Netty4学习笔记（2）-- Bootstrap
Netty4的代码比我想象的要复杂的多，不过Netty4很好的将这种复杂性隐藏了起来，暴露出来的，是一个相对容易使用的接口。Bootstrap就是Netty试图隐藏这种复杂性的一个例子。
bootstrap包
Bootstrap继承结构
AbstractBootstrap是抽象类，有两个具体的实现，Bootstrap和ServerBootstrap：
Bootstrap例子
Netty4自带的例子里有一个EchoClient，看看它是如何使用Bootstrap启动一个客户端程序的：public class EchoClient {
private final String host;
private final int port;
private final int firstMessageSize;
public EchoClient(String host, int port, int firstMessageSize) {
this.host = host;
this.port = port;
this.firstMessageSize = firstMessageSize;
}
public void run() throws Exception {
// 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 {
ch.pipeline().addLast(
//new LoggingHandler(LogLevel.INFO),
new EchoClientHandler(firstMessageSize));
}
});
// 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();
}
}
public static void main(String[] args) throws Exception {
// ...
}
}
Builder模式？
看上面的例子，Bootstrap的使用很像Builder模式，Bootstrap就是Builder，EventLoopGroup、Channel和Handler等是各种Part。稍有不同的是，准备好各种Part后，并不是直接build出一个Product来，而是直接通过connect()方法使用这个Product。
AbstractBootstrap
为了弄清楚Bootstrap如何工作，我们先从AbstractBootstrap入手：
public abstract class AbstractBootstrap<B extends AbstractBootstrap<B, C>, C extends Channel> implements Cloneable {
private volatile EventLoopGroup group;
private volatile ChannelFactory<? extends C> channelFactory;
private volatile SocketAddress localAddress;
private final Map<ChannelOption<?>, Object> options = new LinkedHashMap<ChannelOption<?>, Object>();
private final Map<AttributeKey<?>, Object> attrs = new LinkedHashMap<AttributeKey<?>, Object>();
private volatile ChannelHandler handler;
// ...
}
可以看到，AbstractBootstrap这个抽象Builder一共需要有6个Part，如下图所示：
设置各个Part
AbstractBootstrap有一组方法用来设置各个Part，例如下面这些：
public B group(EventLoopGroup group) public B channel(Class<? extends C> channelClass) public B channelFactory(ChannelFactory<? extends C> channelFactory) public B localAddress(SocketAddress localAddress) public <T> B option(ChannelOption<T> option, T value) public <T> B attr(AttributeKey<T> key, T value) public B handler(ChannelHandler handler)
还有一组对应方法获得各个Part，如下：
final SocketAddress localAddress() final ChannelFactory<? extends C> channelFactory() final ChannelHandler handler() public final EventLoopGroup group() final Map<ChannelOption<?>, Object> options() final Map<AttributeKey<?>, Object> attrs()
ChannelFactory
AbstractBootstrap通过ChannelFactory创建Channel实例，channel(channelClass)方法看起来好像是设置了一个Channel，但实际上只是设置了默认的ChannelFactory实现：
public B channel(Class<? extends C> channelClass) {
if (channelClass == null) {
throw new NullPointerException("channelClass");
}
return channelFactory(new BootstrapChannelFactory<C>(channelClass));
}
默认的ChannelFactory实现使用反射创建Channel实例：
private static final class BootstrapChannelFactory<T extends Channel> implements ChannelFactory<T> {
private final Class<? extends T> clazz;
BootstrapChannelFactory(Class<? extends T> clazz) {
this.clazz = clazz;
}
@Override
public T newChannel() {
try {
return clazz.newInstance();
} catch (Throwable t) {
throw new ChannelException("Unable to create Channel from class " + clazz, t);
}
}
}
Bootstrap.connect()
再来看Bootstrap类的connect()方法：
public ChannelFuture connect(SocketAddress remoteAddress, SocketAddress localAddress) {
if (remoteAddress == null) {
throw new NullPointerException("remoteAddress");
}
validate();
return doConnect(remoteAddress, localAddress);
}
connect()方法调用validate()方法看各个Part是否准备就绪，然后调用doConnect()方法：
private ChannelFuture doConnect(final SocketAddress remoteAddress, final SocketAddress localAddress) {
final ChannelFuture regFuture = initAndRegister();
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);
} else {
regFuture.addListener(new ChannelFutureListener() {
@Override
public void operationComplete(ChannelFuture future) throws Exception {
doConnect0(regFuture, channel, remoteAddress, localAddress, promise);
}
});
}
return promise;
}
doConnect()方法首先调用了initAndRegister()方法，然后又调用了doConnect0()方法，方法调用示意图如下：
AbstractBootstrap.initAndRegister()
final ChannelFuture initAndRegister() {
final Channel channel = channelFactory().newChannel();
try {
init(channel);
} catch (Throwable t) {
channel.unsafe().closeForcibly();
return channel.newFailedFuture(t);
}
ChannelPromise regPromise = channel.newPromise();
group().register(channel, regPromise);
// ...
}initAndRegister()方法用ChannelFactory创建了一个Channel的实例，然后调用init()方法初始化Channel，最后将Channel注册到EventLoopGroup上：
而Channel在实例化的时候已经自动关联了Pipeline，这点从AbstractChannel的构造函数可以看出：
protected AbstractChannel(Channel parent) {
this.parent = parent;
unsafe = newUnsafe();
pipeline = new DefaultChannelPipeline(this);
}
Bootstrap.init()方法
void init(Channel channel) throws Exception {
ChannelPipeline p = channel.pipeline();
p.addLast(handler());
// ...
}Bootstrap.init()方法把Handler添加到了Pipeline的末尾，到这里，Channel就准备就绪了：
继续Bootstrap.doConnect()
initAndRegister()方法结束之后，doConnect()方法紧接着调用了doConnect0()方法，doConnect0()方法继而调用了Channel.connect()方法，这样Channel就接通服务器，可以收发消息了！
