Examples of java.nio.channels.Selector

java.nio.channels.Selector
A multiplexor of {@link SelectableChannel} objects.
A selector may be created by invoking the {@link #open open} method ofthis class, which will use the system's default {@link java.nio.channels.spi.SelectorProvider selector provider} tocreate a new selector. A selector may also be created by invoking the {@link java.nio.channels.spi.SelectorProvider#openSelector openSelector}method of a custom selector provider. A selector remains open until it is closed via its {@link #close close} method.
A selectable channel's registration with a selector is represented by a {@link SelectionKey} object. A selector maintains three sets of selectionkeys:
All three sets are empty in a newly-created selector. A key is added to a selector's key set as a side effect of registering a channel via the channel's {@link SelectableChannel#register(Selector,int) register} method. Cancelled keys are removed from the key set duringselection operations. The key set itself is not directly modifiable. A key is added to its selector's cancelled-key set when it is cancelled, whether by closing its channel or by invoking its {@link SelectionKey#cancel cancel} method. Cancelling a key will cause its channel to be deregisteredduring the next selection operation, at which time the key will removed from all of the selector's key sets.
Keys are added to the selected-key set by selection operations. A key may be removed directly from the selected-key set by invoking the set's {@link java.util.Set#remove(java.lang.Object) remove}method or by invoking the {@link java.util.Iterator#remove() remove} methodof an {@link java.util.Iterator iterator} obtained from theset. Keys are never removed from the selected-key set in any other way; they are not, in particular, removed as a side effect of selection operations. Keys may not be added directly to the selected-key set.
Selection During each selection operation, keys may be added to and removed from a selector's selected-key set and may be removed from its key and cancelled-key sets. Selection is performed by the {@link #select()}, {@link #select(long)}, and {@link #selectNow()} methods, and involves three steps: Each key in the cancelled-key set is removed from each key set of which it is a member, and its channel is deregistered. This step leaves the cancelled-key set empty. The underlying operating system is queried for an update as to the readiness of each remaining channel to perform any of the operations identified by its key's interest set as of the moment that the selection operation began. For a channel that is ready for at least one such operation, one of the following two actions is performed: If the channel's key is not already in the selected-key set then it is added to that set and its ready-operation set is modified to identify exactly those operations for which the channel is now reported to be ready. Any readiness information previously recorded in the ready set is discarded. Otherwise the channel's key is already in the selected-key set, so its ready-operation set is modified to identify any new operations for which the channel is reported to be ready. Any readiness information previously recorded in the ready set is preserved; in other words, the ready set returned by the underlying system is bitwise-disjoined into the key's current ready set. If all of the keys in the key set at the start of this step have empty interest sets then neither the selected-key set nor any of the keys' ready-operation sets will be updated. If any keys were added to the cancelled-key set while step (2) was in progress then they are processed as in step (1). Whether or not a selection operation blocks to wait for one or more channels to become ready, and if so for how long, is the only essential difference between the three selection methods. Concurrency Selectors are themselves safe for use by multiple concurrent threads; their key sets, however, are not. The selection operations synchronize on the selector itself, on the key set, and on the selected-key set, in that order. They also synchronize on the cancelled-key set during steps (1) and (3) above. Changes made to the interest sets of a selector's keys while a selection operation is in progress have no effect upon that operation; they will be seen by the next selection operation. Keys may be cancelled and channels may be closed at any time. Hence the presence of a key in one or more of a selector's key sets does not imply that the key is valid or that its channel is open. Application code should be careful to synchronize and check these conditions as necessary if there is any possibility that another thread will cancel a key or close a channel. A thread blocked in one of the {@link #select()} or {@link #select(long)} methods may be interrupted by some other thread in one ofthree ways: By invoking the selector's {@link #wakeup wakeup} method, By invoking the selector's {@link #close close} method, or By invoking the blocked thread's {@link java.lang.Thread#interrupt() interrupt} method, in which case itsinterrupt status will be set and the selector's {@link #wakeup wakeup}method will be invoked. The {@link #close close} method synchronizes on the selector and allthree key sets in the same order as in a selection operation. A selector's key and selected-key sets are not, in general, safe for use by multiple concurrent threads. If such a thread might modify one of these sets directly then access should be controlled by synchronizing on the set itself. The iterators returned by these sets' {@link java.util.Set#iterator() iterator} methods are fail-fast: If the setis modified after the iterator is created, in any way except by invoking the iterator's own {@link java.util.Iterator#remove() remove} method, then a{@link java.util.ConcurrentModificationException} will be thrown. @author Mark Reinhold @author JSR-51 Expert Group @version 1.38, 05/11/17 @since 1.4 @see SelectableChannel @see SelectionKey




