上篇博文分析了selector的建立，在这之后selector跟ServerSocketChannel，SocketChannel配合使用，在这里我将会介绍在selector.open()；之后，我们以服务器端为切入点，重点关注下面两个方法的实现
ServerSocketChannel serverSocketChannel = ServerSocketChannel.open();
serverSocketChannel.register(selector, SelectionKeyImpl.OP_ACCEPT);
先看ServerSocket.open();
public static ServerSocketChannel open() throws IOException {
return SelectorProvider.provider().openServerSocketChannel();
}
provider这个我们很熟悉了吧？默认就是WindowsSelectorProvider的实例，继续。它的openServerSocketChannel方法的实现在它父类SelectorProviderImpl中。
public ServerSocketChannel openServerSocketChannel() throws IOException {
return new ServerSocketChannelImpl(this);
}其实这里我们都分析过，在创建Pipe时有用到这。既然来了就继续看下构造。
ServerSocketChannelImpl(SelectorProvider var1) throws IOException {
super(var1);
this.fd = Net.serverSocket(true);
this.fdVal = IOUtil.fdVal(this.fd);
this.state = 0;
}
里面无非构造了socket，并保存其fd。我们可以知道serverChannel=ServerSocketChannel.open();其实是指向其子类ServerSocketChannelImpl的实例，里面保存了socket实例，跟fd。继续。
serverChannel.configureBlocking(false);
serverChannel.socket().bind(new InetSocketAddress(port));我们使用serverChannel时设置其blocking为非阻塞，并为其serverSocket绑定一个ip地址跟端口。我们重点看下第二行，先看ServerSocketChannelImpl的.socket()
public ServerSocket socket() {
Object var1 = this.stateLock;
synchronized(this.stateLock) {
if(this.socket == null) {
this.socket = ServerSocketAdaptor.create(this);
}
return this.socket;
}
}socket是ServerSocktAdaptor(门面模式，他无非是ServerSocketChannelImpl的门面)的实例，利用同步保证其多线程安全且单例，其内部bind方法还是调用ServerSocketChannelImpl的bind方法。
public ServerSocketChannel bind(SocketAddress var1, int var2) throws IOException {
Object var3 = this.lock;
synchronized(this.lock) {
if(!this.isOpen()) {
throw new ClosedChannelException();
} else if(this.isBound()) {
throw new AlreadyBoundException();
} else {
InetSocketAddress var4 = var1 == null?new InetSocketAddress(0):Net.checkAddress(var1);
SecurityManager var5 = System.getSecurityManager();
if(var5 != null) {
var5.checkListen(var4.getPort());
}
NetHooks.beforeTcpBind(this.fd, var4.getAddress(), var4.getPort());
Net.bind(this.fd, var4.getAddress(), var4.getPort());
Net.listen(this.fd, var2 < 1?50:var2);
Object var6 = this.stateLock;
synchronized(this.stateLock) {
this.localAddress = Net.localAddress(this.fd);
}
return this;
}
}
}无非先验证channel是否打开，是否绑定过，最终调用Net底层的bind方法为其绑定端口跟地址。
private static native void bind0(FileDescriptor var0, boolean var1, boolean var2, InetAddress var3, int var4) throws IOException;总的来说，就是将 ServerSocketChannel 中的 ServerSocket绑定到指定的IP地址和端口上。
我们继续往下看这次的重点，channel.register(Selector sel, int ops,Object att)，其具体逻辑在AbstractSelectableChannel上
public final SelectionKey register(Selector sel, int ops,
Object att)
throws ClosedChannelException
{
synchronized (regLock) {
if (!isOpen())
throw new ClosedChannelException();
if ((ops & ~validOps()) != 0)
throw new IllegalArgumentException();
if (blocking)
throw new IllegalBlockingModeException();
SelectionKey k = findKey(sel);
if (k != null) {
k.interestOps(ops);
k.attach(att);
}
if (k == null) {
// New registration
synchronized (keyLock) {
if (!isOpen())
throw new ClosedChannelException();
k = ((AbstractSelector)sel).register(this, ops, att);
addKey(k);
}
}
return k;
}
}
我们一点点看，if (blocking)throw new IllegalBlockingModeException();（在这里，我们能看到为什么之前需要设置其blocking为非阻塞）。在channel中，有维护一个SelectionKey的数组，一个SelectionKey代表了一个channel跟一个selectord的注册关系。一开始现在keys中找，判断这个channel是否注册过该Selector，如果这个channel是第一次注册这个Selector，那么肯定找不到，逻辑很简单。
