1.准备ChannelFactory
Netty服务端的ServerBootstrap引导类的channel方法来指定channel类型,并且在bind调用后,会调用此工厂对象来生成一个新channel。
// 设置并绑定服务端Channel,指定所使用的NIO传输的Channel
.channel(NioServerSocketChannel.class)
channel方法的内部细节如下代码:
public B channel(Class<? extends C> channelClass) {
if (channelClass == null) {
throw new NullPointerException("channelClass");
}
return channelFactory(new ReflectiveChannelFactory<C>(channelClass));
}
public ReflectiveChannelFactory(Class<? extends T> clazz) {
if (clazz == null) {
throw new NullPointerException("clazz");
}
this.clazz = clazz;
}
public B channelFactory(ChannelFactory<? extends C> channelFactory) {
if (channelFactory == null) {
throw new NullPointerException("channelFactory");
}
if (this.channelFactory != null) {
throw new IllegalStateException("channelFactory set already");
}
this.channelFactory = channelFactory;
return self();
}
首先明确一点,channel方法是在AbstractBootstrap中,也就是说这个方法client和server端调用时一样的效果,只是参数不同而已,也就是最终实例化的channel类型不同。
接下来分析这段代码:这段代码其实就是设置了AbstractBootstrap中的channelFactory属性,并且实例化好的ReflectiveChannelFactory对象中有一个Class的属性被设置为我们参数NioServerSocketChannel。为后面的实例化NioServerSocketChannel类做准备。
2. 在哪实例化NioServerSocketChannel?
在启动过程的文章中也讲到过,实例化是在AbstractBootstrap中的initAndRegister()方法中:
channel = channelFactory.newChannel();
此处用到的channelFactory正式我们在第一节中讲到的实例化的ReflectiveChannelFactory实例,newChannel方法如下:
public T newChannel() {
try {
return clazz.getConstructor().newInstance();
} catch (Throwable t) {
throw new ChannelException("Unable to create Channel from class " + clazz, t);
}
}
可以看出上面的代码通过反射生成NioServerSocketChannel实例,并且调用的是无参构造函数。
NioServerSocketChannel类的继承结构图如下所示:
3. 构造函数链
NioServerSocketChanne的构造函数如下:
public NioServerSocketChannel() {
this(newSocket(DEFAULT_SELECTOR_PROVIDER));
}
其中DEFAULT_SELECTOR_PROVIDER是一个静态的变量,
private static final SelectorProvider DEFAULT_SELECTOR_PROVIDER = SelectorProvider.provider();
看到Selector想起了NIO中的Selector,没错!默认的多路复用器就是这个。和NIO中的的selector获取方式一样。接下来是newSocket(DEFAULT_SELECTOR_PROVIDER)方法。
private static ServerSocketChannel newSocket(SelectorProvider provider) {
try {
return provider.openServerSocketChannel();
} catch (IOException e) {
throw new ChannelException(
"Failed to open a server socket.", e);
}
}
通过newSocket创建ServerSocketChannel,回到无参的构造函数,接下来调用了另一个有参构造函数:
public NioServerSocketChannel(ServerSocketChannel channel) {
super(null, channel, SelectionKey.OP_ACCEPT);
config = new NioServerSocketChannelConfig(this, javaChannel().socket());
}
主要有调用了父类的构造函数和设置config属性,下面将一步一步的分析。
接着分析,父类AbstractNioMessageChannel的构造方法如下:
protected AbstractNioMessageChannel(Channel parent, SelectableChannel ch, int readInterestOp) {
super(parent, ch, readInterestOp);
}
因为是服务端新生成的channel,第一个参数指定为null,表示没有父channel,第二个参数指定为ServerSocketChannel,第三个参数指定ServerSocketChannel关心的事件类型为SelectionKey.OP_ACCEPT。
续调用父类AbstractNioChannel的构造方法:
protected AbstractNioChannel(Channel parent, SelectableChannel ch, int readInterestOp) {
super(parent);
//将ServerSocketChannel对象保存
this.ch = ch;
this.readInterestOp = readInterestOp;
try {
// 设置当前通道为非阻塞的
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);
}
}
在AbstractNioChannel中做了下面几件事:
