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SelectionKey的引入

如我们在前面内容所讲,在学生确定之后,我们就要对其状态进行设定,然后再交由Selector进行管理,其状态的设定我们就通过SelectionKey来进行。

那这里我们先通过之前在Channel中并未仔细讲解的SelectableChannel下的register方法。我们前面有提到过, SelectableChannelchannel打造成可以通过Selector来进行多路复用。作为管理者,channel想要实现复用,就必须在管理者这里进行注册登记。所以,SelectableChannel下的register方法也就是我们值得二次关注的核心了,也是对接我们接下来内容的切入点,对于register方法的解读,请看我们之前的文章BIO到NIO源码的一些事儿之NIO 上赋予Channel可被多路复用的能力这一节的内容。

这里要记住的是SelectableChannel是对接channel特征(即SelectionKey)的关键所在,这有点类似于表设计,原本可以将特征什么的设定在一张表内,但为了操作更加具有针对性,即为了让代码功能更易于管理,就进行抽取并设计了第二张表,这个就有点像人体器官,整体上大家共同协作完成一件事,但器官内部自己专注于自己的主要特定功能,偶尔也具备其他器官的一些小功能。

由此,我们也就可以知道,SelectionKey表示一个SelectableChannelSelector关联的标记,可以简单理解为一个token。就好比是我们做权限管理系统用户登录后前台会从后台拿到的一个token一样,用户可以凭借此token来访问操作相应的资源信息。

//java.nio.channels.spi.AbstractSelectableChannel#register
public final SelectionKey register(Selector sel, int ops, Object att)
throws ClosedChannelException
{ ...
synchronized (regLock) {
...
synchronized (keyLock) {
...
SelectionKey k = findKey(sel);
if (k != null) {
k.attach(att);
k.interestOps(ops);
} else {
// New registration
k = ((AbstractSelector)sel).register(this, ops, att);
addKey(k);
}
return k;
}
}
}

结合上下两段源码,在每次Selector使用register方法注册channel时,都会创建并返回一个SelectionKey

//sun.nio.ch.SelectorImpl#register
@Override
protected final SelectionKey register(AbstractSelectableChannel ch,
int ops,
Object attachment)
{
if (!(ch instanceof SelChImpl))
throw new IllegalSelectorException();
SelectionKeyImpl k = new SelectionKeyImpl((SelChImpl)ch, this);
k.attach(attachment);

// register (if needed) before adding to key set
implRegister(k);

// add to the selector's key set, removing it immediately if the selector
// is closed. The key is not in the channel's key set at this point but
// it may be observed by a thread iterating over the selector's key set.
keys.add(k);
try {
k.interestOps(ops);
} catch (ClosedSelectorException e) {
assert ch.keyFor(this) == null;
keys.remove(k);
k.cancel();
throw e;
}
return k;
}

我们在BIO到NIO源码的一些事儿之NIO 上赋予Channel可被多路复用的能力这一节的内容知道,一旦注册到Selector上,Channel将一直保持注册直到其被解除注册。在解除注册的时候会解除Selector分配给Channel的所有资源。
也就是SelectionKey在其调用SelectionKey#channel方法,或这个key所代表的channel 关闭,抑或此key所关联的Selector关闭之前,都是有效。我们在前面的文章分析中也知道,取消一个SelectionKey,不会立刻从Selector移除,它将被添加到SelectorcancelledKeys这个Set集合中,以便在下一次选择操作期间删除,我们可以通过java.nio.channels.SelectionKey#isValid判断一个SelectionKey是否有效。

SelectionKey包含四个操作集,每个操作集用一个Int来表示,int值中的低四位的bit 用于表示channel支持的可选操作种类。

 
/**
* Operation-set bit for read operations.
*/
public static final int OP_READ = 1 << 0;

/**
* Operation-set bit for write operations.
*/
public static final int OP_WRITE = 1 << 2;

/**
* Operation-set bit for socket-connect operations.
*/
public static final int OP_CONNECT = 1 << 3;

/**
* Operation-set bit for socket-accept operations.
*/
public static final int OP_ACCEPT = 1 << 4;

interestOps

通过interestOps来确定了selector在下一个选择操作的过程中将测试哪些操作类别的准备情况,操作事件是否是channel关注的。interestOpsSelectionKey创建时,初始化为注册Selector时的ops值,这个值可通过sun.nio.ch.SelectionKeyImpl#interestOps(int)来改变,这点我们在SelectorImpl#register可以清楚的看到。

 //sun.nio.ch.SelectionKeyImpl
public final class SelectionKeyImpl
extends AbstractSelectionKey
{
private static final VarHandle INTERESTOPS =
ConstantBootstraps.fieldVarHandle(
MethodHandles.lookup(),
"interestOps",
VarHandle.class,
SelectionKeyImpl.class, int.class);

private final SelChImpl channel;
private final SelectorImpl selector;

private volatile int interestOps;
private volatile int readyOps;

// registered events in kernel, used by some Selector implementations
private int registeredEvents;

// index of key in pollfd array, used by some Selector implementations
private int index;

SelectionKeyImpl(SelChImpl ch, SelectorImpl sel) {
channel = ch;
selector = sel;
}
...
}

readyOps

readyOps表示通过Selector检测到channel已经准备就绪的操作事件。在SelectionKey创建时(即上面源码所示),readyOps值为0,在Selectorselect操作中可能会更新,但是需要注意的是我们不能直接调用来更新。

SelectionKeyreadyOps表示一个channel已经为某些操作准备就绪,但不能保证在针对这个就绪事件类型的操作过程中不会发生阻塞,即该操作所在线程有可能会发生阻塞。在完成select操作后,大部分情况下会立即对readyOps更新,此时readyOps值最准确,如果外部的事件或在该channel有IO操作,readyOps可能不准确。所以,我们有看到其是volatile类型。

SelectionKey定义了所有的操作事件,但是具体channel支持的操作事件依赖于具体的channel,即具体问题具体分析。
所有可选择的channel(即SelectableChannel的子类)都可以通过SelectableChannel#validOps方法,判断一个操作事件是否被channel所支持,即每个子类都会有对validOps的实现,返回一个数字,仅标识channel支持的哪些操作。尝试设置或测试一个不被channel所支持的操作设定,将会抛出相关的运行时异常。
不同应用场景下,其所支持的Ops是不同的,摘取部分如下所示:

//java.nio.channels.SocketChannel#validOps
public final int validOps() {
//即1|4|8 1101
return (SelectionKey.OP_READ
| SelectionKey.OP_WRITE
| SelectionKey.OP_CONNECT);
}
//java.nio.channels.ServerSocketChannel#validOps
public final int validOps() {
// 16
return SelectionKey.OP_ACCEPT;
}
//java.nio.channels.DatagramChannel#validOps
public final int validOps() {
// 1|4
return (SelectionKey.OP_READ
| SelectionKey.OP_WRITE);
}

