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397 lines
15 KiB
397 lines
15 KiB
/*
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* Copyright (c) 2000, 2013, Oracle and/or its affiliates. All rights reserved.
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* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
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*/
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package java.nio.channels;
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import java.io.Closeable;
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import java.io.IOException;
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import java.nio.channels.spi.SelectorProvider;
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import java.util.Set;
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/**
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* A multiplexor of {@link SelectableChannel} objects.
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*
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* <p> A selector may be created by invoking the {@link #open open} method of
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* this class, which will use the system's default {@link
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* java.nio.channels.spi.SelectorProvider selector provider} to
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* create a new selector. A selector may also be created by invoking the
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* {@link java.nio.channels.spi.SelectorProvider#openSelector openSelector}
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* method of a custom selector provider. A selector remains open until it is
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* closed via its {@link #close close} method.
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*
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* <a name="ks"></a>
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*
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* <p> A selectable channel's registration with a selector is represented by a
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* {@link SelectionKey} object. A selector maintains three sets of selection
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* keys:
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*
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* <ul>
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*
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* <li><p> The <i>key set</i> contains the keys representing the current
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* channel registrations of this selector. This set is returned by the
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* {@link #keys() keys} method. </p></li>
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*
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* <li><p> The <i>selected-key set</i> is the set of keys such that each
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* key's channel was detected to be ready for at least one of the operations
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* identified in the key's interest set during a prior selection operation.
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* This set is returned by the {@link #selectedKeys() selectedKeys} method.
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* The selected-key set is always a subset of the key set. </p></li>
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*
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* <li><p> The <i>cancelled-key</i> set is the set of keys that have been
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* cancelled but whose channels have not yet been deregistered. This set is
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* not directly accessible. The cancelled-key set is always a subset of the
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* key set. </p></li>
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*
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* </ul>
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*
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* <p> All three sets are empty in a newly-created selector.
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*
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* <p> A key is added to a selector's key set as a side effect of registering a
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* channel via the channel's {@link SelectableChannel#register(Selector,int)
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* register} method. Cancelled keys are removed from the key set during
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* selection operations. The key set itself is not directly modifiable.
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*
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* <p> A key is added to its selector's cancelled-key set when it is cancelled,
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* whether by closing its channel or by invoking its {@link SelectionKey#cancel
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* cancel} method. Cancelling a key will cause its channel to be deregistered
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* during the next selection operation, at which time the key will removed from
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* all of the selector's key sets.
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*
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* <a name="sks"></a><p> Keys are added to the selected-key set by selection
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* operations. A key may be removed directly from the selected-key set by
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* invoking the set's {@link java.util.Set#remove(java.lang.Object) remove}
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* method or by invoking the {@link java.util.Iterator#remove() remove} method
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* of an {@link java.util.Iterator iterator} obtained from the
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* set. Keys are never removed from the selected-key set in any other way;
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* they are not, in particular, removed as a side effect of selection
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* operations. Keys may not be added directly to the selected-key set. </p>
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*
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*
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* <a name="selop"></a>
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* <h2>Selection</h2>
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*
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* <p> During each selection operation, keys may be added to and removed from a
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* selector's selected-key set and may be removed from its key and
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* cancelled-key sets. Selection is performed by the {@link #select()}, {@link
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* #select(long)}, and {@link #selectNow()} methods, and involves three steps:
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* </p>
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*
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* <ol>
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*
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* <li><p> Each key in the cancelled-key set is removed from each key set of
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* which it is a member, and its channel is deregistered. This step leaves
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* the cancelled-key set empty. </p></li>
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*
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* <li><p> The underlying operating system is queried for an update as to the
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* readiness of each remaining channel to perform any of the operations
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* identified by its key's interest set as of the moment that the selection
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* operation began. For a channel that is ready for at least one such
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* operation, one of the following two actions is performed: </p>
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*
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* <ol>
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*
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* <li><p> If the channel's key is not already in the selected-key set then
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* it is added to that set and its ready-operation set is modified to
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* identify exactly those operations for which the channel is now reported
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* to be ready. Any readiness information previously recorded in the ready
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* set is discarded. </p></li>
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*
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* <li><p> Otherwise the channel's key is already in the selected-key set,
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* so its ready-operation set is modified to identify any new operations
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* for which the channel is reported to be ready. Any readiness
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* information previously recorded in the ready set is preserved; in other
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* words, the ready set returned by the underlying system is
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* bitwise-disjoined into the key's current ready set. </p></li>
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*
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* </ol>
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*
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* If all of the keys in the key set at the start of this step have empty
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* interest sets then neither the selected-key set nor any of the keys'
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* ready-operation sets will be updated.
