The Selector.java Java example source code
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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.
*
* <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 name="ks">
*
* <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> The key set contains the keys representing the current
* channel registrations of this selector. This set is returned by the
* {@link #keys() keys} method. </p>
*
* <li>
The selected-key set 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.
* This set is returned by the {@link #selectedKeys() selectedKeys} method.
* The selected-key set is always a subset of the key set. </p>
*
* <li>
The cancelled-key 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>
*
* </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 name="sks">
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. </p>
*
*
* <a name="selop">
* <h2>Selection
*
* <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>
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>
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>
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>
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>
*
* </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>
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>
*
* </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>
*
*
* <h2>Concurrency
*
* <p> Selectors are themselves safe for use by multiple concurrent threads;
* their key sets, however, are not.
*
* <p> 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.
*
* <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 one of the {@link #select()} or {@link
* #select(long)} methods may be interrupted by some other thread in one of
* three ways:
*
* <ul>
*
* <li>
By invoking the selector's {@link #wakeup wakeup} method,
* </p>
*
* <li>
By invoking the selector's {@link #close close} method, or
* </p>
*
* <li>
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>
*
* </ul>
*
* <p> The {@link #close close} method synchronizes on the selector and all
* three key sets in the same order as in a selection operation.
*
* <a name="ksc">
*
* <p> 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 <i>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. </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 <tt>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 key set is not thread-safe.
*
* @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 not thread-safe.
*
* @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 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 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 <tt>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 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;
/**
* Causes the first selection operation that has not yet returned to return
* immediately.
*
* <p> 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.
*
* <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;
}
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