Java tutorial
/* * Copyright (c) 2000, 2018, Oracle and/or its affiliates. All rights reserved. * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. * * This code is free software; you can redistribute it and/or modify it * under the terms of the GNU General Public License version 2 only, as * published by the Free Software Foundation. Oracle designates this * particular file as subject to the "Classpath" exception as provided * by Oracle in the LICENSE file that accompanied this code. * * This code is distributed in the hope that it will be useful, but WITHOUT * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License * version 2 for more details (a copy is included in the LICENSE file that * accompanied this code). * * You should have received a copy of the GNU General Public License version * 2 along with this work; if not, write to the Free Software Foundation, * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. * * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA * or visit www.oracle.com if you need additional information or have any * questions. */ package java.nio.channels; import java.io.Closeable; import java.io.IOException; import java.nio.channels.spi.SelectorProvider; import java.util.Objects; import java.util.Set; import java.util.function.Consumer; /** * A multiplexor of {@link SelectableChannel} objects. * * <p> A selector may be created by invoking the {@link #open open} method of * this class, which will use the system's default {@link * java.nio.channels.spi.SelectorProvider selector provider} to * create 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 id="ks"></a> * * <p> A selectable channel's registration with a selector is represented by a * {@link SelectionKey} object. A selector maintains three sets of selection * keys: * * <ul> * * <li><p> The <i>key set</i> contains the keys representing the current * channel registrations of this selector. This set is returned by the * {@link #keys() keys} method. </p></li> * * <li><p> The <i>selected-key set</i> is the set of keys such that each * key's channel was detected to be ready for at least one of the operations * identified in the key's interest set during a prior selection operation * that adds keys or updates keys in the set. * This set is returned by the {@link #selectedKeys() selectedKeys} method. * The selected-key set is always a subset of the key set. </p></li> * * <li><p> The <i>cancelled-key</i> set is the set of keys that have been * cancelled but whose channels have not yet been deregistered. This set is * not directly accessible. The cancelled-key set is always a subset of the * key set. </p></li> * * </ul> * * <p> All three sets are empty in a newly-created selector. * * <p> 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 during * selection operations. The key set itself is not directly modifiable. * * <p> 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 deregistered * during the next selection operation, at which time the key will be removed * from all of the selector's key sets. * * <a id="sks"></a><p> 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} method * of an {@link java.util.Iterator iterator} obtained from the set. * All keys may be removed from the selected-key set by invoking the set's * {@link java.util.Set#clear() clear} method. Keys may not be added directly * to the selected-key set. </p> * * <a id="selop"></a> * <h2>Selection</h2> * * <p> A selection operation queries the underlying operating system for an * update as to the readiness of each registered channel to perform any of the * operations identified by its key's interest set. There are two forms of * selection operation: * * <ol> * * <li><p> The {@link #select()}, {@link #select(long)}, and {@link #selectNow()} * methods add the keys of channels ready to perform an operation to the * selected-key set, or update the ready-operation set of keys already in the * selected-key set. </p></li> * * <li><p> The {@link #select(Consumer)}, {@link #select(Consumer, long)}, and * {@link #selectNow(Consumer)} methods perform an <i>action</i> on the key * of each channel that is ready to perform an operation. These methods do * not add to the selected-key set. </p></li> * * </ol> * * <h3>Selection operations that add to the selected-key set</h3> * * <p> 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: * </p> * * <ol> * * <li><p> 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. </p></li> * * <li><p> 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: </p> * * <ol> * * <li><p> 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. </p></li> * * <li><p> 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. </p></li> * * </ol> * * 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. * * <li><p> If any keys were added to the cancelled-key set while step (2) was * in progress then they are processed as in step (1). </p></li> * * </ol> * * <p> 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. </p> * * * <h3>Selection operations that perform an action on selected keys</h3> * * <p> During each selection operation, keys may be removed from the selector's * key, selected-key, and cancelled-key sets. Selection is performed by the * {@link #select(Consumer)}, {@link #select(Consumer,long)}, and {@link * #selectNow(Consumer)} methods, and involves three steps: </p> * * <ol> * * <li><p> 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. </p></li> * * <li><p> 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. * * <p> For a channel that is ready for at least one such operation, the * ready-operation set of the channel's key is set to identify exactly the * operations for which the channel is ready and the <i>action</i> specified * to the {@code select} method is invoked to consume the channel's key. Any * readiness information previously recorded in the ready set is discarded * prior to invoking the <i>action</i>. * * <p> Alternatively, where a channel is ready for more than one operation, * the <i>action</i> may be invoked more than once with the channel's key and * ready-operation set modified to a subset of the operations for which the * channel is ready. Where the <i>action</i> is invoked more than once for * the same key then its ready-operation set never contains operation bits * that were contained in the set at previous calls to the <i>action</i> * in the same selection operation. </p></li> * * <li><p> If any keys were added to the cancelled-key set while step (2) was * in progress then they are processed as in step (1). </p></li> * * </ol> * * * <h2>Concurrency</h2> * * <p> A Selector and its key set are safe for use by multiple concurrent * threads. Its selected-key set and cancelled-key set, however, are not. * * <p> The selection operations synchronize on the selector itself, on the * selected-key set, in that order. They also synchronize on the cancelled-key * set during steps (1) and (3) above. * * <p> 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. * * <p> 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. * * <p> A thread blocked in a selection operation may be interrupted by some * other thread in one of three ways: * * <ul> * * <li><p> By invoking the selector's {@link #wakeup wakeup} method, * </p></li> * * <li><p> By invoking the selector's {@link #close close} method, or * </p></li> * * <li><p> By invoking the blocked thread's {@link * java.lang.Thread#interrupt() interrupt} method, in which case its * interrupt status will be set and the selector's {@link #wakeup wakeup} * method will be invoked. </p></li> * * </ul> * * <p> The {@link #close close} method synchronizes on the selector and its * selected-key set in the same order as in a selection operation. * * <a id="ksc"></a> * <p> A Selector's key set is safe for use by multiple concurrent threads. * Retrieval operations from the key set do not generally block and so may * overlap with new registrations that add to the set, or with the cancellation * steps of selection operations that remove keys from the set. Iterators and * spliterators return elements reflecting the state of the set at some point at * or since the creation of the iterator/spliterator. They do not throw * {@link java.util.ConcurrentModificationException ConcurrentModificationException}. * * <a id="sksc"></a> * <p> A selector's selected-key set is not, in general, safe for use by * multiple concurrent threads. If such a thread might modify the set directly * then access should be controlled by synchronizing on the set itself. The * iterators returned by the set's {@link java.util.Set#iterator() iterator} * methods are <i>fail-fast:</i> If the set is 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. </p> * * @author Mark Reinhold * @author JSR-51 Expert Group * @since 1.4 * * @see SelectableChannel * @see SelectionKey */ public abstract class Selector implements Closeable { /** * Initializes a new instance of this class. */ protected Selector() { } /** * Opens a selector. * * <p> The new selector is created by invoking the {@link * java.nio.channels.spi.SelectorProvider#openSelector openSelector} method * of the system-wide default {@link * java.nio.channels.spi.SelectorProvider} object. </p> * * @return A new selector * * @throws IOException * If an I/O error occurs */ public static Selector open() throws IOException { return SelectorProvider.provider().openSelector(); } /** * Tells whether or not this selector is open. * * @return {@code true} if, and only if, this selector is open */ public abstract boolean isOpen(); /** * Returns the provider that created this channel. * * @return The provider that created this channel */ public abstract SelectorProvider provider(); /** * Returns this selector's key set. * * <p> The key set is not directly modifiable. A key is removed only after * it has been cancelled and its channel has been deregistered. Any * attempt to modify the key set will cause an {@link * UnsupportedOperationException} to be thrown. * * <p> The set is <a href="#ksc">safe</a> for use by multiple concurrent * threads. </p> * * @return This selector's key set * * @throws ClosedSelectorException * If this selector is closed */ public abstract Set<SelectionKey> keys(); /** * Returns this selector's selected-key set. * * <p> Keys may be removed from, but not directly added to, the * selected-key set. Any attempt to add an object to the key set will * cause an {@link UnsupportedOperationException} to