看了netty源码后发现，它不过是封装在jdk的nio之上的框架，虽然大致猜到nio的原理，但还是忍不住要去jdk底层一探究竟。
要用selector，第一句话无非Selector selector = Selector.open();但里面如何实现的？
public static Selector open() throws IOException {
return SelectorProvider.provider().openSelector();
}
我们看到这里需要先使用SelectorProvider的provider()方法来取得相应的SelectorProvider。
public static SelectorProvider provider() {
synchronized (lock) {
if (provider != null)
return provider;
return AccessController.doPrivileged(
new PrivilegedAction<SelectorProvider>() {
public SelectorProvider run() {
if (loadProviderFromProperty())
return provider;
if (loadProviderAsService())
return provider;
provider = sun.nio.ch.DefaultSelectorProvider.create();
return provider;
}
});
}
}
先从系统属性中去找provider，如果没取到，看是否在规定路径下写了配置文件，安装在jar文件下对系统加载器可见，则用系统类加载器加载，若该类在META-INF / services下则用serviceLoader加载，若还是没找到，则使用DefaultSelectorProvider的creat()方法来获取默认的SelectorProvider。
public static SelectorProvider create() {
return new WindowsSelectorProvider();
}如果没有配置，就采用这个类，其构造方法是空的，其openSelect()
public AbstractSelector openSelector() throws IOException {
return new WindowsSelectorImpl(this);
}
WindowsSelectorImpl有没很熟悉？对啊，netty默认的selesctor用的就是这个。jdk的nio默认情况下就是这个。
WindowsSelectorImpl(SelectorProvider var1) throws IOException {
super(var1);
this.wakeupSourceFd = ((SelChImpl)this.wakeupPipe.source()).getFDVal();
SinkChannelImpl var2 = (SinkChannelImpl)this.wakeupPipe.sink();
var2.sc.socket().setTcpNoDelay(true);
this.wakeupSinkFd = var2.getFDVal();
this.pollWrapper.addWakeupSocket(this.wakeupSourceFd, 0);
}
看起构造，好像都用到wakeupPipe，看其成员private final Pipe wakeupPipe = Pipe.open();
public static Pipe open() throws IOException {
return SelectorProvider.provider().openPipe();
}继续往下看吧，具体逻辑在SelectorProviderImpl类中
public Pipe openPipe() throws IOException {
return new PipeImpl(this);
}我们看下PipImpl的构造方法
PipeImpl(SelectorProvider var1) throws IOException {
try {
AccessController.doPrivileged(new PipeImpl.Initializer(var1));
} catch (PrivilegedActionException var3) {
throw (IOException)var3.getCause();
}
}（稍微提一下成员private static final Random RANDOM_NUMBER_GENERATOR = new SecureRandom();无非生成一个随机数。）可以看到AccessController的doPrivileged是个native方法，从注释中可以得出它调用了传入的action中的run方法，并保证其内部所涉及的权限问题。我们直接来看下PipeImpl的内部类的Initializer的run()方法
public Void run() throws IOException {
PipeImpl.Initializer.LoopbackConnector var1 = new PipeImpl.Initializer.LoopbackConnector();
var1.run();
if(this.ioe instanceof ClosedByInterruptException) {
this.ioe = null;
Thread var2 = new Thread(var1) {
public void interrupt() {
}
};
var2.start();
while(true) {
try {
var2.join();
break;
} catch (InterruptedException var4) {
;
}
}
Thread.currentThread().interrupt();
}
if(this.ioe != null) {
throw new IOException("Unable to establish loopback connection", this.ioe);
} else {
return null;
}
}
该方法主要是启动了个线程？看LoopbackConnector()，内部类Initializer的内部类LoopbackConnector。看其run方法吧。
public void run() {
ServerSocketChannel var1 = null;
SocketChannel var2 = null;
SocketChannel var3 = null;
try {
ByteBuffer var4 = ByteBuffer.allocate(16);
ByteBuffer var5 = ByteBuffer.allocate(16);
