Back to index...

	/*
	* 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 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;
	}

Back to index...