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153         // allocate an unbound server socket channel
        ServerSocketChannel serverChannel = ServerSocketChannel.open();
        // Get the associated ServerSocket to bind it with
        ServerSocket serverSocket = serverChannel.socket();
        // create a new Selector for use below
        Selector selector = Selector.open();
        // set the port the server channel will listen to
        serverSocket.bind (new InetSocketAddress (bind,tcpListenPort));
        // set non-blocking mode for the listening socket
        serverChannel.configureBlocking (false);
        // register the ServerSocketChannel with the Selector
        serverChannel.register (selector, SelectionKey.OP_ACCEPT);
        while (doListen) {
            // this may block for a long time, upon return the
            // selected set contains keys of the ready channels
            try {


                int n = selector.select(tcpSelectorTimeout);
                if (n == 0) {
                    continue; // nothing to do
                }
                // get an iterator over the set of selected keys
                Iterator it = selector.selectedKeys().iterator();
                // look at each key in the selected set
                while (it.hasNext()) {
                    SelectionKey key = (SelectionKey) it.next();
                    // Is a new connection coming in?
                    if (key.isAcceptable()) {
                        ServerSocketChannel server =
                            (ServerSocketChannel) key.channel();
                        SocketChannel channel = server.accept();
                        registerChannel(selector,
                                        channel,
                                        SelectionKey.OP_READ,
                                        new ObjectReader(channel, selector,
                            callback));
                    }
                    // is there data to read on this channel?
                    if (key.isReadable()) {
                        readDataFromSocket(key);
                    } else {
                        key.interestOps(key.interestOps() & (~key.OP_WRITE));
                    }


                    // remove key from selected set, it's been handled
                    it.remove();
                }
            }
            catch (java.nio.channels.CancelledKeyException nx) {
                log.warn(
                    "Replication client disconnected, error when polling key. Ignoring client.");
            }
            catch (Exception x) {
                log.error("Unable to process request in ReplicationListener", x);
            }


        } //while
        serverChannel.close();
        selector.close();
    }

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185         if( localAddress == null )
        {
            throw new NullPointerException( "localAddress" );
        }
        
        Selector selector = this.selector;
        DatagramChannel ch = ( DatagramChannel ) channels.get( localAddress );
        if( selector == null || ch == null )
        {
            throw new IllegalArgumentException( "Unknown localAddress: " + localAddress );
        }

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315         writeBuffer.put(src, offset, length);
        
        writeBuffer.flip();


        long writeTimeout = att.getWriteTimeout();
        Selector selector = null;
        try {
            selector = pool.get();
        } catch ( IOException x ) {
            //ignore
        }

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363         ByteBuffer readBuffer =
                socketWrapper.getSocket().getBufHandler().getReadBuffer();
        readBuffer.clear();
        readBuffer.limit(n);
        if ( block ) {
            Selector selector = null;
            try {
                selector = pool.get();
            } catch ( IOException x ) {
                // Ignore
            }

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161 
        int written = 0;
        NioEndpoint.KeyAttachment att = (NioEndpoint.KeyAttachment)socket.getAttachment(false);
        if ( att == null ) throw new IOException("Key must be cancelled");
        long writeTimeout = att.getWriteTimeout();
        Selector selector = null;
        try {
            selector = pool.get();
        } catch ( IOException x ) {
            //ignore
        }

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208     protected long lastCheck = System.currentTimeMillis();
    protected void socketTimeouts() {
        long now = System.currentTimeMillis();
        if ( (now-lastCheck) < getSelectorTimeout() ) return;
        //timeout
        Selector tmpsel = selector;
        Set<SelectionKey> keys =  (isListening()&&tmpsel!=null)?tmpsel.keys():null;
        if ( keys == null ) return;
        for (Iterator<SelectionKey> iter = keys.iterator(); iter.hasNext();) {
            SelectionKey key = iter.next();
            try {
//                if (key.interestOps() == SelectionKey.OP_READ) {

