* @param parent the {@link Channel} which created this instance or {@code null} if it was created by the user
* @param socket the {@link SocketChannel} which will be used
*/
public NioSocketChannel(Channel parent, SocketChannel socket) {
super(parent, socket);
config = new DefaultSocketChannelConfig(this, socket.socket());
}
@Override
protected SocketChannel javaChannel() {
return (SocketChannel) super.javaChannel();
}
@Override
public boolean isActive() {
SocketChannel ch = javaChannel();
return ch.isOpen() && ch.isConnected();
}
@Override
public boolean isInputShutdown() {
return super.isInputShutdown();
}
@Override
public InetSocketAddress localAddress() {
return (InetSocketAddress) super.localAddress();
}
@Override
public InetSocketAddress remoteAddress() {
return (InetSocketAddress) super.remoteAddress();
}
@Override
public boolean isOutputShutdown() {
return javaChannel().socket().isOutputShutdown() || !isActive();
}
@Override
public ChannelFuture shutdownOutput() {
return shutdownOutput(newPromise());
}
@Override
public ChannelFuture shutdownOutput(final ChannelPromise promise) {
EventLoop loop = eventLoop();
if (loop.inEventLoop()) {
try {
javaChannel().socket().shutdownOutput();
promise.setSuccess();
} catch (Throwable t) {
promise.setFailure(t);
}
} else {
loop.execute(new Runnable() {
@Override
public void run() {
shutdownOutput(promise);
}
});
}
return promise;
}
@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);
if (!connected) {
selectionKey().interestOps(SelectionKey.OP_CONNECT);
}
success = true;
return connected;
} finally {
if (!success) {
doClose();
}
}
}
@Override
protected void doFinishConnect() throws Exception {
if (!javaChannel().finishConnect()) {
throw new Error();
}
}
@Override
protected void doDisconnect() throws Exception {
doClose();
}
@Override
protected void doClose() throws Exception {
javaChannel().close();
}
@Override
protected int doReadBytes(ByteBuf byteBuf) throws Exception {
return byteBuf.writeBytes(javaChannel(), byteBuf.writableBytes());
}
@Override
protected int doWriteBytes(ByteBuf buf) throws Exception {
final int expectedWrittenBytes = buf.readableBytes();
final int writtenBytes = buf.readBytes(javaChannel(), expectedWrittenBytes);
return writtenBytes;
}
@Override
protected long doWriteFileRegion(FileRegion region) throws Exception {
final long position = region.transfered();
final long writtenBytes = region.transferTo(javaChannel(), position);
return writtenBytes;
}
@Override
protected void doWrite(ChannelOutboundBuffer in) throws Exception {
for (;;) {
// Do non-gathering write for a single buffer case.
final int msgCount = in.size();
if (msgCount <= 1) {
super.doWrite(in);
return;
}
// Ensure the pending writes are made of ByteBufs only.
ByteBuffer[] nioBuffers = in.nioBuffers();
if (nioBuffers == null) {
super.doWrite(in);
return;
}
int nioBufferCnt = in.nioBufferCount();
long expectedWrittenBytes = in.nioBufferSize();
final SocketChannel ch = javaChannel();
long writtenBytes = 0;
boolean done = false;
for (int i = config().getWriteSpinCount() - 1; i >= 0; i --) {
final long localWrittenBytes = ch.write(nioBuffers, 0, nioBufferCnt);
if (localWrittenBytes == 0) {
break;
}
expectedWrittenBytes -= localWrittenBytes;
writtenBytes += localWrittenBytes;
if (expectedWrittenBytes == 0) {
done = true;
break;
}
}
if (done) {
// Release all buffers
for (int i = msgCount; i > 0; i --) {
in.remove();
}
// Finish the write loop if no new messages were flushed by in.remove().
if (in.isEmpty()) {
clearOpWrite();
break;
}
} else {
// Did not write all buffers completely.
// Release the fully written buffers and update the indexes of the partially written buffer.
for (int i = msgCount; i > 0; i --) {
final ByteBuf buf = (ByteBuf) in.current();
final int readerIndex = buf.readerIndex();
final int readableBytes = buf.writerIndex() - readerIndex;
if (readableBytes < writtenBytes) {
in.progress(readableBytes);
in.remove();
writtenBytes -= readableBytes;
} else if (readableBytes > writtenBytes) {
buf.readerIndex(readerIndex + (int) writtenBytes);
in.progress(writtenBytes);
break;
} else { // readableBytes == writtenBytes
in.progress(readableBytes);
in.remove();
break;
}
}
setOpWrite();
break;
}
}
}
}
从代码中可以看出，调用了SocketChannel.open();创建SocketChannel对象，将NioSocketChannel和SocketChannel关联。主要是实现了发送数据的doWrite函数。
3、总结
NioSocketChannel和NioServerSocketChannel这两个具体类是提供给开发者使用的。从上面的分析可以看出，实际上他们底层关联的还是JDK的SocketChannel和ServerSocketChannel。netty的Socket Channel是对JDK的Socket Channel的封装，它将Channel和loop关联，在loop中处理Channel的事件通知。
备注：Channel是netty的核心数据结构，这篇文章只是对Channel的Socket部分进行简单分析，不过通过它基本上已经能够了解netty是如何将它的Channel和上一篇的event关联的，以及它是如何将channel和JDK的channel关联的。