Java NIO中Selector类源码

Java NIO中Selector源码

/** * 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 </code>selector provider<code>} 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><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. *   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 name="sks"><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 </code>iterator<code>} 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"> * <h4>Selection</h4> * * <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 type=a> * *     <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></li> * *   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> * * * <h4>Concurrency</h4> * * <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><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 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:</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> */public abstract class Selector {    protected Selector() { }    public static Selector open() throws IOException {return SelectorProvider.provider().openSelector();    }    public abstract boolean isOpen();    public abstract SelectorProvider provider();    public abstract Set<SelectionKey> keys();    public abstract Set<SelectionKey> selectedKeys();    public abstract int selectNow() throws IOException;    public abstract int select(long timeout)throws IOException;    public abstract int select() throws IOException;    /**     * Causes the first selection operation that has not yet returned to return     * immediately.     */    public abstract Selector wakeup();    /**     * Closes this selector.     */    public abstract void close() throws IOException;}