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384             }
        }
    }


    private void closeSelector() throws IOException {
        Selector selector = this.selector;
        this.selector = null;
        if (selector==null) return;
        try {
            Iterator<SelectionKey> it = selector.keys().iterator();
            // look at each key in the selected set
            while (it.hasNext()) {
                SelectionKey key = it.next();
                key.channel().close();
                key.attach(null);
                key.cancel();
            }
        }catch ( IOException ignore ){
            if (log.isWarnEnabled()) {
                log.warn("Unable to cleanup on selector close.",ignore);
            }
        }catch ( ClosedSelectorException ignore){}
        selector.close();
    }

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142 
    void register(NioSocketChannel channel, ChannelFuture future) {


        boolean server = !(channel instanceof NioClientSocketChannel);
        Runnable registerTask = new RegisterTask(channel, future, server);
        Selector selector;


        synchronized (startStopLock) {
            if (!started) {
                // Open a selector if this worker didn't start yet.
                try {
                    this.selector = selector = Selector.open();
                } catch (Throwable t) {
                    throw new ChannelException(
                            "Failed to create a selector.", t);
                }


                // Start the worker thread with the new Selector.
                String threadName =
                    (server ? "New I/O server worker #"
                            : "New I/O client worker #") + bossId + '-' + id;


                boolean success = false;
                try {
                    executor.execute(new ThreadRenamingRunnable(this, threadName));
                    success = true;
                } finally {
                    if (!success) {
                        // Release the Selector if the execution fails.
                        try {
                            selector.close();
                        } catch (Throwable t) {
                            logger.warn("Failed to close a selector.", t);
                        }
                        this.selector = selector = null;
                        // The method will return to the caller at this point.
                    }
                }
            } else {
                // Use the existing selector if this worker has been started.
                selector = this.selector;
            }


            assert selector != null && selector.isOpen();


            started = true;
            boolean offered = registerTaskQueue.offer(registerTask);
            assert offered;
        }


        if (wakenUp.compareAndSet(false, true)) {
            selector.wakeup();
        }
    }

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194 
    public void run() {
        thread = Thread.currentThread();


        boolean shutdown = false;
        Selector selector = this.selector;
        for (;;) {
            wakenUp.set(false);


            if (CONSTRAINT_LEVEL != 0) {
                selectorGuard.writeLock().lock();
                    // This empty synchronization block prevents the selector
                    // from acquiring its lock.
                selectorGuard.writeLock().unlock();
            }


            try {
                int selectedKeyCount = selector.select(500);


                // Wake up immediately in the next turn if someone might
                // have waken up the selector between 'wakenUp.set(false)'
                // and 'selector.select(...)'.
                if (wakenUp.get()) {
                    selector.wakeup();
                }


                processRegisterTaskQueue();
                processWriteTaskQueue();


                if (selectedKeyCount > 0) {
                    processSelectedKeys(selector.selectedKeys());
                }


                // Exit the loop when there's nothing to handle.
                // The shutdown flag is used to delay the shutdown of this
                // loop to avoid excessive Selector creation when
                // connections are registered in a one-by-one manner instead of
                // concurrent manner.
                if (selector.keys().isEmpty()) {
                    if (shutdown ||
                        executor instanceof ExecutorService && ((ExecutorService) executor).isShutdown()) {


                        synchronized (startStopLock) {
                            if (registerTaskQueue.isEmpty() && selector.keys().isEmpty()) {
                                started = false;
                                try {
                                    selector.close();
                                } catch (IOException e) {
                                    logger.warn(
                                            "Failed to close a selector.", e);
                                } finally {
                                    this.selector = null;

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348         if (workerThread == null || Thread.currentThread() != workerThread) {
            if (channel.writeTaskInTaskQueue.compareAndSet(false, true)) {
                boolean offered = worker.writeTaskQueue.offer(channel.writeTask);
                assert offered;
            }
            Selector workerSelector = worker.selector;
            if (workerSelector != null) {
                if (worker.wakenUp.compareAndSet(false, true)) {
                    workerSelector.wakeup();
                }
            }
            return true;
        }


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Examples of java.nio.channels.Selector

Selection

Concurrency

Related Classes of java.nio.channels.Selector