如果需要经常关联一些我们程序中指定数据到SelectionKey,比如一个我们使用一个object表示上层的一种高级协议的状态,object用于通知实现协议处理器。所以,SelectionKey支持通过attach方法将一个对象附加到SelectionKeyattachment上。attachment可以通过java.nio.channels.SelectionKey#attachment方法进行访问。如果要取消该对象,则可以通过该种方式:selectionKey.attach(null)

需要注意的是如果附加的对象不再使用,一定要人为清除,如果没有,假如此SelectionKey一直存在,由于此处属于强引用,那么垃圾回收器不会回收该对象,若不清除的话会成内存泄漏。

SelectionKey在由多线程并发使用时,是线程安全的。我们只需要知道,Selectorselect操作会一直使用在调用该操作开始时当前的interestOps所设定的值。

Selector探究

到现在为止,我们已经多多少少接触了Selector,其是一个什么样的角色,想必都很清楚了,那我们就在我们已经接触到的来进一步深入探究Selector的设计运行机制。

Selector的open方法

从命名上就可以知道 SelectableChannel对象是依靠Selector来实现多路复用的。
我们可以通过调用java.nio.channels.Selector#open来创建一个selector对象:

//java.nio.channels.Selector#open
public static Selector open() throws IOException {
return SelectorProvider.provider().openSelector();
}

关于这个SelectorProvider.provider(),其使用了根据所在系统的默认实现,我这里是windows系统,那么其默认实现为sun.nio.ch.WindowsSelectorProvider,这样,就可以调用基于相应系统的具体实现了。

//java.nio.channels.spi.SelectorProvider#provider
public static SelectorProvider provider() {
synchronized (lock) {
if (provider != null)
return provider;
return AccessController.doPrivileged(
new PrivilegedAction<>() {
public SelectorProvider run() {
if (loadProviderFromProperty())
return provider;
if (loadProviderAsService())
return provider;
provider = sun.nio.ch.DefaultSelectorProvider.create();
return provider;
}
});
}
}
//sun.nio.ch.DefaultSelectorProvider
public class DefaultSelectorProvider {

/**
* Prevent instantiation.
*/
private DefaultSelectorProvider() { }

/**
* Returns the default SelectorProvider.
*/
public static SelectorProvider create() {
return new sun.nio.ch.WindowsSelectorProvider();
}

}

基于windows来讲,selector这里最终会使用sun.nio.ch.WindowsSelectorImpl来做一些核心的逻辑。

public class WindowsSelectorProvider extends SelectorProviderImpl {

public AbstractSelector openSelector() throws IOException {
return new WindowsSelectorImpl(this);
}
}

这里,我们需要来看一下WindowsSelectorImpl的构造函数:

//sun.nio.ch.WindowsSelectorImpl#WindowsSelectorImpl
WindowsSelectorImpl(SelectorProvider sp) throws IOException {
super(sp);
pollWrapper = new PollArrayWrapper(INIT_CAP);
wakeupPipe = Pipe.open();
wakeupSourceFd = ((SelChImpl)wakeupPipe.source()).getFDVal();

// Disable the Nagle algorithm so that the wakeup is more immediate
SinkChannelImpl sink = (SinkChannelImpl)wakeupPipe.sink();
(sink.sc).socket().setTcpNoDelay(true);
wakeupSinkFd = ((SelChImpl)sink).getFDVal();

pollWrapper.addWakeupSocket(wakeupSourceFd, 0);
}

我们由Pipe.open()就可知道selector会保持打开的状态,直到其调用它的close方法:

//java.nio.channels.spi.AbstractSelector#close
public final void close() throws IOException {
boolean open = selectorOpen.getAndSet(false);
if (!open)
return;
implCloseSelector();
}
//sun.nio.ch.SelectorImpl#implCloseSelector
@Override
public final void implCloseSelector() throws IOException {
wakeup();
synchronized (this) {
implClose();
synchronized (publicSelectedKeys) {
// Deregister channels
Iterator<SelectionKey> i = keys.iterator();
while (i.hasNext()) {
SelectionKeyImpl ski = (SelectionKeyImpl)i.next();
deregister(ski);
SelectableChannel selch = ski.channel();
if (!selch.isOpen() && !selch.isRegistered())
((SelChImpl)selch).kill();
selectedKeys.remove(ski);
i.remove();
}
assert selectedKeys.isEmpty() && keys.isEmpty();
}
}
}
//sun.nio.ch.WindowsSelectorImpl#implClose
@Override
protected void implClose() throws IOException {
assert !isOpen();
assert Thread.holdsLock(this);

// prevent further wakeup
synchronized (interruptLock) {
interruptTriggered = true;
}

wakeupPipe.sink().close();
wakeupPipe.source().close();
pollWrapper.free();

// Make all remaining helper threads exit
for (SelectThread t: threads)
t.makeZombie();
startLock.startThreads();
}

可以看到,前面的wakeupPipe在close方法中关闭掉了。这里的close方法中又涉及了wakeupPipe.sink()wakeupPipe.source()的关闭与pollWrapper.free()的释放,此处也是我们本篇的难点所在,这里,我们来看看它们到底是什么样的存在。
首先,我们对WindowsSelectorImpl(SelectorProvider sp)这个构造函数做下梳理:

  • 创建一个PollArrayWrapper对象(pollWrapper);
  • Pipe.open()打开一个管道;
  • 拿到wakeupSourceFdwakeupSinkFd两个文件描述符;
  • 把pipe内Source端的文件描述符(wakeupSourceFd)放到pollWrapper里;

    Pipe.open()的解惑

    这里我们会有疑惑,为什么要创建一个管道,它是用来做什么的。

我们来看Pipe.open()源码实现:

//java.nio.channels.Pipe#open
public static Pipe open() throws IOException {
return SelectorProvider.provider().openPipe();
}
//sun.nio.ch.SelectorProviderImpl#openPipe
public Pipe openPipe() throws IOException {
return new PipeImpl(this);
}
//sun.nio.ch.PipeImpl#PipeImpl
PipeImpl(final SelectorProvider sp) throws IOException {
try {
AccessController.doPrivileged(new Initializer(sp));
} catch (PrivilegedActionException x) {
throw (IOException)x.getCause();
}
}
private class Initializer
implements PrivilegedExceptionAction<Void>
{

private final SelectorProvider sp;

private IOException ioe = null;

private Initializer(SelectorProvider sp) {
this.sp = sp;
}

@Override
public Void run() throws IOException {
LoopbackConnector connector = new LoopbackConnector();
connector.run();
if (ioe instanceof ClosedByInterruptException) {
ioe = null;
Thread connThread = new Thread(connector) {
@Override
public void interrupt() {}
};
connThread.start();
for (;;) {
try {
connThread.join();
break;
} catch (InterruptedException ex) {}
}
Thread.currentThread().interrupt();
}

if (ioe != null)
throw new IOException("Unable to establish loopback connection", ioe);

return null;
}

从上述源码我们可以知道,创建了一个PipeImpl对象, 在PipeImpl的构造函数里会执行AccessController.doPrivileged,在它调用后紧接着会执行Initializerrun方法:

