/* * Copyright (c) 2000, 2004, 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.Set; /** * A multiplexor of {@link SelectableChannel} objects. * *

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 selectable channel's registration with a selector is represented by a * {@link SelectionKey} object. A selector maintains three sets of selection * keys: * *

* *

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 during * selection 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 deregistered * during 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} method * of an {@link java.util.Iterator iterator} obtained from the * set. 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: *

* *
    * *

  1. 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.

    2. * *

  2. 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:

    * *
      * *

    1. 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.

      2. * *

    2. 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.

      3. * *
    3. * * 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. * *

  3. If any keys were added to the cancelled-key set while step (2) was * in progress then they are processed as in step (1).

    4. * *
* *

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 of * three ways: * *

* *

The {@link #close close} method synchronizes on the selector and all * three 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 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.

* * * @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. * *

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.

* * @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 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. * *

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. * *

The key set is not thread-safe.

* * @return This selector's key set * * @throws ClosedSelectorException * If this selector is closed */ public abstract Set keys(); /** * Returns this selector's selected-key set. * *

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. * *

The selected-key set is not thread-safe.

* * @return This selector's selected-key set * * @throws ClosedSelectorException * If this selector is closed */ public abstract Set selectedKeys(); /** * Selects a set of keys whose corresponding channels are ready for I/O * operations. * *

This method performs a non-blocking selection * operation. If no channels have become selectable since the previous * selection operation then this method immediately returns zero. * *

Invoking this method clears the effect of any previous invocations * of the {@link #wakeup wakeup} method.

* * @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. * *

This method performs a blocking selection * operation. 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. * *

This method does not offer real-time guarantees: It schedules the * timeout as if by invoking the {@link Object#wait(long)} method.

* * @param timeout If positive, block for up to 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. * *

This method performs a blocking selection * operation. 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.

* * @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; /** * Causes the first selection operation that has not yet returned to return * immediately. * *

If another thread is currently blocked in an invocation of the * {@link #select()} or {@link #select(long)} methods then that invocation * will return immediately. If no selection operation is currently in * progress then the next invocation of one of these methods will return * immediately unless the {@link #selectNow()} method is invoked in the * meantime. In any case the value returned by that invocation may be * non-zero. Subsequent invocations of the {@link #select()} or {@link * #select(long)} methods will block as usual unless this method is invoked * again in the meantime. * *

Invoking this method more than once between two successive selection * operations has the same effect as invoking it just once.

* * @return This selector */ public abstract Selector wakeup(); /** * Closes this selector. * *

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. * *

Any uncancelled keys still associated with this selector are * invalidated, their channels are deregistered, and any other resources * associated with this selector are released. * *

If this selector is already closed then invoking this method has no * effect. * *

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.

* * @throws IOException * If an I/O error occurs */ public abstract void close() throws IOException; }