netty技术内幕一(Selector,SelectionKey)

Java Nio注意事项 # selector

Selector类的使用(一)

SelectionKey类的使用

/*
package java.nio.channels;import java.io.Closeable;
import java.io.IOException;
import java.nio.channels.spi.SelectorProvider;
import java.util.Set;/**=============================================channel的多路复用器=============================================* A multiplexor of {@link SelectableChannel} objects.=============================================1个selector可以通过调用Selector#open方法创建(使用系统默认实现),也可以通过SelectorProvider#open方法提供自定义的实现。selector会一直处于打开状态，一直到调用该selector#close方法=============================================* <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"></a>=============================================当1个channel注册到selector时，就会得到1个SelectionKey来代表这个注册关系,1个selector维护了3个这个的SelectionKey的set集合=============================================   * <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>=============================================第1个是 key set 集合, 它代表了注册到当前selector的所有channel注册关系, 可以通过selector#key()方法返回=============================================*   <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>=============================================第2个是 selected-key set集合, 它包含了通过上一次的查询操作, 当注册在该selector上的channel的任何感兴趣的事件已就绪时, 所对应的selectionKey。 可以通过调用Selector#selectedKeys()返回这个集合永远时key set集合的子集=============================================*   <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>=============================================第3个是 cancelled-key set集合, 它代表着所有被取消的SelectionKey, 但是这些被取消的key对应的channel还没有从selector上注销, 这个集合不能直接被访问,这个集合永远时key set集合的子集=============================================*   <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>=============================================在新建的selector中, 这3个set集合都是空的=============================================* <p> All three sets are empty in a newly-created selector.=============================================当把1个channel注册到selector上时，就会得到1个SelectionKey来代表这个注册关系。被取消的key在查询操作期间, 才会从三个集合中移除掉。key set集合是不能被直接修改的。=============================================* <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.=============================================无论是调用SelectionKey#cancel方法还是关闭channel, 其对应的SelectionKey都将会被添加到cancelled-key set集合中。但是这并不会使得对应channel立即从selector中注销, 而是在selector下一次作查询操作的时候, 这个channel才会从这个selector中注销, 并且注销的selectionKey将会从3个set集合中移除掉=============================================* <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.=============================================在selector的查询操作浅见, 会将发生感兴趣事件的SelectionKey添加到 slected-key 集合中,注意: 不能直接往slected-key 集合中添加SelectionKey.可以通过调用remove方法或者对应的迭代器的remove方法, 将某个selectionKey从slected-key 集合中移除=============================================* <a name="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.  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"></a>* <h2>Selection</h2>=============================================在selector的每一次查询操作期间, selectionKey可能被添加到selected-key集合当中，也可能从selected-key集合中移除,selectionKey也可能从key set集合中移除,selectionKey也可能从cancelled-key中移除。可以通过调用selector#select()、selector#select(long)、selector#selectNow()开启查询, 它包含3个步骤.=============================================* <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>=============================================第一步: cancelled-key集合中的selectionKey从3个集合中全部移除掉, 对应的channel也从selector中注销掉,这个步骤会让cancelled-key集合变为空的=============================================*   <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>=============================================第二步: 底层操作系统会去查询剩余未被注销的channel是否发生了注册时所感兴趣的事件，当1个channel的任何1个感兴趣事件已就绪时, 会执行下面2个动作中的1个=============================================*   <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>=============================================如果这个channel的selectionKey还没有在selected-key集合中, 则它会被添加到这个集合中,并且该selectionKey的就绪事件集被修改为channel当前已就绪事件集。selectionKey之前的就绪集将会被丢弃。=============================================*     <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>=============================================如果这个channel的selectionKey已经在selected-key集合中, 之前的就绪事件集将会保留=============================================*     <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>=============================================如果selector的key set集合中的所有SelectionKey在selector查询步骤开始的时候就没有任何感兴趣的事件,那么就不会更新selected-key集合， 也不会更新selectionKey中的任何就绪事件集=============================================*   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.