Java Nio之Selector源码分析

具体过程图，上面写出了核心步骤，可对照下面观看。

Pipe——单向管道

Pipe使用两组Channel，SinkChannel用于发送数据，SourceChannel用于接收数据。

public class PipeDemo {
    public static void main(String[] args) throws Exception {
        String msg = "Hello world!";
        Pipe pipe = Pipe.open();
        // 用于发送数据的SinkChannel
        Pipe.SinkChannel sinkChannel = pipe.sink();
        sinkChannel.write(ByteBuffer.wrap((msg).getBytes())); // 发送数据
        // 用于接收数据的SourceChannel
        Pipe.SourceChannel sourceChannel = pipe.source();
        ByteBuffer byteBuffer = ByteBuffer.allocate(msg.length());
        sourceChannel.read(byteBuffer);                       // 读取数据
        System.out.println(new String(byteBuffer.array()));   // 打印数据
    }
}
// 输出结果：Hello world!

由于Pipe在下面会使用到，所以这里先介绍一下。

创建好一个Selector

（1）首先，我们会通过Selector.open()创建Selector

Selector selector = Selector.open();

（2）SelectorProvider.provider()最终会创建new WindowsSelectorProvider()对象
openSelector()最终会创建new WindowsSelectorImpl(this)对象
——这两步跟JDK操作系统版本有关，这里是Windows。

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

public class WindowsSelectorProvider extends SelectorProviderImpl {
    public AbstractSelector openSelector() throws IOException {
        return new WindowsSelectorImpl(this);
    }
}

这里我们已经认识到通过Selector.open()创建的其实是一个WindowsSelectorImpl对象，下面深入其构造函数进行分析。

WindowsSelectorImpl构造函数

WindowsSelectorImpl(SelectorProvider selectorProvider) throws IOException {
	super(selectorProvider);
	// 省略，下面讲解
}

（1）先来看看super()中的方法
在此之前，先来看看Selector的关系图

SelectorImpl会为我们初始化publicKets和publicSelectedKeys

public abstract class SelectorImpl extends AbstractSelector {
	/** 已选择集合，select()添加到该键集并返回 */
	protected Set<SelectionKey> selectedKeys = new HashSet();
	/** 键集，register()时添加到该键级 */
	protected HashSet<SelectionKey> keys = new HashSet();
    protected SelectorImpl(SelectorProvider selectorProvider) {
        super(selectorProvider);
        // 省略
    }
}

AbstractSelector会初始化provider，这里是WindowsSelectorProvider

public abstract class AbstractSelector extends Selector {
	private final SelectorProvider provider;
	/** 已取消键集，已被取消但其通道尚未注销的键的集合 */
	private final Set<SelectionKey> cancelledKeys = new HashSet<SelectionKey>();
    protected AbstractSelector(SelectorProvider provider) {
        this.provider = provider;
    }
}

这里不需要清晰的明白上面各变量的具体含义，只需记得有下面这三种即可：

keys：键集
selectedKeys：已选择键集
cancelledKeys：已取消键集

（2）看完了父类的构造方法，接下来看WindowsSelectorImpl的构造函数和局部变量。

在介绍构造函数之前，先来介绍两个布局变量，上面介绍了Pipe的使用。并了解了Pipe使用两组Channel，SinkChannel用于发送数据，SourceChannel用于接收数据。这里介绍Pipe是如何创建的。

private final Pipe wakeupPipe = Pipe.open();

其核心在PipeImpl内部类LoopbackConnector的run()方法中

private class LoopbackConnector implements Runnable {
	private LoopbackConnector() {
	}

	public void run() {
		ServerSocketChannel serverSocketChannel = null;
		SocketChannel socketChannel1 = null;
		SocketChannel socketChannel2 = null;