//sun.nio.ch.PipeImpl.Initializer.LoopbackConnector
private class LoopbackConnector implements Runnable {

@Override
public void run() {
ServerSocketChannel ssc = null;
SocketChannel sc1 = null;
SocketChannel sc2 = null;

try {
// Create secret with a backing array.
ByteBuffer secret = ByteBuffer.allocate(NUM_SECRET_BYTES);
ByteBuffer bb = ByteBuffer.allocate(NUM_SECRET_BYTES);

// Loopback address
InetAddress lb = InetAddress.getLoopbackAddress();
assert(lb.isLoopbackAddress());
InetSocketAddress sa = null;
for(;;) {
// Bind ServerSocketChannel to a port on the loopback
// address
if (ssc == null || !ssc.isOpen()) {
ssc = ServerSocketChannel.open();
ssc.socket().bind(new InetSocketAddress(lb, 0));
sa = new InetSocketAddress(lb, ssc.socket().getLocalPort());
}

// Establish connection (assume connections are eagerly
// accepted)
sc1 = SocketChannel.open(sa);
RANDOM_NUMBER_GENERATOR.nextBytes(secret.array());
do {
sc1.write(secret);
} while (secret.hasRemaining());
secret.rewind();

// Get a connection and verify it is legitimate
sc2 = ssc.accept();
do {
sc2.read(bb);
} while (bb.hasRemaining());
bb.rewind();

if (bb.equals(secret))
break;

sc2.close();
sc1.close();
}

// Create source and sink channels
source = new SourceChannelImpl(sp, sc1);
sink = new SinkChannelImpl(sp, sc2);
} catch (IOException e) {
try {
if (sc1 != null)
sc1.close();
if (sc2 != null)
sc2.close();
} catch (IOException e2) {}
ioe = e;
} finally {
try {
if (ssc != null)
ssc.close();
} catch (IOException e2) {}
}
}
}
}

这里即为创建pipe的过程,windows下的实现是创建两个本地的socketChannel,然后连接(连接的过程通过写一个随机数据做两个socket的连接校验),两个socketChannel分别实现了管道pipesourcesink端。
而我们依然不清楚这个pipe到底干什么用的,
假如大家熟悉系统调用的C/C++的话,就可以知道,一个阻塞在select上的线程有以下三种方式可以被唤醒:

  1. 有数据可读/写,或出现异常。
  2. 阻塞时间到,即time out
  3. 收到一个non-block的信号。可由killpthread_kill发出。

所以,Selector.wakeup()要唤醒阻塞的select,那么也只能通过这三种方法,其中:

  • 第二种方法可以排除,因为select一旦阻塞,无法修改其time out时间。
  • 而第三种看来只能在Linux上实现,Windows上没有这种信号通知的机制。

看来只有第一种方法了。假如我们多次调用Selector.open(),那么在Windows上会每调用一次,就会建立一对自己和自己的loopbackTCP连接;在Linux上的话,每调用一次,会开一对pipe(pipe在Linux下一般都成对打开),到这里,估计我们能够猜得出来——那就是如果想要唤醒select,只需要朝着自己的这个loopback连接发点数据过去,于是,就可以唤醒阻塞在select上的线程了。

我们对上面所述做下总结:在Windows下,Java虚拟机在Selector.open()时会自己和自己建立loopbackTCP连接;在Linux下,Selector会创建pipe。这主要是为了Selector.wakeup()可以方便唤醒阻塞在select()系统调用上的线程(通过向自己所建立的TCP链接和管道上随便写点什么就可以唤醒阻塞线程)。

PollArrayWrapper解读

WindowsSelectorImpl构造器最后,我们看到这一句代码:pollWrapper.addWakeupSocket(wakeupSourceFd, 0);,即把pipe内Source端的文件描述符(wakeupSourceFd)放到pollWrapper里。pollWrapper作为PollArrayWrapper的实例,它到底是什么,这一节,我们就来对其探索一番。

class PollArrayWrapper {

private AllocatedNativeObject pollArray; // The fd array

long pollArrayAddress; // pollArrayAddress

@Native private static final short FD_OFFSET = 0; // fd offset in pollfd
@Native private static final short EVENT_OFFSET = 4; // events offset in pollfd

static short SIZE_POLLFD = 8; // sizeof pollfd struct

private int size; // Size of the pollArray

PollArrayWrapper(int newSize) {
int allocationSize = newSize * SIZE_POLLFD;
pollArray = new AllocatedNativeObject(allocationSize, true);
pollArrayAddress = pollArray.address();
this.size = newSize;
}

...

// Access methods for fd structures
void putDescriptor(int i, int fd) {
pollArray.putInt(SIZE_POLLFD * i + FD_OFFSET, fd);
}

void putEventOps(int i, int event) {
pollArray.putShort(SIZE_POLLFD * i + EVENT_OFFSET, (short)event);
}
...
// Adds Windows wakeup socket at a given index.
void addWakeupSocket(int fdVal, int index) {
putDescriptor(index, fdVal);
putEventOps(index, Net.POLLIN);
}
}

这里将wakeupSourceFdPOLLIN事件标识为pollArrayEventOps的对应的值,这里使用的是unsafe直接操作的内存,也就是相对于这个pollArray所在内存地址的偏移量SIZE_POLLFD * i + EVENT_OFFSET这个位置上写入Net.POLLIN所代表的值,即参考下面本地方法相关源码所展示的值。putDescriptor同样是这种类似操作。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。

//java.base/windows/native/libnio/ch/nio_util.h
/* WSAPoll()/WSAPOLLFD and the corresponding constants are only defined */
/* in Windows Vista / Windows Server 2008 and later. If we are on an */
/* older release we just use the Solaris constants as this was previously */
/* done in PollArrayWrapper.java. */
#define POLLIN 0x0001
#define POLLOUT 0x0004
#define POLLERR 0x0008
#define POLLHUP 0x0010
#define POLLNVAL 0x0020
#define POLLCONN 0x0002