be thrown. * * <p> The selected-key set is <a href="#sksc">not thread-safe</a>. </p> * * @return This selector's selected-key set * * @throws ClosedSelectorException * If this selector is closed */ public abstract Set<SelectionKey> selectedKeys(); /** * Selects a set of keys whose corresponding channels are ready for I/O * operations. * * <p> This method performs a non-blocking <a href="#selop">selection * operation</a>. If no channels have become selectable since the previous * selection operation then this method immediately returns zero. * * <p> Invoking this method clears the effect of any previous invocations * of the {@link #wakeup wakeup} method. </p> * * @return The number of keys, possibly zero, whose ready-operation sets * were updated by the selection operation * * @throws IOException * If an I/O error occurs * * @throws ClosedSelectorException * If this selector is closed */ public abstract int selectNow() throws IOException; /** * Selects a set of keys whose corresponding channels are ready for I/O * operations. * * <p> This method performs a blocking <a href="#selop">selection * operation</a>. It returns only after at least one channel is selected, * this selector's {@link #wakeup wakeup} method is invoked, the current * thread is interrupted, or the given timeout period expires, whichever * comes first. * * <p> This method does not offer real-time guarantees: It schedules the * timeout as if by invoking the {@link Object#wait(long)} method. </p> * * @param timeout If positive, block for up to {@code timeout} * milliseconds, more or less, while waiting for a * channel to become ready; if zero, block indefinitely; * must not be negative * * @return The number of keys, possibly zero, * whose ready-operation sets were updated * * @throws IOException * If an I/O error occurs * * @throws ClosedSelectorException * If this selector is closed * * @throws IllegalArgumentException * If the value of the timeout argument is negative */ public abstract int select(long timeout) throws IOException; /** * Selects a set of keys whose corresponding channels are ready for I/O * operations. * * <p> This method performs a blocking <a href="#selop">selection * operation</a>. It returns only after at least one channel is selected, * this selector's {@link #wakeup wakeup} method is invoked, or the current * thread is interrupted, whichever comes first. </p> * * @return The number of keys, possibly zero, * whose ready-operation sets were updated * * @throws IOException * If an I/O error occurs * * @throws ClosedSelectorException * If this selector is closed */ public abstract int select() throws IOException; /** * Selects and performs an action on the keys whose corresponding channels * are ready for I/O operations. * * <p> This method performs a blocking <a href="#selop">selection * operation</a>. It wakes up from querying the operating system only when * at least one channel is selected, this selector's {@link #wakeup wakeup} * method is invoked, the current thread is interrupted, or the given * timeout period expires, whichever comes first. * * <p> The specified <i>action</i>'s {@link Consumer#accept(Object) accept} * method is invoked with the key for each channel that is ready to perform * an operation identified by its key's interest set. The {@code accept} * method may be invoked more than once for the same key but with the * ready-operation set containing a subset of the operations for which the * channel is ready (as described above). The {@code accept} method is * invoked while synchronized on the selector and its selected-key set. * Great care must be taken to avoid deadlocking with other threads that * also synchronize on these objects. Selection operations are not reentrant * in general and consequently the <i>action</i> should take great care not * to attempt a selection operation on the same selector. The behavior when * attempting a reentrant selection operation is implementation specific and * therefore not specified. If the <i>action</i> closes the selector then * {@code ClosedSelectorException} is thrown when the action completes. * The <i>action</i> is not prohibited from closing channels registered with * the selector, nor prohibited from cancelling keys or changing a key's * interest set. If a channel is selected but its key is cancelled or its * interest set changed before the <i>action</i> is performed on the key * then it is implementation specific as to whether the <i>action</i> is * invoked (it may be invoked with an {@link SelectionKey#isValid() invalid} * key). Exceptions thrown by the action are relayed to the caller. * * <p> This method does not offer real-time guarantees: It schedules the * timeout as if by invoking the {@link Object#wait(long)} method. * * @implSpec The default implementation removes all keys from the * selected-key set, invokes {@link #select(long) select(long)} with the * given timeout and then performs the action for each key added to the * selected-key set. The default implementation does not detect the action * performing a reentrant selection operation. The selected-key set may * or may not be empty on completion of the default implementation. * * @param action The action