		try {
			ByteBuffer byteBuffer1 = ByteBuffer.allocate(16);
			ByteBuffer byteBuffer2 = ByteBuffer.allocate(16);
			inetAddressetAddress inetAddress = InetAddress.getByName("127.0.0.1");
			assert inetAddress.isLoopbackAddress();
			InetSocketAddress inetSocketAddress = null;
			while(true) {
				// ServerSocketChannel绑定端口0
				if (serverSocketChannel == null || !serverSocketChannel.isOpen()) {
					serverSocketChannel = ServerSocketChannel.open();
					serverSocketChannel.socket().bind(new InetSocketAddress(inetAddress, 0)); 
					inetSocketAddress = new InetSocketAddress(inetAddress, serverSocketChannel.socket().getLocalPort());
				}
				// 创建socketChnnel1，用于发送数据
				socketChannel1 = SocketChannel.open();
				// 生成随机字节写到byteBuffer1
				PipeImpl.RANDOM_NUMBER_GENERATOR.nextBytes(byteBuffer1.array());
				do {
					socketChannel1.write(byteBuffer1); // 将byteBuffer1数据写到socketChannel1
				} while(byteBuffer1.hasRemaining());
				byteBuffer1.rewind();
				// serverSocketChannel调用accept()接收一个SocketChannel，用于读取数据
				socketChannel2 = serverSocketChannel.accept();
				do {
					socketChannel2.read(byteBuffer2); // socketChannel2将数据写道byteBuffer2
				} while(byteBuffer2.hasRemaining());
				byteBuffer2.rewind();
				// 判断是否能正常地读写数据
				if (byteBuffer2.equals(byteBuffer1)) { 
					PipeImpl.this.source = new SourceChannelImpl(Initializer.this.sp, socketChannel1);
					PipeImpl.this.sink = new SinkChannelImpl(Initializer.this.sp, socketChannel2);
					break;
				}
				// 不能正确读写数据则关闭SocketChannel资源
				socketChannel2.close();
				socketChannel1.close();
			}
		} catch (IOException e) {
			// 关闭SocketChannel
		} finally {
			// 关闭ServerSocketChannel
		}
	}
}

如果发送的数据与接收的数据内容相同，则将发送的SocketChannel作为source（SourceChannelImpl），使用接收的SocketChannel作为sink（SinkChannelImpl）。

如果看源码，SourceChannelImpl继承了SourceChannel，而SourceChannel实现了ReadableByteChannel, ScatteringByteChannel两个接口，而这两个接口只提供了只读的方法，因此SourceChannel只能接受数据。

public static abstract class SourceChannel
    extends AbstractSelectableChannel
    implements ReadableByteChannel, ScatteringByteChannel

SinkChannelImpl继承了SinkChannel，同样只能写数据。

public static abstract class SinkChannel
    extends AbstractSelectableChannel
    implements WritableByteChannel, GatheringByteChannel

wakeuppipe是我们介绍的第一个WindowsSelectorImpl局部变量，第二个局部变量是pollWrapper，用于存储socket句柄fd以及事件events。

private PollArrayWrapper pollWrapper = new PollArrayWrapper(8);

这里先看一下构造函数，具体的方法使用到再看。

class PollArrayWrapper {
	// 存储和获取操作
	private AllocatedNativeObject pollArray;
    PollArrayWrapper(int var1) {
        int var2 = var1 * SIZE_POLLFD;
		// 初始化时调用unsafe.allocateMemory()申请一块内存
        this.pollArray = new AllocatedNativeObject(var2, true);
		// 用pollArrayAddress记录内存地址
        this.pollArrayAddress = this.pollArray.address();
        this.size = var1;
    }
}

现在我们回过来看WindowsSelectorImpl的构造函数

WindowsSelectorImpl(SelectorProvider selectorProvider) throws IOException {
	super(selectorProvider);
	// 这里的wakeupPipe就是上面介绍的局部变量PipeImpl
	// 获取保存SourceChannel的socket句柄
	this.wakeupSourceFd = ((SelChImpl)this.wakeupPipe.source()).getFDVal();
	// 获取SinkChannel
	SinkChannelImpl sinkChannelImpl = (SinkChannelImpl)this.wakeupPipe.sink();
	sinkChannelImpl.sc.socket().setTcpNoDelay(true);
	// 获取保存SinkChannel的socket句柄
	this.wakeupSinkFd = sinkChannelImpl.getFDVal();
	this.pollWrapper.addWakeupSocket(this.wakeupSourceFd, 0);
}

addWakeupSocket：当事件触发，会通知对应的socket

class PollArrayWrapper {
    void addWakeupSocket(int var1, int var2) { // (SourceChannel的socket句柄, 0)
    	// 添加描述符
        this.putDescriptor(var2, var1); // (0, SourceChannel的socket句柄)
        // 添加事件, Net.POLLIN表示有数据可读
        this.putEventOps(var2, Net.POLLIN); // (0, Net.POLLIN)
    }
    void putDescriptor(int var1, int var2) {
        this.pollArray.putInt(SIZE_POLLFD * var1 + 0, var2); // (0, socket句柄)
    }
    void putEventOps(int var1, int var2) {
        this.pollArray.putShort(SIZE_POLLFD * var1 + 4, (short)var2); // (4, socket句柄)
    }
}

