来看看selector最核心的select()方法吧
我们之前分析了Selector默认是WindowSelectorImpl的一个实例，调用selector(timeout)，timout是给Selector设置的时间参数，之后调用lockAndDoSelector，lockAndDoSelector方法核心就是调用抽象方法doSelect，此时加了synchronize同步。我们看下子类的doSelect的实现，默认实现是在WindowsSelectorImpl类下。
protected int doSelect(long var1) throws IOException {
if(this.channelArray == null) {
throw new ClosedSelectorException();
} else {
this.timeout = var1;
this.processDeregisterQueue();
if(this.interruptTriggered) {
this.resetWakeupSocket();
return 0;
} else {
this.adjustThreadsCount();
this.finishLock.reset();
this.startLock.startThreads();
try {
this.begin();
try {
this.subSelector.poll();
} catch (IOException var7) {
this.finishLock.setException(var7);
}
if(this.threads.size() > 0) {
this.finishLock.waitForHelperThreads();
}
} finally {
this.end();
}
this.finishLock.checkForException();
this.processDeregisterQueue();
int var3 = this.updateSelectedKeys();
this.resetWakeupSocket();
return var3;
}
}
}
首先调用processDeregisterQueue()方法，把已经解除注册的channel进行注销注册。
void processDeregisterQueue() throws IOException {
Set var1 = this.cancelledKeys();
synchronized(var1) {
if(!var1.isEmpty()) {
Iterator var3 = var1.iterator();
while(var3.hasNext()) {
SelectionKeyImpl var4 = (SelectionKeyImpl)var3.next();
try {
this.implDereg(var4);
} catch (SocketException var11) {
throw new IOException("Error deregistering key", var11);
} finally {
var3.remove();
}
}
}
}
}
遍历加到cancelledKeys中的SelectorKey(需要解除注册的key)，依次调用implDereg()进行注销。
protected void implDereg(SelectionKeyImpl var1) throws IOException {
int var2 = var1.getIndex();
assert var2 >= 0;
Object var3 = this.closeLock;
synchronized(this.closeLock) {
if(var2 != this.totalChannels - 1) {
SelectionKeyImpl var4 = this.channelArray[this.totalChannels - 1];
this.channelArray[var2] = var4;
var4.setIndex(var2);
this.pollWrapper.replaceEntry(this.pollWrapper, this.totalChannels - 1, this.pollWrapper, var2);
}
var1.setIndex(-1);
}
this.channelArray[this.totalChannels - 1] = null;
--this.totalChannels;
if(this.totalChannels != 1 && this.totalChannels % 1024 == 1) {
--this.totalChannels;
--this.threadsCount;
}
this.fdMap.remove(var1);
this.keys.remove(var1);
this.selectedKeys.remove(var1);
this.deregister(var1);
SelectableChannel var7 = var1.channel();
if(!var7.isOpen() && !var7.isRegistered()) {
((SelChImpl)var7).kill();
}
}
如果当前解除注册key的channel是Selector中最后一个，那么直接移除，否则，需要将当前需移除的channel跟最后一个channel调换位置，再移除，以保证数组的连续性，剩下无非是注册的逆操作。
完成真正注销后，然后调用adjustThreadCount()
private void adjustThreadsCount() {
int var1;
if(this.threadsCount > this.threads.size()) {
for(var1 = this.threads.size(); var1 < this.threadsCount; ++var1) {
WindowsSelectorImpl.SelectThread var2 = new WindowsSelectorImpl.SelectThread(var1);
this.threads.add(var2);
var2.setDaemon(true);
var2.start();
}
} else if(this.threadsCount < this.threads.size()) {
for(var1 = this.threads.size() - 1; var1 >= this.threadsCount; --var1) {
((WindowsSelectorImpl.SelectThread)this.threads.remove(var1)).makeZombie();
}
}
}
该方法是对于线程数量进行调整，要求的线程数量是根据channel数目统计的，每1024个channel注册，则多一个线程来负责。根据现有线程进行动态调整。
之后调用begin()方法
protected final void begin() {
if (interruptor == null) {
interruptor = new Interruptible() {
public void interrupt(Thread ignore) {
AbstractSelector.this.wakeup();
}};
}
AbstractInterruptibleChannel.blockedOn(interruptor);
Thread me = Thread.currentThread();
if (me.isInterrupted())
interruptor.interrupt(me);
}
这里先判断interruptor是否为空，空的话则创建一个，这个interruptor保证了：当前线程阻塞在io上时，当其被interruptor时，可以从select阻塞中被唤醒。在begin()完成后，将会调用其subSelector的poll，正式开始select过程。
private int poll() throws IOException {
return this.poll0(WindowsSelectorImpl.this.pollWrapper.pollArrayAddress,
Math.min(WindowsSelectorImpl.this.totalChannels, 1024),