AllocatedNativeObject这个类的父类有大量的unsafe类的操作,这些都是直接基于内存级别的操作。从其父类的构造器中,我们能也清楚的看到pollArray是通过unsafe.allocateMemory(size + ps)分配的一块系统内存。

class AllocatedNativeObject                             // package-private
extends NativeObject
{
/**
* Allocates a memory area of at least {@code size} bytes outside of the
* Java heap and creates a native object for that area.
*/
AllocatedNativeObject(int size, boolean pageAligned) {
super(size, pageAligned);
}

/**
* Frees the native memory area associated with this object.
*/
synchronized void free() {
if (allocationAddress != 0) {
unsafe.freeMemory(allocationAddress);
allocationAddress = 0;
}
}

}
//sun.nio.ch.NativeObject#NativeObject(int, boolean)
protected NativeObject(int size, boolean pageAligned) {
if (!pageAligned) {
this.allocationAddress = unsafe.allocateMemory(size);
this.address = this.allocationAddress;
} else {
int ps = pageSize();
long a = unsafe.allocateMemory(size + ps);
this.allocationAddress = a;
this.address = a + ps - (a & (ps - 1));
}
}

至此,我们算是完成了对Selector.open()的解读,其主要任务就是完成建立Pipe,并把pipe source端的wakeupSourceFd放入pollArray中,这个pollArraySelector完成其角色任务的枢纽。本篇主要围绕Windows的实现来进行分析,即在windows下通过两个连接的socketChannel实现了Pipelinux下则直接使用系统的pipe即可。

SelectionKey在selector中的管理

SelectionKey在selector中注册

所谓的注册,其实就是将一个对象放到注册地对象内的一个容器字段上,这个字段可以是数组,队列,也可以是一个set集合,也可以是一个list。这里,同样是这样,只不过,其需要有个返回值,那么把这个要放入集合的对象返回即可。

//sun.nio.ch.SelectorImpl#register
@Override
protected final SelectionKey register(AbstractSelectableChannel ch,
int ops,
Object attachment)
{
if (!(ch instanceof SelChImpl))
throw new IllegalSelectorException();
SelectionKeyImpl k = new SelectionKeyImpl((SelChImpl)ch, this);
k.attach(attachment);

// register (if needed) before adding to key set
implRegister(k);

// add to the selector's key set, removing it immediately if the selector
// is closed. The key is not in the channel's key set at this point but
// it may be observed by a thread iterating over the selector's key set.
keys.add(k);
try {
k.interestOps(ops);
} catch (ClosedSelectorException e) {
assert ch.keyFor(this) == null;
keys.remove(k);
k.cancel();
throw e;
}
return k;
}
//sun.nio.ch.WindowsSelectorImpl#implRegister
@Override
protected void implRegister(SelectionKeyImpl ski) {
ensureOpen();
synchronized (updateLock) {
newKeys.addLast(ski);
}
}

这段代码我们之前已经有看过,这里我们再次温习下。
首先会新建一个SelectionKeyImpl对象,这个对象就是对Channel的包装,不仅如此,还顺带把当前这个Selector对象给收了进去,这样,我们也可以通过SelectionKey的对象来拿到其对应的Selector对象。

接着,基于windows平台实现的implRegister,先通过ensureOpen()来确保该Selector是打开的。接着将这个SelectionKeyImpl加入到WindowsSelectorImpl内针对于新注册SelectionKey进行管理的newKeys之中,newKeys是一个ArrayDeque对象。对于ArrayDeque有不懂的,可以参考Java 容器源码分析之 Deque 与 ArrayDeque这篇文章。

然后再将此这个SelectionKeyImpl加入到sun.nio.ch.SelectorImpl#keys中去,这个Set<SelectionKey>集合代表那些已经注册到当前这个Selector对象上的SelectionKey集合。我们来看sun.nio.ch.SelectorImpl的构造函数:

//sun.nio.ch.SelectorImpl#SelectorImpl
protected SelectorImpl(SelectorProvider sp) {
super(sp);
keys = ConcurrentHashMap.newKeySet();
selectedKeys = new HashSet<>();
publicKeys = Collections.unmodifiableSet(keys);
publicSelectedKeys = Util.ungrowableSet(selectedKeys);
}

也就是说,这里的publicKeys就来源于keys,只是publicKeys属于只读的,我们想要知道当前Selector对象上所注册的keys,就可以调用sun.nio.ch.SelectorImpl#keys来得到:

//sun.nio.ch.SelectorImpl#keys
@Override
public final Set<SelectionKey> keys() {
ensureOpen();
return publicKeys;
}

再回到这个构造函数中,selectedKeys,顾名思义,其属于已选择Keys,即前一次操作期间,已经准备就绪的Channel所对应的SelectionKey。此集合为keys的子集。通过selector.selectedKeys()获取。

//sun.nio.ch.SelectorImpl#selectedKeys
@Override
public final Set<SelectionKey> selectedKeys() {
ensureOpen();
return publicSelectedKeys;
}

我们看到其返回的是publicSelectedKeys,针对这个字段里的元素操作可以做删除,但不能做增加。
在前面的内容中,我们有涉及到SelectionKey的取消,所以,我们在java.nio.channels.spi.AbstractSelector方法内,是有定义cancelledKeys的,也是一个HashSet对象。其代表已经被取消但尚未取消注册(deregister)的SelectionKey。此Set集合无法直接访问,同样,它也是keys()的子集。

对于新的Selector实例,上面几个集合均为空。由上面展示的源码可知,通过channel.registerSelectionKey添加keys中,此为key的来源。
如果某个selectionKey.cancel()被调用,那么此key将会被添加到cancelledKeys这个集合中,然后在下一次调用selector select方法期间,此时canceldKeys不为空,将会触发此SelectionKeyderegister操作(释放资源,并从keys中移除)。无论通过channel.close()还是通过selectionKey.cancel(),都会导致SelectionKey被加入到cannceldKey中.