通过上面代码我们也可以看出socket句柄占用4个字节，事件占用两个字节。

——到这里我们已经清晰地了解创建一个Selector的具体流程。包括常用的三个键集keys、selectedKeys、cancelledKeys，以及Pipe管道和PollArrayWrapper。下面将介绍Selector的常用方法。

WindowsSelectorImpl常用方法

（1）register()
SelectorImpl

protected final SelectionKey register(AbstractSelectableChannel ch, int ops, Object att) {
    if (!(var1 instanceof SelChImpl)) {
        throw new IllegalSelectorException();
    } else {
    	// 创建一个注册标识
        SelectionKeyImpl selectionKey = new SelectionKeyImpl((SelChImpl)ch, this);
        // 为selectionKey附加额外属性
        selectionKey.attach(att);
        synchronized(this.publicKeys) {
        	// 添加socket句柄
            this.implRegister(selectionKey);
        }
        // 添加事件
        selectionKey.interestOps(ops);
        return selectionKey;
    }
}

WindowsSelectorImpl
用到的局部变量

/** 用于存储注册标识 */
private SelectionKeyImpl[] channelArray = new SelectionKeyImpl[8];
/** 用于存储Socket句柄和events */
private PollArrayWrapper pollWrapper = new PollArrayWrapper(8);
/** 可以认为存储的是<socket句柄，SelectionKey> */
private final WindowsSelectorImpl.FdMap fdMap = new WindowsSelectorImpl.FdMap();
/** 已注册的Channel总数，从1开始 */
private int totalChannels = 1;

添加socket句柄

protected void implRegister(SelectionKeyImpl selectionKey) {
    synchronized(this.closeLock) {
        if (this.pollWrapper == null) {
            throw new ClosedSelectorException();
        } else {
        	// 功能（1）判断是否需要扩容，如果需要则对channelArray、pollWrapper进行扩容
        	// 功能（2）每增加1024个Channel则增加一个线程
            this.growIfNeeded();
            // 添加selectionKey到channelArray
            this.channelArray[this.totalChannels] = selectionKey;
            // 设置selectionKey的index属性，添加事件时会用到
            selectionKey.setIndex(this.totalChannels);
            this.fdMap.put(selectionKey);
            // 添加到已注册键集
            this.keys.add(selectionKey);
            // 添加socket句柄到pollWrapper
            this.pollWrapper.addEntry(this.totalChannels, selectionKey);
            // 将已注册的channel总数 + 1
            ++this.totalChannels;
        }
    }
}

void addEntry(int var1, SelectionKeyImpl var2) { // (1, selectionKey) // 下一次为(2, sk)
	this.putDescriptor(var1, var2.channel.getFDVal()); // (1, socket句柄) // 下一次为(2, s)
}

void putDescriptor(int var1, int var2) { // (1, socket句柄) // 下一次为(2, s)
	this.pollArray.putInt(SIZE_POLLFD * var1 + 0, var2); //(8, socket句柄)//下一次为(16, s)
}

selectionKey.interestOps(ops)——添加事件

public SelectionKey interestOps(int ops) {
	this.ensureValid();
	return this.nioInterestOps(ops);
}

public SelectionKey nioInterestOps(int ops) {
	if ((ops & ~this.channel().validOps()) != 0) {
		throw new IllegalArgumentException();
	} else {
		// 添加事件到pollWrapper
		this.channel.translateAndSetInterestOps(ops, this);
		this.interestOps = ops;
		return this;
	}
}
// translateAndSetInterestOps在不同的类实现不同这里主要看ServerSocketChannel和SocketChannel
// SocketChannel
public void translateAndSetInterestOps(int ops, SelectionKeyImpl selectionKey) {
	int finalOps = 0;
	if ((ops & 1) != 0) { // 1 ：读（read）
		finalOps |= Net.POLLIN;
	}
	if ((ops & 4) != 0) { // 2 : 写（write）
		finalOps |= Net.POLLOUT;
	}
	if ((ops & 8) != 0) { // 8 : 连接（connect）
		finalOps |= Net.POLLCONN;
	}
	selectionKey.selector.putEventOps(selectionKey, finalOps);
}
// ServerSocketChannel
public void translateAndSetInterestOps(int ops, SelectionKeyImpl selectionKey) {
	int finalOps = 0;
	if ((ops & 16) != 0) { // 16 ：接收（accept）
		finalOps |= Net.POLLIN;
	}
	selectionKey.selector.putEventOps(selectionKey, finalOps);
}

public void putEventOps(SelectionKeyImpl selectionKey, int ops) {
	synchronized(this.closeLock) {
		if (this.pollWrapper == null) {
			throw new ClosedSelectorException();
		} else {
			int index = selectionKey.getIndex();
			if (index == -1) { // index是大于0的
				throw new CancelledKeyException();
			} else {
				this.pollWrapper.putEventOps(index, ops); // 添加事件（具体看上面）
			}
		}
	}
}