InetAddress var6 = InetAddress.getByName("127.0.0.1");
assert var6.isLoopbackAddress();
InetSocketAddress var7 = null;
while(true) {
if(var1 == null || !var1.isOpen()) {
var1 = ServerSocketChannel.open();
var1.socket().bind(new InetSocketAddress(var6, 0));
var7 = new InetSocketAddress(var6, var1.socket().getLocalPort());
}
var2 = SocketChannel.open(var7);
PipeImpl.RANDOM_NUMBER_GENERATOR.nextBytes(var4.array());
do {
var2.write(var4);
} while(var4.hasRemaining());
var4.rewind();
var3 = var1.accept();
do {
var3.read(var5);
} while(var5.hasRemaining());
var5.rewind();
if(var5.equals(var4)) {
PipeImpl.this.source = new SourceChannelImpl(Initializer.this.sp, var2);
PipeImpl.this.sink = new SinkChannelImpl(Initializer.this.sp, var3);
break;
}
var3.close();
var2.close();
}
} catch (IOException var18) {
try {
if(var2 != null) {
var2.close();
}
if(var3 != null) {
var3.close();
}
} catch (IOException var17) {
;
}
Initializer.this.ioe = var18;
} finally {
try {
if(var1 != null) {
var1.close();
}
} catch (IOException var16) {
;
}
}
}
这里尝试通过两条SocketChannel来联通一个ServerSocketChannel，先看val1通过ServerSocketChannel.open()，通过SelectorProviderImpl的openServerSocketChannel()
public ServerSocketChannel openServerSocketChannel() throws IOException {
return new ServerSocketChannelImpl(this);
}返回ServerSocketChannelImpl
ServerSocketChannelImpl(SelectorProvider var1) throws IOException {
super(var1);
this.fd = Net.serverSocket(true);
this.fdVal = IOUtil.fdVal(this.fd);
this.state = 0;
}
在构造方法中，先创建socket，再得到它的fd跟fdVal。然后将创建的socket绑定本地机的0号端口（var1.socket().bind(new InetSocketAddress(var6, 0));）然后根据这个socket的ip跟端口生成一个SocketChannel(var2)。然后向var2中写之前生成的随机数RANDOM_NUMBER_GENERATOR。然后另一条无非是var1监听连接请求生成的一条socketChannel(var3)。从var3中读取之前var2发送的随机数，如果传输成功(即前后收发的随机数相同)，如果无误，说明一条Pipe被成功建立起来。然后把var2设为source，var3设为sink。成功建立起Pipe后我们回到WindowsSelectorImpl的构造方法中
WindowsSelectorImpl(SelectorProvider var1) throws IOException {
super(var1);
this.wakeupSourceFd = ((SelChImpl)this.wakeupPipe.source()).getFDVal();
SinkChannelImpl var2 = (SinkChannelImpl)this.wakeupPipe.sink();
var2.sc.socket().setTcpNoDelay(true);
this.wakeupSinkFd = var2.getFDVal();
this.pollWrapper.addWakeupSocket(this.wakeupSourceFd, 0);
}得到source的fd跟sink的fd并保存，把wakeupSourceFd加到PoolWrapper中。
void addWakeupSocket(int var1, int var2) {
this.putDescriptor(var2, var1);
this.putEventOps(var2, Net.POLLIN);
}
void putDescriptor(int var1, int var2) {
this.pollArray.putInt(SIZE_POLLFD * var1 + 0, var2);
}
void putEventOps(int var1, int var2) {
this.pollArray.putShort(SIZE_POLLFD * var1 + 4, (short)var2);
}
这里将source的POLLIN事件标识为感兴趣的，当sink端有数据写入时，source对应的文件描述符wakeupSourceFd就会处于就绪状态。
到此为止selector算是创建好了。
下面插一张大图（网上找的），我们看着图把整个过程总结一下。
Selector selector = Selector.open()，默认情况下生成了一个WindowsSelectorImpl实例，并建立了Pipe，同时把Pipe的Source端传入到poolArray中。其中Pipe建立，先生成一个监听本地地址，0端口的serverSocket，再生成一个Socket连接ServerSocket，这个SocketChannel是source端，serverSocket的accept得到一个SocketChannel是sink端，于是从source向sink传一随机数，sink收到正确的后，证明Pipe建立成功。将Source端传入poolArray后，当sink端有数据传入时，source端对应的文件描述符wakeupSourceFd就会处于就绪状态。
真的是配合图片食用，口感更佳。