=============================================如果每个key都被添加到了cacelled-key集合中， 那在第一步的时候就已经清空了=============================================*   <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>=============================================是等待1个channel感兴趣的事件就绪, 还是等待多个channel感兴趣的事件就绪, 等待多长时间是selector查询操作的这3个方法的区别=============================================   * <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</h2>=============================================Selectors本身是线程安全的，但是他们的key set集合不是线程安全的=============================================   * <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.=============================================对selectionKey的感兴趣事件进行修改，不会立即生效，它将会在下一次查询操作时才会生效=============================================   * <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.=============================================key可能会在任何事件取消, channnel也可能在任何时间被关闭。因此,从selector中获取的key set集合并不意味着这些key都是有效的或者这些key对应的channel都是打开状态的,应用代码应该同步并且在必要的时候检查它们的状态, 以确定它们是否被取消或者被关闭了=============================================   * <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.=============================================1个线程调用selector#select()或者selector#select(long)方法可以通过其它线程使用以下的方式中断: =============================================   * <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>=============================================通过调用selector的wakeup方法=============================================   *   <li><p> By invoking the selector's {@link #wakeup wakeup} method,*   </p></li>=============================================通过调用selector的close方法=============================================   *   <li><p> By invoking the selector's {@link #close close} method, or*   </p></li>=============================================通过调用阻塞线程对象的interrupt方法, 在这种情况下, 线程会设置中断标记, 并且selector的wakeup()方法将会被调用=============================================   *   <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"></a>=============================================selector的key set集合与selected-key集合通常对多线程并发来说不是安全的。如果1个线程要直接修改这些set集合，应当要对这些set集合本身同步加锁。如果这个set集合在获取迭代器后，以任何除了调用迭代器本身的remove的方法修改了，那么会抛出并发修改异常，=============================================   	* <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>**** @see SelectableChannel* @see SelectionKey*/public abstract class Selector implements Closeable {protected Selector() { }public static Selector open() throws IOException {return SelectorProvider.provider().openSelector();}// 返回 selector是否处于打开状态public abstract boolean isOpen();public abstract SelectorProvider provider();// 返回 selector的key set集合（注册的所有channel对应的SelectionKey）public abstract Set<SelectionKey> keys();// 返回 selector的key set集合（注册的所有channel对应的SelectionKey）public abstract Set<SelectionKey> selectedKeys();// 查询selector当前注册的channel感兴趣且已就绪的事件// 这个方法是非阻塞的。如果当前没有已就绪的事件，则返回0。// 调用此方法会消耗1次wakeup方法的调用public abstract int selectNow() throws IOException;// 查询selector当前注册的channel感兴趣且已就绪的事件// 这个方法是阻塞的，它会在如下的几种情况下停止阻塞：// 		1. 查询到至少1个channel的感兴趣且已就绪事件//      2. 调用了selector#wakeup方法//      3. 当前线程被中断//      4. 已过超时时间// 这个方法并不保证时间是对的，会有毫秒上的差异。// 指定的时间不应当为负数。如果是0，表示超时时间无限长。public abstract int select(long timeout) throws IOException;// 查询selector当前注册的channel感兴趣且已就绪的事件// 这个方法是阻塞的，它会在如下的几种情况下停止阻塞：// 		1. 查询到至少1个channel的感兴趣且已就绪事件//      2. 调用了selector#wakeup方法//      3. 当前线程被中断public abstract int select() throws IOException;// 调用此方法能让正处于查询状态的selector立即唤醒返回// 如果selector调用了select()或者select(long)处于阻塞状态, 调用此方法会让select方法立即返回。// 如果selector还未调用select()或者select(long)或者selectNow()前，就已经调用了wakeup，// 那么下一次调用这三个方法中的任何一个时，将会立即返回。// 在2个select()期间，调用此方法2次或多次，跟调用1次是一样的。// 作用类似于LockSupport#unpark()中的许可证。public abstract Selector wakeup();// 关闭selector// 如果selector正在select查询阻塞状态，那么它会停止阻塞，类似于调用了wakeup方法。// 任何未被取消的SelectionKey将会失效，对应的channnel也会注销掉，与此selector关联的其它资源也会被释放掉。// 再次调用此方法没有任何效果。// 关闭此selector后，除了调用wakeup方法外，调用任何其它的方法都将会抛出异常。public abstract void close() throws IOException;}