（2）select()

// Selector
public int select() throws IOException {
	return this.select(0L);
}
// SelectorImpl
public int select(long timeout) throws IOException {
	if (timeout < 0L) {
		throw new IllegalArgumentException("Negative timeout");
	} else {
		return this.lockAndDoSelect(timeout == 0L ? -1L : timeout);
	}
}
// SelectorImpl
private int lockAndDoSelect(long timeout) throws IOException {
	// 这里省略了没有意义的
	return this.doSelect(timeout);
}

下面是核心方法，这里只对关键代码进行分析

// WindowsSelectorImpl
protected int doSelect(long timeout) throws IOException {
	if (this.channelArray == null) {
		throw new ClosedSelectorException();
	} else {
		this.timeout = timeout;
		// 检查cancelledKeys，清除里面的selectionKey与channel、selector的联系
		this.processDeregisterQueue();
		if (this.interruptTriggered) {
			this.resetWakeupSocket();
			return 0;
		} else {
			// 调整线程数量，少了就添加，多了就移除
			// 移除是通过修改线程内的volatile boolean zombie标识
			this.adjustThreadsCount();
			// 调整要执行的线程数
			this.finishLock.reset();
			// 唤醒所有SelectThread线程，所有线程在检测到一个就绪socket句柄后返回
			this.startLock.startThreads();
			try {
				this.begin();
				try {
					// 当前线程监听0-1024个socket句柄，其他线程是从1025开始
					this.subSelector.poll();
				} catch (IOException e) {
					this.finishLock.setException(e);
				}
				// waitForHelperThreads()用于阻塞当前线程直到所有线程执行完毕
				// 每个线程执行完毕调用finishLock.threadFinished()，所有线程执行完毕会在该方法调用notify()
				if (this.threads.size() > 0) {
					this.finishLock.waitForHelperThreads();
				}
			} finally {
				this.end();
			}
			this.finishLock.checkForException();
			// 再次检查cancelledKeys
			this.processDeregisterQueue();
			// 更新selectedKeys
			int var3 = this.updateSelectedKeys();
			this.resetWakeupSocket();
			return var3;
		}
	}
}

检查cancelledKeys具体代码

// 省略synchronized、及try-catch等
void processDeregisterQueue() throws IOException {
	Set cancelledKeys = this.cancelledKeys();
	Iterator iterator = cancelledKeys.iterator();
	while(iterator.hasNext()) {
		SelectionKeyImpl selectionKey = (SelectionKeyImpl)iterator.next();
		// 清理Channel、Selector与该SelectionKey的联系
		this.implDereg(selectionKey);
		iterator.remove();
	}
}

protected void implDereg(SelectionKeyImpl selectionKey) throws IOException {
	int index = selectionKey.getIndex();
	assert index >= 0;
	synchronized(this.closeLock) {
		if (index != this.totalChannels - 1) {
			// 获取最后一个注册标识
			SelectionKeyImpl lastSelectionKey = this.channelArray[this.totalChannels - 1];
			// 替换掉channelArray index位置的注册标识
			this.channelArray[index] = lastSelectionKey;
			lastSelectionKey.setIndex(index);
			// 获取最后一个注册标识的socket句柄和事件、替换到index位置
			this.pollWrapper.replaceEntry(this.pollWrapper, this.totalChannels - 1, this.pollWrapper, index);
		}
		selectionKey.setIndex(-1);
	}
	// 下面是一些后置的清理工作
	this.channelArray[this.totalChannels - 1] = null;
	--this.totalChannels;
	if (this.totalChannels != 1 && this.totalChannels % 1024 == 1) {
		--this.totalChannels;
		--this.threadsCount;
	}
	this.fdMap.remove(selectionKey);
	this.keys.remove(selectionKey);
	this.selectedKeys.remove(selectionKey);
	// 清除channel中的注册标识
	this.deregister(selectionKey);
	SelectableChannel selectableChannel = selectionKey.channel();
	if (!selectableChannel.isOpen() && !selectableChannel.isRegistered()) {
		((SelChImpl)selectableChannel).kill();
	}
}