每次选择操作(select)期间,都可以将key添加到selectedKeys中或者将从cancelledKeys中移除。

Selector的select方法的解读

了解了上面的这些,我们来进入到select方法中,观察下它的细节。由Selector的api可知,select操作有两种形式,一种为
select(),selectNow(),select(long timeout);另一种为select(Consumer<SelectionKey> action, long timeout)select(Consumer<SelectionKey> action)selectNow(Consumer<SelectionKey> action)。后者为JDK11新加入的api,主要针对那些准备好进行I/O操作的channels在select过程中对相应的key进行的一个字的自定义的一个操作。
需要注意的是,有Consumer<SelectionKey> action参数的select操作是阻塞的,只有在选择了至少一个Channel的情况下,才会调用此Selector实例的wakeup方法来唤醒,同样,其所在线程被打断也可以。

//sun.nio.ch.SelectorImpl
@Override
public final int select(long timeout) throws IOException {
if (timeout < 0)
throw new IllegalArgumentException("Negative timeout");
return lockAndDoSelect(null, (timeout == 0) ? -1 : timeout);
}

//sun.nio.ch.SelectorImpl
@Override
public final int select(Consumer<SelectionKey> action, long timeout)
throws IOException
{
Objects.requireNonNull(action);
if (timeout < 0)
throw new IllegalArgumentException("Negative timeout");
return lockAndDoSelect(action, (timeout == 0) ? -1 : timeout);
}
//sun.nio.ch.SelectorImpl#lockAndDoSelect
private int lockAndDoSelect(Consumer<SelectionKey> action, long timeout)
throws IOException
{
synchronized (this) {
ensureOpen();
if (inSelect)
throw new IllegalStateException("select in progress");
inSelect = true;
try {
synchronized (publicSelectedKeys) {
return doSelect(action, timeout);
}
} finally {
inSelect = false;
}
}
}

我们可以观察,无论哪种,它们最后都落在了lockAndDoSelect这个方法上,最终会执行特定系统上的doSelect(action, timeout)实现。
这里我们以sun.nio.ch.WindowsSelectorImpl#doSelect为例来讲述其操作执行的步骤:

// sun.nio.ch.WindowsSelectorImpl#doSelect
@Override
protected int doSelect(Consumer<SelectionKey> action, long timeout)
throws IOException
{
assert Thread.holdsLock(this);
this.timeout = timeout; // set selector timeout
processUpdateQueue(); // <1>
processDeregisterQueue(); // <2>
if (interruptTriggered) {
resetWakeupSocket();
return 0;
}
// Calculate number of helper threads needed for poll. If necessary
// threads are created here and start waiting on startLock
adjustThreadsCount();
finishLock.reset(); // reset finishLock
// Wakeup helper threads, waiting on startLock, so they start polling.
// Redundant threads will exit here after wakeup.
startLock.startThreads();
// do polling in the main thread. Main thread is responsible for
// first MAX_SELECTABLE_FDS entries in pollArray.
try {
begin();
try {
subSelector.poll(); // <3>
} catch (IOException e) {
finishLock.setException(e); // Save this exception
}
// Main thread is out of poll(). Wakeup others and wait for them
if (threads.size() > 0)
finishLock.waitForHelperThreads();
} finally {
end();
}
// Done with poll(). Set wakeupSocket to nonsignaled for the next run.
finishLock.checkForException();
processDeregisterQueue(); // <4>
int updated = updateSelectedKeys(action); // <5>
// Done with poll(). Set wakeupSocket to nonsignaled for the next run.
resetWakeupSocket(); // <6>
return updated;
}

processUpdateQueue解读

  1. 首先通过相应操作系统实现类(此处是WindowsSelectorImpl)的具体实现我们可以知道,通过<1> 处的 processUpdateQueue()获得关于每个剩余Channel(有些Channel取消了)的在此刻的interestOps,这里包括新注册的和updateKeys,并对其进行pollWrapper的管理操作。

    • 即对于新注册的SelectionKeyImpl,我们在相对于这个pollArray所在内存地址的偏移量SIZE_POLLFD * totalChannels + FD_OFFSETSIZE_POLLFD * totalChannels + EVENT_OFFSET分别存入SelectionKeyImpl的文件描述符fd与其对应的EventOps(初始为0)。

    • updateKeys,因为是其之前已经在pollArray的某个相对位置上存储过,这里我们还需要对拿到的key的有效性进行判断,如果有效,只需要将正在操作的这个SelectionKeyImpl对象的interestOps写入到在pollWrapper中的存放它的EventOps位置上。

    注意: 在对newKeys进行key的有效性判断之后,如果有效,会调用growIfNeeded()方法,这里首先会判断channelArray.length == totalChannels,此为一个SelectionKeyImpl的数组,初始容量大小为8。channelArray其实就是方便Selector管理在册SelectionKeyImpl数量的一个数组而已,通过判断它的数组长度大小,如果和totalChannels(初始值为1)相等,不仅仅是为了channelArray扩容,更重要的是为了辅助pollWrapper,让pollWrapper扩容才是这里的目的所在。
    而当totalChannels % MAX_SELECTABLE_FDS == 0时,则多开一个线程处理selectorwindowsselect系统调用有最大文件描述符限制,一次只能轮询1024个文件描述符,如果多于1024个,需要多线程进行轮询。通过ski.setIndex(totalChannels)选择键记录下在数组中的索引位置SelectionKeyImpl选择键的映射关系,以待后续使用。同时调用pollWrapper.addWakeupSocket(wakeupSourceFd, totalChannels)在相对于这个pollArray所在内存地址的偏移量SIZE_POLLFD * totalChannels + FD_OFFSET这个位置上写入wakeupSourceFd所代表的fdVal值。这样在新起的线程就可以通过MAX_SELECTABLE_FDS来确定这个用来监控的wakeupSourceFd

       /**
    * sun.nio.ch.WindowsSelectorImpl#processUpdateQueue
    * Process new registrations and changes to the interest ops.
    */
    private void processUpdateQueue() {
    assert Thread.holdsLock(this);

    synchronized (updateLock) {
    SelectionKeyImpl ski;

    // new registrations
    while ((ski = newKeys.pollFirst()) != null) {
    if (ski.isValid()) {
    growIfNeeded();
    channelArray[totalChannels] = ski;
    ski.setIndex(totalChannels);
    pollWrapper.putEntry(totalChannels, ski);
    totalChannels++;
    MapEntry previous = fdMap.put(ski);
    assert previous == null;
    }
    }

    // changes to interest ops
    while ((ski = updateKeys.pollFirst()) != null) {
    int events = ski.translateInterestOps();
    int fd = ski.getFDVal();
    if (ski.isValid() && fdMap.containsKey(fd)) {
    int index = ski.getIndex();
    assert index >= 0 && index < totalChannels;
    pollWrapper.putEventOps(index, events);
    }
    }
    }
    }