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*
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* <li><p> If any keys were added to the cancelled-key set while step (2) was
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* in progress then they are processed as in step (1). </p></li>
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*
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* </ol>
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*
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* <p> Whether or not a selection operation blocks to wait for one or more
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* channels to become ready, and if so for how long, is the only essential
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* difference between the three selection methods. </p>
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*
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*
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* <h2>Concurrency</h2>
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*
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* <p> Selectors are themselves safe for use by multiple concurrent threads;
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* their key sets, however, are not.
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*
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* <p> The selection operations synchronize on the selector itself, on the key
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* set, and on the selected-key set, in that order. They also synchronize on
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* the cancelled-key set during steps (1) and (3) above.
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*
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* <p> Changes made to the interest sets of a selector's keys while a
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* selection operation is in progress have no effect upon that operation; they
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* will be seen by the next selection operation.
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*
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* <p> Keys may be cancelled and channels may be closed at any time. Hence the
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* presence of a key in one or more of a selector's key sets does not imply
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* that the key is valid or that its channel is open. Application code should
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* be careful to synchronize and check these conditions as necessary if there
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* is any possibility that another thread will cancel a key or close a channel.
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*
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* <p> A thread blocked in one of the {@link #select()} or {@link
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* #select(long)} methods may be interrupted by some other thread in one of
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* three ways:
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*
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* <ul>
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*
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* <li><p> By invoking the selector's {@link #wakeup wakeup} method,
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* </p></li>
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*
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* <li><p> By invoking the selector's {@link #close close} method, or
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* </p></li>
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*
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* <li><p> By invoking the blocked thread's {@link
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* java.lang.Thread#interrupt() interrupt} method, in which case its
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* interrupt status will be set and the selector's {@link #wakeup wakeup}
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* method will be invoked. </p></li>
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*
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* </ul>
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*
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* <p> The {@link #close close} method synchronizes on the selector and all
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* three key sets in the same order as in a selection operation.
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*
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* <a name="ksc"></a>
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*
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* <p> A selector's key and selected-key sets are not, in general, safe for use
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* by multiple concurrent threads. If such a thread might modify one of these
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* sets directly then access should be controlled by synchronizing on the set
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* itself. The iterators returned by these sets' {@link
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* java.util.Set#iterator() iterator} methods are <i>fail-fast:</i> If the set
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* is modified after the iterator is created, in any way except by invoking the
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* iterator's own {@link java.util.Iterator#remove() remove} method, then a
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* {@link java.util.ConcurrentModificationException} will be thrown. </p>
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*
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*
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* @author Mark Reinhold
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* @author JSR-51 Expert Group
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* @since 1.4
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*
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* @see SelectableChannel
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* @see SelectionKey
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*/
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public abstract class Selector implements Closeable {
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/**
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* Initializes a new instance of this class.
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*/
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protected Selector() { }
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/**
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* Opens a selector.
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*
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* <p> The new selector is created by invoking the {@link
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* java.nio.channels.spi.SelectorProvider#openSelector openSelector} method
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* of the system-wide default {@link
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* java.nio.channels.spi.SelectorProvider} object. </p>
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*
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* @return A new selector
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*
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* @throws IOException
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* If an I/O error occurs
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*/
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public static Selector open() throws IOException {
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return SelectorProvider.provider().openSelector();
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}
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/**
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* Tells whether or not this selector is open.
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*
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* @return <tt>true</tt> if, and only if, this selector is open
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*/
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public abstract boolean isOpen();
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/**
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* Returns the provider that created this channel.
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*
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* @return The provider that created this channel
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*/
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public abstract SelectorProvider provider();
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/**
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* Returns this selector's key set.
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*
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* <p> The key set is not directly modifiable. A key is removed only after
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* it has been cancelled and its channel has been deregistered. Any
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* attempt to modify the key set will cause an {@link
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* UnsupportedOperationException} to be thrown.
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*
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* <p> The key set is <a href="#ksc">not thread-safe</a>. </p>
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*
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* @return This selector's key set
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*
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* @throws ClosedSelectorException
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* If this selector is closed
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*/
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public abstract Set<SelectionKey> keys();
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/**
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* Returns this selector's selected-key set.