执行真正的select具体代码跟三个WindowsSelectorImpl的内部类有关
StartLock

private final class StartLock {
	/** 执行select()方法的次数 */
	private long runsCounter;
	private StartLock() {}
	/** 唤醒所有阻塞在该锁上的线程 */
	private synchronized void startThreads() {
		++this.runsCounter;
		this.notifyAll();
	}
	private synchronized boolean waitForStart(WindowsSelectorImpl.SelectThread selectThread) {
		// 合法判断，不合法阻塞线程
		while(this.runsCounter == selectThread.lastRun) {
			try {
				WindowsSelectorImpl.this.startLock.wait();
			} catch (InterruptedException e) {
				Thread.currentThread().interrupt();
			}
		}
		// 如果是僵尸线程，返回true，之后线程会跳出for循环
		// 移除线程就是通过这种方式
		if (selectThread.isZombie()) {
			return true;
		} else {
			// 修改线程最后一次执行次数
			selectThread.lastRun = this.runsCounter;
			return false;
		}
	}
}

FinishLock

private final class FinishLock {
	/** 剩余多少线程还没执行完本次select */
	private int threadsToFinish;

	private synchronized void threadFinished() {
		// 如果要执行的线程数与总的线程数相等
		if (this.threadsToFinish == WindowsSelectorImpl.this.threads.size()) {
			// 调用sink发送数据唤醒阻塞在poll上的selector线程
			WindowsSelectorImpl.this.wakeup();
		}
		--this.threadsToFinish;
		// 如果本轮select执行完毕，是否阻塞在该锁上的线程
		if (this.threadsToFinish == 0) {
			this.notify();
		}
	}

	private synchronized void waitForHelperThreads() {
		// 如果要执行的线程数与总的线程数相等
		if (this.threadsToFinish == WindowsSelectorImpl.this.threads.size()) {
			WindowsSelectorImpl.this.wakeup();
		}
		// 如果其他线程该没执行完，阻塞当前线程
		while(this.threadsToFinish != 0) {
			try {
				WindowsSelectorImpl.this.finishLock.wait();
			} catch (InterruptedException e) {
				Thread.currentThread().interrupt();
			}
		}
	}
}

// 调用sink发送数据唤醒阻塞在poll上的selector线程
public Selector wakeup() {
	synchronized(this.interruptLock) {
		if (!this.interruptTriggered) {
			this.setWakeupSocket();
			this.interruptTriggered = true;
		}
		return this;
	}
}

SelectThread

private final class SelectThread extends Thread {
	private final int index;
	/** 真正执行select操作的执行器 */
	final WindowsSelectorImpl.SubSelector subSelector;
	private long lastRun;
	/** 为true会跳出for循环 */
	private volatile boolean zombie;

	private SelectThread(int index) {
		this.lastRun = 0L;
		this.index = index;
		this.subSelector = WindowsSelectorImpl.this.new SubSelector(index);
		this.lastRun = WindowsSelectorImpl.this.startLock.runsCounter;
	}

	void makeZombie() {this.zombie = true;}
	boolean isZombie() {return this.zombie;}

	public void run() {
		// waitForStart()会让僵尸线程跳出for循环
		// threadFinished()会在检测到就绪事件之后让其他线程poll()返回
		for(; !WindowsSelectorImpl.this.startLock.waitForStart(this); WindowsSelectorImpl.this.finishLock.threadFinished()) {
			try {
				// 应用操作系统监听pollWrapper有没有就绪的socket句柄(阻塞)
				this.subSelector.poll(this.index);
			} catch (IOException e) {
				WindowsSelectorImpl.this.finishLock.setException(e);
			}
		}
	}
}

private final class SubSelector {
	// poll的索引开始处，pollWrapper
    private final int pollArrayIndex;
    // 存放可读事件fd
    private final int[] readFds;
    // 存放可写事件fd
    private final int[] writeFds;
    // 存放异常事件fd
    private final int[] exceptFds;
    // ... 
}

selectNow()与select()区别就是最后的timeout，selectNow()的timeout为0，select()的timeout为-1，该timeout在SubSelector的poll()方法使用，区别就是阻塞和非阻塞。

完毕，以上如果有误，欢迎指正。部分涉及到native方法并没有深入分析，因此有的地方没有解释清楚。

Java Nio之Selector源码分析

具体过程图，上面写出了核心步骤，可对照下面观看。

Pipe——单向管道

创建好一个Selector

WindowsSelectorImpl构造函数

WindowsSelectorImpl常用方法

DAPPER 事务 TRANSACTION

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Java Nio之Selector源碼分析

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