    //sun.nio.ch.PollArrayWrapper#putEntry
    // Prepare another pollfd struct for use.
    void putEntry(int index, SelectionKeyImpl ski) {
    putDescriptor(index, ski.getFDVal());
    putEventOps(index, 0);
    }
    //sun.nio.ch.WindowsSelectorImpl#growIfNeeded
    private void growIfNeeded() {
    if (channelArray.length == totalChannels) {
    int newSize = totalChannels * 2; // Make a larger array
    SelectionKeyImpl temp[] = new SelectionKeyImpl[newSize];
    System.arraycopy(channelArray, 1, temp, 1, totalChannels - 1);
    channelArray = temp;
    pollWrapper.grow(newSize);
    }
    if (totalChannels % MAX_SELECTABLE_FDS == 0) { // more threads needed
    pollWrapper.addWakeupSocket(wakeupSourceFd, totalChannels);
    totalChannels++;
    threadsCount++;
    }
    }
    // Initial capacity of the poll array
    private final int INIT_CAP = 8;
    // Maximum number of sockets for select().
    // Should be INIT_CAP times a power of 2
    private static final int MAX_SELECTABLE_FDS = 1024;

    // The list of SelectableChannels serviced by this Selector. Every mod
    // MAX_SELECTABLE_FDS entry is bogus, to align this array with the poll
    // array, where the corresponding entry is occupied by the wakeupSocket
    private SelectionKeyImpl[] channelArray = new SelectionKeyImpl[INIT_CAP];
    // The number of valid entries in poll array, including entries occupied
    // by wakeup socket handle.
    private int totalChannels = 1;

    //sun.nio.ch.PollArrayWrapper#grow
    // Grows the pollfd array to new size
    void grow(int newSize) {
    PollArrayWrapper temp = new PollArrayWrapper(newSize);
    for (int i = 0; i < size; i++)
    replaceEntry(this, i, temp, i);
    pollArray.free();
    pollArray = temp.pollArray;
    this.size = temp.size;
    pollArrayAddress = pollArray.address();
    }

    // Maps file descriptors to their indices in pollArray
    private static final class FdMap extends HashMap<Integer, MapEntry> {
    static final long serialVersionUID = 0L;
    private MapEntry get(int desc) {
    return get(Integer.valueOf(desc));
    }
    private MapEntry put(SelectionKeyImpl ski) {
    return put(Integer.valueOf(ski.getFDVal()), new MapEntry(ski));
    }
    private MapEntry remove(SelectionKeyImpl ski) {
    Integer fd = Integer.valueOf(ski.getFDVal());
    MapEntry x = get(fd);
    if ((x != null) && (x.ski.channel() == ski.channel()))
    return remove(fd);
    return null;
    }
    }

    // class for fdMap entries
    private static final class MapEntry {
    final SelectionKeyImpl ski;
    long updateCount = 0;
    MapEntry(SelectionKeyImpl ski) {
    this.ski = ski;
    }
    }
    private final FdMap fdMap = new FdMap();

processDeregisterQueue解读

  1. 接着通过上面WindowsSelectorImpl#doSelect展示源码中<2> 处的 processDeregisterQueue()
    • cancelledKeys进行清除,遍历cancelledKeys,并对每个key进行deregister操作,然后从cancelledKeys集合中删除,从keys集合与selectedKeys中删除,以此来释放引用,方便gc回收,
    • 其内调用implDereg方法,将会从channelArray中移除对应的Channel代表的SelectionKeyImpl,调整totalChannels和线程数,从mapkeys中移除SelectionKeyImpl,移除Channel上的SelectionKeyImpl并关闭Channel
    • 同时还发现该processDeregisterQueue()方法在调用poll方法前后都进行调用,这是确保能够正确处理在调用poll方法阻塞的这一段时间之内取消的键能被及时清理。
    • 最后,还会判断这个cancelledKey所代表的channel是否打开和解除注册,如果关闭并解除注册,则应该将相应的文件描述符对应占用的资源给关闭掉。
         /**
      * sun.nio.ch.SelectorImpl#processDeregisterQueue
      * Invoked by selection operations to process the cancelled-key set
      */
      protected final void processDeregisterQueue() throws IOException {
      assert Thread.holdsLock(this);
      assert Thread.holdsLock(publicSelectedKeys);

      Set<SelectionKey> cks = cancelledKeys();
      synchronized (cks) {
      if (!cks.isEmpty()) {
      Iterator<SelectionKey> i = cks.iterator();
      while (i.hasNext()) {
      SelectionKeyImpl ski = (SelectionKeyImpl)i.next();
      i.remove();

      // remove the key from the selector
      implDereg(ski);

      selectedKeys.remove(ski);
      keys.remove(ski);

      // remove from channel's key set
      deregister(ski);

      SelectableChannel ch = ski.channel();
      if (!ch.isOpen() && !ch.isRegistered())
      ((SelChImpl)ch).kill();
      }
      }
      }
      }
      //sun.nio.ch.WindowsSelectorImpl#implDereg
      @Override
      protected void implDereg(SelectionKeyImpl ski) {
      assert !ski.isValid();
      assert Thread.holdsLock(this);

      if (fdMap.remove(ski) != null) {
      int i = ski.getIndex();
      assert (i >= 0);

      if (i != totalChannels - 1) {
      // Copy end one over it
      SelectionKeyImpl endChannel = channelArray[totalChannels-1];
      channelArray[i] = endChannel;
      endChannel.setIndex(i);
      pollWrapper.replaceEntry(pollWrapper, totalChannels-1, pollWrapper, i);
      }
      ski.setIndex(-1);

      channelArray[totalChannels - 1] = null;
      totalChannels--;
      if (totalChannels != 1 && totalChannels % MAX_SELECTABLE_FDS == 1) {
      totalChannels--;
      threadsCount--; // The last thread has become redundant.
      }
      }
      }

      //sun.nio.ch.SocketChannelImpl#kill
      @Override
      public void kill() throws IOException {
      synchronized (stateLock) {
      if (state == ST_KILLPENDING) {
      state = ST_KILLED;
      nd.close(fd);
      }
      }
      }
      //C:/Program Files/Java/jdk-11.0.1/lib/src.zip!/java.base/sun/nio/ch/SocketChannelImpl.java:1126
      static {
      IOUtil.load();
      nd = new SocketDispatcher();
      }
      //sun.nio.ch.SocketDispatcher#close
      void close(FileDescriptor fd) throws IOException {
      close0(fd);
      }

adjustThreadsCount解读

  1. 接着我们来看到上面WindowsSelectorImpl#doSelect展示源码中adjustThreadsCount()方法的调用。
    • 前面有提到如果totalChannels % MAX_SELECTABLE_FDS == 0,则多开一个线程处理selector。这里就是根据分配的线程数量值来增加或减少线程,其实就是针对操作系统的最大select操作的文件描述符限制对线程个数进行调整。
    • 我们来观察所建线程做了什么事情,即观察SelectThreadrun方法实现。通过观察其源码可以看到它首先是while (true),通过startLock.waitForStart(this)来控制该线程是否运行还是等待,运行状态的话,会进而调用subSelector.poll(index)(这个我们后面内容详细解读),
    • 当此线程poll结束,而且相对于当前主线程假如有多条SelectThread子线程的话,当前这条SelectThread线程第一个结束poll的话,就调用finishLock.threadFinished()来通知主线程。在刚新建这个线程并调用其run方法的时候,此时lastRun = 0,在第一次启动的时候sun.nio.ch.WindowsSelectorImpl.StartLock#runsCounter同样为0,所以会调用startLock.wait()进而进入等待状态。

