轉(zhuǎn)載自： 秦漢郵俠 https://www.jianshu.com/p/d33f2f6cdba0

要點(diǎn)

• 是一個抽象類,表示selectableChannel在Selector中注冊的標(biāo)識.每個Channel向Selector注冊時,都將會創(chuàng)建一個selectionKey
• 選擇鍵將Channel與Selector建立了關(guān)系,并維護(hù)了channel事件.
• 可以通過cancel方法取消鍵,取消的鍵不會立即從selector中移除,而是添加到cancelledKeys中,在下一次select操作時移除它.所以在調(diào)用某個key時,需要使用isValid進(jìn)行校驗(yàn).

操作集

• interest 集合:當(dāng)前channel感興趣的操作,此類操作將會在下一次選擇器select操作時被交付,可以通過selectionKey.interestOps(int)進(jìn)行修改.
• ready 集合:表示此選擇鍵上,已經(jīng)就緒的操作.每次select時,選擇器都會對ready集合進(jìn)行更新;外部程序無法修改此集合.

操作屬性

• OP_ACCEPT:連接可接受操作,僅ServerSocketChannel支持
• OP_CONNECT:連接操作,Client端支持的一種操作
• OP_READ/OP_WRITE

0表示什么？

• 這些opts都不為0，如果向selector之中register一個為“0”的opts，表示此channel不關(guān)注任何類型的事件。（言外之意，register方法只是獲取一個selectionKey，具體這個Channel對何種事件感興趣，可以在稍后操作）

方法列表

• public abstract SelectableChannel channel():返回此選擇鍵所關(guān)聯(lián)的通道.即使此key已經(jīng)被取消,仍然會返回.
• public abstract Selector selector():返回此選擇鍵所關(guān)聯(lián)的選擇器,即使此鍵已經(jīng)被取消,仍然會返回.
• public abstract boolean isValid():檢測此key是否有效.當(dāng)key被取消,或者通道被關(guān)閉,或者selector被關(guān)閉,都將導(dǎo)致此key無效.在AbstractSelector.removeKey(key)中,會導(dǎo)致selectionKey被置為無效.
• public abstract void cancel():請求將此鍵取消注冊.一旦返回成功,那么該鍵就是無效的,被添加到selector的cancelledKeys中.cancel操作將key的valid屬性置為false,并執(zhí)行selector.cancel(key)(即將key加入cancelledkey集合)
• public abstract int interesOps():獲得此鍵的interes集合.
• public abstract SelectionKey interestOps(int ops):將此鍵的interst設(shè)置為指定值.此操作會對ops和channel.validOps進(jìn)行校驗(yàn).如果此ops不會當(dāng)前channel支持,將拋出異常.
• public abstract int readyOps():獲取此鍵上ready操作集合.即在當(dāng)前通道上已經(jīng)就緒的事件.
• public final boolean isReadable(): 檢測此鍵是否為"read"事件.等效于:k.,readyOps() & OP_READ != 0;還有isWritable(),isConnectable(),isAcceptable()
• public final Object attach(Object ob):將給定的對象作為附件添加到此key上.在key有效期間,附件可以在多個ops事件中傳遞.
• public final Object attachment():獲取附件.一個channel的附件,可以再當(dāng)前Channel(或者說是SelectionKey)生命周期中共享,但是attachment數(shù)據(jù)不會作為socket數(shù)據(jù)在網(wǎng)絡(luò)中傳輸.

參考代碼

• Reactor

public class Reactor implements Runnable {final Selector selector;final ServerSocketChannel serverSocket;Reactor(int port) throws IOException {selector = Selector.open();serverSocket = ServerSocketChannel.open();InetSocketAddress address = new InetSocketAddress(InetAddress.getLocalHost(), port);serverSocket.socket().bind(address);serverSocket.configureBlocking(false);//向selector注冊該channelSelectionKey sk = serverSocket.register(selector, SelectionKey.OP_ACCEPT);System.out.println("-->Start serverSocket.register!");//利用sk的attache功能綁定Acceptor 如果有事情，觸發(fā)Acceptorsk.attach(new Acceptor());System.out.println("-->attach(new Acceptor()!");}public void run() { // normally in a new Threadtry {while (!Thread.interrupted()) {selector.select();Set selected = selector.selectedKeys();Iterator it = selected.iterator();//Selector如果發(fā)現(xiàn)channel有OP_ACCEPT或READ事件發(fā)生，下列遍歷就會進(jìn)行。while (it.hasNext()) {//來一個事件 第一次觸發(fā)一個accepter線程//以后觸發(fā)SocketReadHandlerdispatch((SelectionKey) (it.next()));}selected.clear();}} catch (IOException ex) {System.out.println("reactor stop!" + ex);}}//運(yùn)行Acceptor或SocketReadHandlervoid dispatch(SelectionKey k) {Runnable r = (Runnable) (k.attachment());if (r != null) {r.run();}}class Acceptor implements Runnable { // innerpublic void run() {try {System.out.println("-->ready for accept!");SocketChannel c = serverSocket.accept();if (c != null)//調(diào)用Handler來處理channelnew Handler(selector, c);} catch (IOException ex) {}}} }

• Handler

public class Handler implements Runnable {final SocketChannel socket;final SelectionKey sk;ByteBuffer input = ByteBuffer.allocate(Integer.MAX_VALUE);ByteBuffer output = ByteBuffer.allocate(Integer.MAX_VALUE);static final int READING = 0, SENDING = 1;int state = READING;public Handler(Selector sel, SocketChannel c) throws IOException {socket = c;//設(shè)置為非阻塞模式c.configureBlocking(false);//此處的0，表示不關(guān)注任何時間sk = socket.register(sel, 0);//將SelectionKey綁定為本Handler 下一步有事件觸發(fā)時，將調(diào)用本類的run方法sk.attach(this);//將SelectionKey標(biāo)記為可讀，以便讀取，不可關(guān)注可寫事件sk.interestOps(SelectionKey.OP_READ);sel.wakeup();}boolean inputIsComplete() {return false;}boolean outputIsComplete() {return false;}//這里可以通過線程池處理數(shù)據(jù)void process() {}public void run() {try {if (state == READING) {read();} else if (state == SENDING) {send();}} catch (IOException ex) { /* ... */ }}void read() throws IOException {socket.read(input);if (inputIsComplete()) {process();state = SENDING;// Normally also do first write nowsk.interestOps(SelectionKey.OP_WRITE);}}void send() throws IOException {socket.write(output);if (outputIsComplete()) {//sk.cancel();}}}

參考來源

• Selector類詳解

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