to perform * * @param timeout If positive, block for up to {@code timeout} * milliseconds, more or less, while waiting for a * channel to become ready; if zero, block indefinitely; * must not be negative * * @return The number of unique keys consumed, possibly zero * * @throws IOException * If an I/O error occurs * * @throws ClosedSelectorException * If this selector is closed or is closed by the action * * @throws IllegalArgumentException * If the value of the timeout argument is negative * * @since 11 */ public int select(Consumer<SelectionKey> action, long timeout) throws IOException { if (timeout < 0) throw new IllegalArgumentException("Negative timeout"); return doSelect(Objects.requireNonNull(action), timeout); } /** * Selects and performs an action on the keys whose corresponding channels * are ready for I/O operations. * * <p> This method performs a blocking <a href="#selop">selection * operation</a>. It wakes up from querying the operating system only when * at least one channel is selected, this selector's {@link #wakeup wakeup} * method is invoked, or the current thread is interrupted, whichever comes * first. * * <p> This method is equivalent to invoking the 2-arg * {@link #select(Consumer, long) select} method with a timeout of {@code 0} * to block indefinitely. </p> * * @implSpec The default implementation invokes the 2-arg {@code select} * method with a timeout of {@code 0}. * * @param action The action to perform * * @return The number of unique keys consumed, possibly zero * * @throws IOException * If an I/O error occurs * * @throws ClosedSelectorException * If this selector is closed or is closed by the action * * @since 11 */ public int select(Consumer<SelectionKey> action) throws IOException { return select(action, 0); } /** * Selects and performs an action on the keys whose corresponding channels * are ready for I/O operations. * * <p> This method performs a non-blocking <a href="#selop">selection * operation</a>. * * <p> Invoking this method clears the effect of any previous invocations * of the {@link #wakeup wakeup} method. </p> * * @implSpec The default implementation removes all keys from the * selected-key set, invokes {@link #selectNow() selectNow()} and then * performs the action for each key added to the selected-key set. The * default implementation does not detect the action performing a reentrant * selection operation. The selected-key set may or may not be empty on * completion of the default implementation. * * @param action The action to perform * * @return The number of unique keys consumed, possibly zero * * @throws IOException * If an I/O error occurs * * @throws ClosedSelectorException * If this selector is closed or is closed by the action * * @since 11 */ public int selectNow(Consumer<SelectionKey> action) throws IOException { return doSelect(Objects.requireNonNull(action), -1); } /** * Default implementation of select(Consumer) and selectNow(Consumer). */ private int doSelect(Consumer<SelectionKey> action, long timeout) throws IOException { synchronized (this) { Set<SelectionKey> selectedKeys = selectedKeys(); synchronized (selectedKeys) { selectedKeys.clear(); int numKeySelected; if (timeout < 0) { numKeySelected = selectNow(); } else { numKeySelected = select(timeout); } // copy selected-key set as action may remove keys Set<SelectionKey> keysToConsume = Set.copyOf(selectedKeys); assert keysToConsume.size() == numKeySelected; selectedKeys.clear(); // invoke action for each selected key keysToConsume.forEach(k -> { action.accept(k); if (!isOpen()) throw new ClosedSelectorException(); }); return numKeySelected; } } } /** * Causes the first selection operation that has not yet returned to return * immediately. * * <p> If another thread is currently blocked in a selection operation then * that invocation will return immediately. If no selection operation is * currently in progress then the next invocation of a selection operation * will return immediately unless {@link #selectNow()} or {@link * #selectNow(Consumer)} is invoked in the meantime. In any case the value * returned by that invocation may be non-zero. Subsequent selection * operations will block as usual unless this method is invoked again in the * meantime. * * <p> Invoking this method more than once between two successive selection * operations has the same effect as invoking it just once. </p> * * @return This selector */ public abstract Selector wakeup(); /** * Closes this selector. * * <p> If a thread is currently blocked in one of this selector's selection * methods then it is interrupted as if by invoking the selector's {@link * #wakeup wakeup} method. * * <p> Any uncancelled keys still associated with this selector are * invalidated, their channels are deregistered, and any other resources * associated with this selector are released. * * <p> If this selector is already closed then invoking this method has no * effect. * * <p> After a selector is closed, any further attempt to use it, except by * invoking this method or the {@link #wakeup wakeup} method, will cause a * {@link ClosedSelectorException} to be thrown. </p> * * @throws IOException * If an I/O error occurs */ public abstract void close() throws IOException; }