Selector学习笔记 (未完待续)

目录

• Selector open流程
  • Selector.open()
    • SelectorProvider.provider()
    • WindowsSelectorProvider.openSelector()
• register流程
  • AbstractSelector::register
  • SelectorImpl::register
    • WindowsSelectorImpl::implRegister
• select流程
  • SelectorImpl.doSelect(long timeout)
    • WindowsSelectorImpl::doSelect

Selector open流程

Selector.open()

可以看到Selector是由Selector.provider()提供的provider提供的.所以接下来看看SelectorProvider::provider方法

    public static Selector open() throws IOException {
        return SelectorProvider.provider().openSelector();
    }

SelectorProvider.provider()

windows这边基本上都是走WindowsSelectorProvider了.另一种linux的还没看.

    public static SelectorProvider provider() {
        synchronized (lock) {
            if (provider != null)
                return provider;
            return AccessController.doPrivileged(
                new PrivilegedAction<SelectorProvider>() {
                    public SelectorProvider run() {
                            //从系统环境变量中"java.nio.channels.spi.SelectorProvider"加载Provider实现.
                            if (loadProviderFromProperty())
                                return provider;
                            //通过JAVA SPI加载Provider的实现
                            if (loadProviderAsService())
                                return provider;
                            //默认Provider,我看win版jdk里返回的是WindowsSelectorProvider
                            provider = sun.nio.ch.DefaultSelectorProvider.create();
                            return provider;
                        }
                    });
        }
    }

WindowsSelectorProvider.openSelector()

返回WindowsSelectorImpl.java

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

register流程

AbstractSelector::register

如上图所示,首先只有SelectableChannel可以注册至selector上,而我们常用到的大部分channel基本都继承自SelectableChannel.
接下来看看AbstractSelectableChannel::register的API介绍

使用给定的Selector注册此Channel，并返回SelectionKey。
如果此Channel已在给定的Selector中注册，则将其兴趣(ops参数)设置为给定值后，返回表示注册的SelectionKey.
若此Channel尚未在给定的Selector中注册,则在保持适当的锁的同时调用AbstractSelector::register方法进行注册.返回key之前先添加进channel的key集合中.(每个channel也维护了自己注册过的selector集合)

    public final SelectionKey register(Selector sel, int ops,
                                       Object att) throws ClosedChannelException
    {
        synchronized (regLock) {
            //验证通道是否打开
            if (!isOpen())
                throw new ClosedChannelException();
            //validOps返回子类支持的ops,即对应的位数值为1. 此处校验ops是否有支持权限外的位数值为1
            if ((ops & ~validOps()) != 0)
                throw new IllegalArgumentException();
            //selector不支持阻塞channel注册.此处检查了channel是否为阻塞模式.
            if (blocking)
                throw new IllegalBlockingModeException();
            //检查此channel是否已经注册过Selector了.
            SelectionKey k = findKey(sel);
            //如果注册过,就更新ops和attachment
            if (k != null) {
                k.interestOps(ops);
                k.attach(att);
            }
            //如果没有注册过
            if (k == null) {
                // New registration
                synchronized (keyLock) {
                    //检查连接是否是打开的
                    if (!isOpen())
                        throw new ClosedChannelException();
                    //走selector的register方法注册此channel.
                    k = ((AbstractSelector)sel).register(this, ops, att);
                    //并添加至此channel记录的注册的Selector集合.
                    addKey(k);
                }
            }
            return k;
        }
    }

看下来,发现channel最后还是通过调用Selector::register方法进行注册,所以接下来看Selector的register方法.

SelectorImpl::register

SelectorImpl实现了Selector的register接口.SelectionKey的创建是在这一步进行的.

    protected final SelectionKey register(AbstractSelectableChannel var1, int var2, Object var3) {
        //只允许SelChImpl实现类可以注册.
        if (!(var1 instanceof SelChImpl)) {
            throw new IllegalSelectorException();
        } else {
            //创建SelectionKey并返回.
            SelectionKeyImpl var4 = new SelectionKeyImpl((SelChImpl)var1, this);
            var4.attach(var3);
            synchronized(this.publicKeys) {
                //注册到Selector实现里.
                this.implRegister(var4);
            }
            var4.interestOps(var2);
            return var4;
        }
    }

WindowsSelectorImpl::implRegister

windows下的JDK8默认实现是这个. 总的来说selector维护了一批SelectionKey.

    //从SelectorImpl下继承来的.
    protected HashSet<SelectionKey> keys = new HashSet();
    //一个存放 selectionKey的Map(继承于HashMap)
    private final WindowsSelectorImpl.FdMap fdMap = new 
WindowsSelectorImpl.FdMap();
    //一个用于存放SelectionKey的数组. 此处自己用数组实现增删可能是为了快?
    private SelectionKeyImpl[] channelArray = new 
SelectionKeyImpl[8];
    private int totalChannels = 1;

    protected void implRegister(SelectionKeyImpl var1) {
        synchronized(this.closeLock) {
            if (this.pollWrapper == null) {
                throw new ClosedSelectorException();
            } else {
                //判断数组是否需要扩容
                this.growIfNeeded();
                //追加至channelArray数组的末尾.
                this.channelArray[this.totalChannels] = var1;
                //并且selectionKey自己也记录着自己在selector中的哪个位置.
                var1.setIndex(this.totalChannels);
                //往map集合里存一个
                this.fdMap.put(var1);
                //往set集合里存一个
                this.keys.add(var1);
                //存放文件句柄信息( fd ( file description ))
                this.pollWrapper.addEntry(this.totalChannels, var1);
                ++this.totalChannels;
            }
        }
    }

select流程

SelectorImpl.doSelect(long timeout)

WindowsSelectorImpl::doSelect

windows JDK8 默认自带的Selector的doSelect()方法.

    
    //存放着文件句柄,基本用unsafe直接操作内存.
    private PollArrayWrapper pollWrapper = new PollArrayWrapper(8);

    protected int doSelect(long var1) throws IOException {
        if (this.channelArray == null) {
            throw new ClosedSelectorException();
        } else {
            this.timeout = var1;
            //清除注销队列(有个集合记录了cancel的key.通过这个方法将其正式清除)
            this.processDeregisterQueue();
            if (this.interruptTriggered) {
                this.resetWakeupSocket();
                return 0;
            } else {
                //启动/销毁WindowsSelectorImpl.SelectThread线程,保证有threadsCount(每增加1024个连接,这个值+1)个线程在工作.每个SelectThread都有一个SubSelector去poll这个Selector里的PollArrayWrapper里的句柄.  *推测*  并发分段轮询机制.
                this.adjustThreadsCount();
                this.finishLock.reset();
                this.startLock.startThreads();

                try {
                    this.begin();

                    try {
                        //负责一开始1024个channel的poll工作.后面的每1024个连接都将给SelectThread的SubSelector负责poll. 据说系统调用的poll会因为句柄太多影响性能,所以进行拆分.(待证)
                        this.subSelector.poll();
                    } catch (IOException var7) {
                        this.finishLock.setException(var7);
                    }

                    if (this.threads.size() > 0) {
                        this.finishLock.waitForHelperThreads();
                    }
                } finally {
                    this.end();
                }

                this.finishLock.checkForException();
                //清除Cancel了的连接.
                this.processDeregisterQueue();
                //更新keys事件. 这个方法的核心在于SubSelector的processSelectedKeys()方法
                int var3 = this.updateSelectedKeys();
                this.resetWakeupSocket();
                return var3;
            }
        }
    }