注意:

  • sun.nio.ch.WindowsSelectorImpl.StartLock同样会判断当前其所检测的线程是否废弃,废弃的话就返回true,这样被检测线程也就能跳出其内run方法的while循环从而结束线程运行。
  • 在调整线程的时候(调用adjustThreadsCount方法)与Selector调用close方法会间接调用到sun.nio.ch.WindowsSelectorImpl#implClose,这两个方法都会涉及到Selector线程的释放,即调用sun.nio.ch.WindowsSelectorImpl.SelectThread#makeZombie
  • finishLock.threadFinished()会调用wakeup()方法来通知主线程,这里,我们可以学到一个细节,如果线程正阻塞在select方法上,就可以调用wakeup方法会使阻塞的选择操作立即返回,通过Windows的相关实现,原理其实是向pipesink端写入了一个字节,source文件描述符就会处于就绪状态,poll方法会返回,从而导致select方法返回。而在其他solaris或者linux系统上其实采用系统调用pipe来完成管道的创建,相当于直接用了系统的管道。通过wakeup()相关实现还可以看出,调用wakeup会设置interruptTriggered的标志位,所以连续多次调用wakeup的效果等同于一次调用,不会引起无所谓的bug出现。
//sun.nio.ch.WindowsSelectorImpl#adjustThreadsCount
// After some channels registered/deregistered, the number of required
// helper threads may have changed. Adjust this number.
private void adjustThreadsCount() {
if (threadsCount > threads.size()) {
// More threads needed. Start more threads.
for (int i = threads.size(); i < threadsCount; i++) {
SelectThread newThread = new SelectThread(i);
threads.add(newThread);
newThread.setDaemon(true);
newThread.start();
}
} else if (threadsCount < threads.size()) {
// Some threads become redundant. Remove them from the threads List.
for (int i = threads.size() - 1 ; i >= threadsCount; i--)
threads.remove(i).makeZombie();
}
}

//sun.nio.ch.WindowsSelectorImpl.SelectThread
// Represents a helper thread used for select.
private final class SelectThread extends Thread {
private final int index; // index of this thread
final SubSelector subSelector;
private long lastRun = 0; // last run number
private volatile boolean zombie;
// Creates a new thread
private SelectThread(int i) {
super(null, null, "SelectorHelper", 0, false);
this.index = i;
this.subSelector = new SubSelector(i);
//make sure we wait for next round of poll
this.lastRun = startLock.runsCounter;
}
void makeZombie() {
zombie = true;
}
boolean isZombie() {
return zombie;
}
public void run() {
while (true) { // poll loop
// wait for the start of poll. If this thread has become
// redundant, then exit.
if (startLock.waitForStart(this))
return;
// call poll()
try {
subSelector.poll(index);
} catch (IOException e) {
// Save this exception and let other threads finish.
finishLock.setException(e);
}
// notify main thread, that this thread has finished, and
// wakeup others, if this thread is the first to finish.
finishLock.threadFinished();
}
}
}

// sun.nio.ch.WindowsSelectorImpl.FinishLock#threadFinished
// Each helper thread invokes this function on finishLock, when
// the thread is done with poll().
private synchronized void threadFinished() {
if (threadsToFinish == threads.size()) { // finished poll() first
// if finished first, wakeup others
wakeup();
}
threadsToFinish--;
if (threadsToFinish == 0) // all helper threads finished poll().
notify(); // notify the main thread
}

//sun.nio.ch.WindowsSelectorImpl#wakeup
@Override
public Selector wakeup() {
synchronized (interruptLock) {
if (!interruptTriggered) {
setWakeupSocket();
interruptTriggered = true;
}
}
return this;
}
//sun.nio.ch.WindowsSelectorImpl#setWakeupSocket
// Sets Windows wakeup socket to a signaled state.
private void setWakeupSocket() {
setWakeupSocket0(wakeupSinkFd);
}
private native void setWakeupSocket0(int wakeupSinkFd);

JNIEXPORT void JNICALL
Java_sun_nio_ch_WindowsSelectorImpl_setWakeupSocket0(JNIEnv *env, jclass this,
jint scoutFd)
{
/* Write one byte into the pipe */
const char byte = 1;
send(scoutFd, &byte, 1, 0);
}

subSelector的poll方法解读

  1. subSelector.poll() 是select的核心,由native函数poll0实现,并把pollWrapper.pollArrayAddress作为参数传给poll0readFdswriteFdsexceptFds数组用来保存底层select的结果,数组的第一个位置都是存放发生事件的socket的总数,其余位置存放发生事件的socket句柄fd
    我们通过下面的代码可知:
    这个poll0()会监听pollWrapper中的FD有没有数据进出,这里会造成IO阻塞,直到有数据读写事件发生。由于pollWrapper中保存的也有ServerSocketChannelFD,所以只要ClientSocket发一份数据到ServerSocket,那么poll0()就会返回;又由于pollWrapper中保存的也有pipewrite端的FD,所以只要pipewrite端向FD发一份数据,也会造成poll0()返回;如果这两种情况都没有发生,那么poll0()就一直阻塞,也就是selector.select()会一直阻塞;如果有任何一种情况发生,那么selector.select()就会返回,所有在SelectThreadrun()里要用while (true) {},这样就可以保证在selector接收到数据并处理完后继续监听poll();

可以看出,NIO依然是阻塞式的IO,那么它和BIO的区别究竟在哪呢。
其实它的区别在于阻塞的位置不同,BIO是阻塞在read方法(recvfrom),而NIO阻塞在select方法。那么这样做有什么好处呢。如果单纯的改变阻塞的位置,自然是没有什么变化的,但epoll等的实现的巧妙之处就在于,它利用回调机制,让监听能够只需要知晓哪些socket上的数据已经准备好了,只需要处理这些线程上面的数据就行了。采用BIO,假设有1000个连接,需要开1000个线程,然后有1000read的位置在阻塞(我们在讲解BIO部分已经通过Demo体现),采用NIO编程,只需要1个线程,它利用select的轮询策略配合epoll的事件机制及红黑树数据结构,降低了其内部轮询的开销,同时极大的减小了线程上下文切换的开销。