- 通过this.ch = ch 保存ServerSocketChannel, 因为NioServerSocketChannel是Netty封装的对象,而ServerSocketChannel是有前面默认selector_provider生成的,是java nio的, 其实“this.ch = ch”可以被认为是绑定原生的ServerSocketChannel到NioServerSocketChannel对象中。
- 设置ServerSocketChannel关心的事件类型
- 设置ServerSocketChannel为非阻塞的
继续进入到父类AbstractChannel的构造函数:
protected AbstractChannel(Channel parent) {
this.parent = parent;
id = newId();
unsafe = newUnsafe();
pipeline = newChannelPipeline();
}
此构造方法中,主要做了三件事:
- 给channel生成一个新的id
- 通过newUnsafe初始化channel的unsafe属性
- newChannelPipeline初始化channel的pipeline属性
Id是通过当前时间以及Mac地址等生成的标识,此处不再赘述。主要分析newUnsafe和newChannelPipleLine方法。
4. newUnsafe();
io.netty.channel.Channel.Unsafe
和sun.misc.Unsafe
类似,我们知道后者是操作Java底层的API,前者是操作Netty底层的API,Netty底层就是NIO,说白了就是操作Channel,进行读和写等操作。
在AbstractChannel类中,newUnsafe()是一个抽象方法,通过上面的继承关系,AbstractNioMessageChannel类中有newUnsafe()的实现。
private final class NioMessageUnsafe extends AbstractNioUnsafe {
private final List<Object> readBuf = new ArrayList<Object>();
@Override
public void read() {
assert eventLoop().inEventLoop();
final ChannelConfig config = config();
final ChannelPipeline pipeline = pipeline();
final RecvByteBufAllocator.Handle allocHandle = unsafe().recvBufAllocHandle();
allocHandle.reset(config);
boolean closed = false;
Throwable exception = null;
try {
try {
do {
int localRead = doReadMessages(readBuf);
if (localRead == 0) {
break;
}
if (localRead < 0) {
closed = true;
break;
}
allocHandle.incMessagesRead(localRead);
} while (allocHandle.continueReading());
} catch (Throwable t) {
exception = t;
}
int size = readBuf.size();
for (int i = 0; i < size; i ++) {
readPending = false;
pipeline.fireChannelRead(readBuf.get(i));
}
readBuf.clear();
allocHandle.readComplete();
pipeline.fireChannelReadComplete();
if (exception != null) {
closed = closeOnReadError(exception);
pipeline.fireExceptionCaught(exception);
}
if (closed) {
inputShutdown = true;
if (isOpen()) {
close(voidPromise());
}
}
} finally {
if (!readPending && !config.isAutoRead()) {
removeReadOp();
}
}
}
}
NioMessageUnsafe 只覆盖了 父类AbstractNioUnsafe中的read方法,其实unsafe对象是真正的负责底层channel的连接/读/写等操作的,unsafe就好比一个底层channel操作的代理对象。
5. newChannelPipeline();
newChannelPipeline直接在AbstractChannel内实现,该方法返回了DefaultChannelPipeline对象
protected DefaultChannelPipeline newChannelPipeline() {
return new DefaultChannelPipeline(this);
}
//构造方法
protected DefaultChannelPipeline(Channel channel) {
this.channel = ObjectUtil.checkNotNull(channel, "channel");
succeededFuture = new SucceededChannelFuture(channel, null);
voidPromise = new VoidChannelPromise(channel, true);
tail = new TailContext(this);
head = new HeadContext(this);
head.next = tail;
tail.prev = head;
}
此DefaultChannelPipeline对象会绑定NioServerSocketChannel对象,并初始化了HeadContext及TailContext对象。head及tail初始化完成后,它们会相互连接。pipleLine的更多细节在下篇源码分析中分享。
父类的构造函数执行完毕之后,继续config = new NioServerSocketChannelConfig(this, javaChannel().socket());
此config对象就是就会对底层ServerSocket一些配置设置行为的封装。
小结
NioServerSocketChannel对象内部绑定了Java NIO创建的ServerSocketChannel对象;
Netty中,每个channel都有一个unsafe对象,此对象封装了Java NIO底层channel的操作细节;
Netty中,每个channel都有一个pipeline对象,此对象就是一个双向链表;