this.readFds, this.writeFds, this.exceptFds, WindowsSelectorImpl.this.timeout);
}
private int poll(int var1) throws IOException {
return this.poll0(WindowsSelectorImpl.this.pollWrapper.pollArrayAddress +
(long)(this.pollArrayIndex * PollArrayWrapper.SIZE_POLLFD),
Math.min(1024, WindowsSelectorImpl.this.totalChannels - (var1 + 1) * 1024), this.readFds,
this.writeFds, this.exceptFds, WindowsSelectorImpl.this.timeout);
}
private native int poll0(long var1, int var3, int[] var4, int[] var5, int[] var6, long var7);
这里的poll0()是native方法。主要是为了监听pollWrapper中所保存的fd是否有数据进出，如果没有进出，则会在在此处在timeout的时间里一直保持阻塞状态。
当poll取得数据后，会重新检查一遍已经被取消注册的channel，并注销它，之后调用updateSelectedKeys()
private int updateSelectedKeys() {
++this.updateCount;
byte var1 = 0;
int var4 = var1 + this.subSelector.processSelectedKeys(this.updateCount);
WindowsSelectorImpl.SelectThread var3;
for(Iterator var2 = this.threads.iterator(); var2.hasNext();
var4 += var3.subSelector.processSelectedKeys(this.updateCount)) {
var3 = (WindowsSelectorImpl.SelectThread)var2.next();
}
return var4;
}这里对所有的线程调用processSelectedKeys来处理所有线程在poll过程中取得的结果并处理。然后返回所有线程中处理的channel数量的总和。我们来看下processSelectedKeys()方法
private int processSelectedKeys(long var1) {
byte var3 = 0;
int var4 = var3 + this.processFDSet(var1, this.readFds, Net.POLLIN, false);
var4 += this.processFDSet(var1, this.writeFds, Net.POLLCONN | Net.POLLOUT, false);
var4 += this.processFDSet(var1, this.exceptFds, Net.POLLIN | Net.POLLCONN | Net.POLLOUT, true);
return var4;
}
private int processFDSet(long var1, int[] var3, int var4, boolean var5) {
int var6 = 0;
for(int var7 = 1; var7 <= var3[0]; ++var7) {
int var8 = var3[var7];
if(var8 == WindowsSelectorImpl.this.wakeupSourceFd) {
synchronized(WindowsSelectorImpl.this.interruptLock) {
WindowsSelectorImpl.this.interruptTriggered = true;
}
} else {
WindowsSelectorImpl.MapEntry var9 = WindowsSelectorImpl.this.fdMap.get(var8);
if(var9 != null) {
SelectionKeyImpl var10 = var9.ski;
if(!var5 || !(var10.channel() instanceof SocketChannelImpl) ||
!WindowsSelectorImpl.this.discardUrgentData(var8)) {
if(WindowsSelectorImpl.this.selectedKeys.contains(var10)) {
if(var9.clearedCount != var1) {
if(var10.channel.translateAndSetReadyOps(var4, var10) && var9.updateCount != var1) {
var9.updateCount = var1;
++var6;
}
} else if(var10.channel.translateAndUpdateReadyOps(var4, var10)
&& var9.updateCount != var1) {
var9.updateCount = var1;
++var6;
}
var9.clearedCount = var1;
} else {
if(var9.clearedCount != var1) {
var10.channel.translateAndSetReadyOps(var4, var10);
if((var10.nioReadyOps() & var10.nioInterestOps()) != 0) {
WindowsSelectorImpl.this.selectedKeys.add(var10);
var9.updateCount = var1;
++var6;
}
} else {
var10.channel.translateAndUpdateReadyOps(var4, var10);
if((var10.nioReadyOps() & var10.nioInterestOps()) != 0) {
WindowsSelectorImpl.this.selectedKeys.add(var10);
var9.updateCount = var1;
++var6;
}
}
var9.clearedCount = var1;
}
}
}
}
}
return var6;
}
这里首先根据poll监听到发生io事件所需要处理的fd，根据fd从selector中读取相应的已经注册了的channel，根据所发生的io事件（读，写，异常）更新channel的状态。相应的Selector的select到这里算是结束了。
最后还想看一下WindowsSelectorImpl.wakeup()的具体实现
public Selector wakeup() {
Object var1 = this.interruptLock;
synchronized(this.interruptLock) {
if(!this.interruptTriggered) {
this.setWakeupSocket();
this.interruptTriggered = true;
}
return this;
}
}
private void setWakeupSocket() {
this.setWakeupSocket0(this.wakeupSinkFd);
}
private native void setWakeupSocket0(int var1);
最后找资料，windowsSelectorImpl.c中实现send(scoutFd, (char*)&POLLIN, 1, 0);这儿就跟我们之前selector.open中构造的pipe联系在了一起，向sink端写入了一个自己，注册在pollArray中的wakeupsource端的fd有变化，poll方法返回，并将线程从selector的select中唤醒。
小结
我觉得jdk 的nio的selector底层分析到这里算是完结撒花了，相信看完这几篇后那个图应该算是掌握了。
如果有机会，要去jdk的native方法底层再一探nio的究竟。在jdk层面分析也就到这里结束了。