//sun.nio.ch.WindowsSelectorImpl.SubSelector
private final class SubSelector {
private final int pollArrayIndex; // starting index in pollArray to poll
// These arrays will hold result of native select().
// The first element of each array is the number of selected sockets.
// Other elements are file descriptors of selected sockets.
// 保存发生read的FD
private final int[] readFds = new int [MAX_SELECTABLE_FDS + 1];
// 保存发生write的FD
private final int[] writeFds = new int [MAX_SELECTABLE_FDS + 1];
//保存发生except的FD
private final int[] exceptFds = new int [MAX_SELECTABLE_FDS + 1];

private SubSelector() {
this.pollArrayIndex = 0; // main thread
}

private SubSelector(int threadIndex) { // helper threads
this.pollArrayIndex = (threadIndex + 1) * MAX_SELECTABLE_FDS;
}

private int poll() throws IOException{ // poll for the main thread
return poll0(pollWrapper.pollArrayAddress,
Math.min(totalChannels, MAX_SELECTABLE_FDS),
readFds, writeFds, exceptFds, timeout);
}

private int poll(int index) throws IOException {
// poll for helper threads
return poll0(pollWrapper.pollArrayAddress +
(pollArrayIndex * PollArrayWrapper.SIZE_POLLFD),
Math.min(MAX_SELECTABLE_FDS,
totalChannels - (index + 1) * MAX_SELECTABLE_FDS),
readFds, writeFds, exceptFds, timeout);
}

private native int poll0(long pollAddress, int numfds,
int[] readFds, int[] writeFds, int[] exceptFds, long timeout);
...
}

updateSelectedKeys解读

  1. 接下来将通过上面WindowsSelectorImpl#doSelect展示源码中<5> 处的 updateSelectedKeys(action)来处理每个channel准备就绪的信息。
    • 如果该通道的key尚未在selectedKeys中存在,则将其添加到该集合中。
    • 如果该通道的key已经存在selectedKeys中,即这个channel存在所支持的ReadyOps就绪操作中必须包含一个这种操作(由(ski.nioReadyOps() & ski.nioInterestOps()) != 0来确定),此时修改其ReadyOps为当前所要进行的操作。而我们之前看到的Consumer<SelectionKey>这个动作也是在此处进行。而由下面源码可知,先前记录在ReadyOps中的任何就绪信息在调用此action之前被丢弃掉,直接进行设定。
      //sun.nio.ch.WindowsSelectorImpl#updateSelectedKeys
      private int updateSelectedKeys(Consumer<SelectionKey> action) {
      updateCount++;
      int numKeysUpdated = 0;
      numKeysUpdated += subSelector.processSelectedKeys(updateCount, action);
      for (SelectThread t: threads) {
      numKeysUpdated += t.subSelector.processSelectedKeys(updateCount, action);
      }
      return numKeysUpdated;
      }
      //sun.nio.ch.SelectorImpl#processReadyEvents
      protected final int processReadyEvents(int rOps,
      SelectionKeyImpl ski,
      Consumer<SelectionKey> action) {
      if (action != null) {
      ski.translateAndSetReadyOps(rOps);
      if ((ski.nioReadyOps() & ski.nioInterestOps()) != 0) {
      action.accept(ski);
      ensureOpen();
      return 1;
      }
      } else {
      assert Thread.holdsLock(publicSelectedKeys);
      if (selectedKeys.contains(ski)) {
      if (ski.translateAndUpdateReadyOps(rOps)) {
      return 1;
      }
      } else {
      ski.translateAndSetReadyOps(rOps);
      if ((ski.nioReadyOps() & ski.nioInterestOps()) != 0) {
      selectedKeys.add(ski);
      return 1;
      }
      }
      }
      return 0;
      }
      //sun.nio.ch.WindowsSelectorImpl.SubSelector#processSelectedKeys
      private int processSelectedKeys(long updateCount, Consumer<SelectionKey> action) {
      int numKeysUpdated = 0;
      numKeysUpdated += processFDSet(updateCount, action, readFds,
      Net.POLLIN,
      false);
      numKeysUpdated += processFDSet(updateCount, action, writeFds,
      Net.POLLCONN |
      Net.POLLOUT,
      false);
      numKeysUpdated += processFDSet(updateCount, action, exceptFds,
      Net.POLLIN |
      Net.POLLCONN |
      Net.POLLOUT,
      true);
      return numKeysUpdated;
      }

      /**
      * sun.nio.ch.WindowsSelectorImpl.SubSelector#processFDSet
      * updateCount is used to tell if a key has been counted as updated
      * in this select operation.
      *
      * me.updateCount <= updateCount
      */
      private int processFDSet(long updateCount,
      Consumer<SelectionKey> action,
      int[] fds, int rOps,
      boolean isExceptFds)
      {
      int numKeysUpdated = 0;
      for (int i = 1; i <= fds[0]; i++) {
      int desc = fds[i];
      if (desc == wakeupSourceFd) {
      synchronized (interruptLock) {
      interruptTriggered = true;
      }
      continue;
      }
      MapEntry me = fdMap.get(desc);
      // If me is null, the key was deregistered in the previous
      // processDeregisterQueue.
      if (me == null)
      continue;
      SelectionKeyImpl sk = me.ski;

      // The descriptor may be in the exceptfds set because there is
      // OOB data queued to the socket. If there is OOB data then it
      // is discarded and the key is not added to the selected set.
      if (isExceptFds &&
      (sk.channel() instanceof SocketChannelImpl) &&
      discardUrgentData(desc))
      {
      continue;
      }
      //我们应该关注的
      int updated = processReadyEvents(rOps, sk, action);
      if (updated > 0 && me.updateCount != updateCount) {
      me.updateCount = updateCount;
      numKeysUpdated++;
      }
      }
      return numKeysUpdated;
      }

至此,关于Selector的内容就暂时告一段落,在下一篇中,我会针对Java NIO Buffer进行相关解读。

文章目录
  1. 1. SelectionKey的引入
    1. 1.1. interestOps
    2. 1.2. readyOps
  2. 2. Selector探究
    1. 2.1. Selector的open方法
      1. 2.1.1. Pipe.open()的解惑
    2. 2.2. PollArrayWrapper解读
    3. 2.3. SelectionKey在selector中的管理
      1. 2.3.1. SelectionKey在selector中注册
      2. 2.3.2. Selector的select方法的解读
      3. 2.3.3. processUpdateQueue解读
        1. 2.3.3.1. processDeregisterQueue解读
        2. 2.3.3.2. adjustThreadsCount解读
        3. 2.3.3.3. subSelector的poll方法解读
        4. 2.3.3.4. updateSelectedKeys解读