private SelectionKey findKey(Selector sel) {
synchronized (keyLock) {
if (keys == null)
return null;
for (int i = 0; i < keys.length; i++)
if ((keys[i] != null) && (keys[i].selector() == sel))
return keys[i];
return null;
}
}
如果一开始就找到了SelectorKey，那么只需要更新interestops跟attach。如果没有找到SelectorKey，那就需要对传进来的Selector进行注册。Selector的regist()方法实现在SelectorImpl中
protected final SelectionKey register(AbstractSelectableChannel var1, int var2, Object var3) {
if(!(var1 instanceof SelChImpl)) {
throw new IllegalSelectorException();
} else {
SelectionKeyImpl var4 = new SelectionKeyImpl((SelChImpl)var1, this);
var4.attach(var3);
Set var5 = this.publicKeys;
synchronized(this.publicKeys) {
this.implRegister(var4);
}
var4.interestOps(var2);
return var4;
}
}首先根据channel跟selector生成相应的SelectorKeyIml实例，其构造方法无非对传入的参数保存，然后把所需的attach实例保存至SelectorKey。然后调用子类的implRegister()并传入SelectorKeyIml实例。具体实现我们以windowsSelectorImpl为例子
protected void implRegister(SelectionKeyImpl var1) {
Object var2 = this.closeLock;
synchronized(this.closeLock) {
if(this.pollWrapper == null) {
throw new ClosedSelectorException();
} else {
this.growIfNeeded();
this.channelArray[this.totalChannels] = var1;
var1.setIndex(this.totalChannels);
this.fdMap.put(var1);
this.keys.add(var1);
this.pollWrapper.addEntry(this.totalChannels, var1);
++this.totalChannels;
}
}
}
这里对传入的SelectorKeyIml继续处理，以实现注册。
由于传入新的key，所以可能导致原来数组长度不够，如果需要扩容则扩容。
private void growIfNeeded() {
if(this.channelArray.length == this.totalChannels) {
int var1 = this.totalChannels * 2;
SelectionKeyImpl[] var2 = new SelectionKeyImpl[var1];
System.arraycopy(this.channelArray, 1, var2, 1, this.totalChannels - 1);
this.channelArray = var2;
this.pollWrapper.grow(var1);
}
if(this.totalChannels % 1024 == 0) {
this.pollWrapper.addWakeupSocket(this.wakeupSourceFd, this.totalChannels);
++this.totalChannels;
++this.threadsCount;
}
}
扩容则把数组(channelArray，poolArrayWrapper)长度扩大为原来的两倍。
然后把按照已经注册的channel的数量为下标将SelectorKeyIml(对于Selector来说SelectorKeyIml相当于封装了的channel)存在存channel的数组中，然后在SelectorKeyIml中记录channel注册在Selector中的位置索引，通过fdMap保存SelectorKeyIml
private WindowsSelectorImpl.MapEntry put(SelectionKeyImpl var1) {
return (WindowsSelectorImpl.MapEntry)this.put(new Integer(var1.channel.getFDVal()),
new WindowsSelectorImpl.MapEntry(var1));
}可以看到，fdMap以channel的fdVal为key，与具体的SelectionKey为值的键值对。最后想poolWrapper中添加映射，并自增totalchannels的数目。
void addEntry(int var1, SelectionKeyImpl var2) {
this.putDescriptor(var1, var2.channel.getFDVal());
}
在调用Selector的register后返回SelectorKey给channel，然后回到AbstractSelectableChannel中，最后调用addKey(k);
private void addKey(SelectionKey k) {
assert Thread.holdsLock(keyLock);
int i = 0;
if ((keys != null) && (keyCount < keys.length)) {
// Find empty element of key array
for (i = 0; i < keys.length; i++)
if (keys[i] == null)
break;
} else if (keys == null) {
keys = new SelectionKey[3];
} else {
// Grow key array
int n = keys.length * 2;
SelectionKey[] ks = new SelectionKey[n];
for (i = 0; i < keys.length; i++)
ks[i] = keys[i];
keys = ks;
i = keyCount;
}
keys[i] = k;
keyCount++;
}
在这如果keys大小ok，那么添加key至数组中，如果大小不够，扩容。最后return k到，完毕。
总结一下吧。其实使用channel.register(selector,ops)语句，channel.registe -> AbstractSelectableChannel.regist -> windowsSelectorimpl.implRegister()；将channel注册到指定的selector上，实际上是将channel内部的socket的fd保存到pollWrapper的数组上，那么我们大概能猜到selector.select()方法实现的逻辑就是遍历这个pollWrapper，查看其fd，得到其通道是否准备好(猜的);