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*
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* <p> Keys may be removed from, but not directly added to, the
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* selected-key set. Any attempt to add an object to the key set will
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* cause an {@link UnsupportedOperationException} to be thrown.
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*
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* <p> The selected-key set is <a href="#ksc">not thread-safe</a>. </p>
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*
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* @return This selector's selected-key set
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*
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* @throws ClosedSelectorException
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* If this selector is closed
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*/
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public abstract Set<SelectionKey> selectedKeys();
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/**
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* Selects a set of keys whose corresponding channels are ready for I/O
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* operations.
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*
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* <p> This method performs a non-blocking <a href="#selop">selection
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* operation</a>. If no channels have become selectable since the previous
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* selection operation then this method immediately returns zero.
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*
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* <p> Invoking this method clears the effect of any previous invocations
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* of the {@link #wakeup wakeup} method. </p>
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*
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* @return The number of keys, possibly zero, whose ready-operation sets
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* were updated by the selection operation
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*
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* @throws IOException
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* If an I/O error occurs
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*
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* @throws ClosedSelectorException
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* If this selector is closed
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*/
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public abstract int selectNow() throws IOException;
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/**
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* Selects a set of keys whose corresponding channels are ready for I/O
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* operations.
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*
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* <p> This method performs a blocking <a href="#selop">selection
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* operation</a>. It returns only after at least one channel is selected,
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* this selector's {@link #wakeup wakeup} method is invoked, the current
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* thread is interrupted, or the given timeout period expires, whichever
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* comes first.
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*
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* <p> This method does not offer real-time guarantees: It schedules the
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* timeout as if by invoking the {@link Object#wait(long)} method. </p>
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*
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* @param timeout If positive, block for up to <tt>timeout</tt>
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* milliseconds, more or less, while waiting for a
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* channel to become ready; if zero, block indefinitely;
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* must not be negative
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*
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* @return The number of keys, possibly zero,
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* whose ready-operation sets were updated
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*
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* @throws IOException
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* If an I/O error occurs
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*
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* @throws ClosedSelectorException
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* If this selector is closed
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*
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* @throws IllegalArgumentException
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* If the value of the timeout argument is negative
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*/
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public abstract int select(long timeout)
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throws IOException;
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/**
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* Selects a set of keys whose corresponding channels are ready for I/O
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* operations.
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*
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* <p> This method performs a blocking <a href="#selop">selection
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* operation</a>. It returns only after at least one channel is selected,
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* this selector's {@link #wakeup wakeup} method is invoked, or the current
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* thread is interrupted, whichever comes first. </p>
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*
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* @return The number of keys, possibly zero,
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* whose ready-operation sets were updated
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*
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* @throws IOException
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* If an I/O error occurs
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*
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* @throws ClosedSelectorException
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* If this selector is closed
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*/
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public abstract int select() throws IOException;
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/**
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* Causes the first selection operation that has not yet returned to return
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* immediately.
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*
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* <p> If another thread is currently blocked in an invocation of the
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* {@link #select()} or {@link #select(long)} methods then that invocation
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* will return immediately. If no selection operation is currently in
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* progress then the next invocation of one of these methods will return
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* immediately unless the {@link #selectNow()} method is invoked in the
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* meantime. In any case the value returned by that invocation may be
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* non-zero. Subsequent invocations of the {@link #select()} or {@link
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* #select(long)} methods will block as usual unless this method is invoked
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* again in the meantime.
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*
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* <p> Invoking this method more than once between two successive selection
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* operations has the same effect as invoking it just once. </p>
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*
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* @return This selector
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*/
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public abstract Selector wakeup();
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/**
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* Closes this selector.
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*
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* <p> If a thread is currently blocked in one of this selector's selection
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* methods then it is interrupted as if by invoking the selector's {@link
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* #wakeup wakeup} method.
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*
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* <p> Any uncancelled keys still associated with this selector are
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* invalidated, their channels are deregistered, and any other resources
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* associated with this selector are released.
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*
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* <p> If this selector is already closed then invoking this method has no
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* effect.
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*
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* <p> After a selector is closed, any further attempt to use it, except by
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* invoking this method or the {@link #wakeup wakeup} method, will cause a
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* {@link ClosedSelectorException} to be thrown. </p>
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*
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* @throws IOException
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* If an I/O error occurs
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*/
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public abstract void close() throws IOException;
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}
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