NIO之坑：完全理解NIO Selector

目录

• • Selector是什么
  • 如何创建一个Selector对象
  • 如何将selectable channel注册到selector中
  • selector如何维护selection keys
  • selector如何选择就绪channel
  • selector线程安全吗
  • 原文
  • Selection
  • Concurrency

Selector是什么

• Selector是一个或多个SelectableChannel对象的多路复用器.

如何创建一个Selector对象

• 一个

  selector

  对象可以通过调用Selector.open()来创建，这个工厂方法会使用系统默认的selector provider来创建一个新的selector对象。或者我们还可以通过实现抽象类

  SelectorProvider

  自定义一个selector provider，然后调用它的

  openSelector()

  来创建，

  例如：new SelectorProviderImpl().openSelector()

• 除非调用selector.close()，否则该

  selector

  将会一直保持

  打开状态

  。

如何将selectable channel注册到selector中

• 通过

  channel

  的register方法，将

  channel

  注册到给定的

  selector

  中，并返回一个表示注册关系的SelectionKey 对象。

selector如何维护selection keys

一个

selector

维护着三个selection keys集合:

• key set 包含着所有

  selectionKey

  s，当前所有注册到

  selector

  中的

  channel

  返回的注册关系SelectionKey都包含在内，这个集合可以通过selector.keys() 方法返回。
• selected-key set 包含着一部分

  selectionKey

  s，其中的每个

  selectionKey

  所关联的

  channel

  在

  selection operation

  期间被检测出至少

  准备好

  了一个可以在

  兴趣集

  中匹配到的操作。这个集合可以通过调用selector.selectedKeys()方法返回。selected-key set 一定是 key set 的子集。
• cancelled-key set 也包含着一部分

  selectionKey

  s，其中的每个

  selectionKey

  都已经被

  取消

  ，但是所关联

  channel

  还没有被

  撤销登记

  。cancelled-key set 不能够被直接返回，但也一定是 key set 的子集。

对于一个新创建的

selector

其中这三个集合都是空着的。

通过

channel

的register方法，一个

selectionKey

被增加到

selector

的 key set 中。

无论通过channel.close()还是通过selectionKey.cancel()来

取消

一个

selectionKey

，这个

selectionKey

都会被立即添加到

selector

的 cancelled-key set 中，但是所关联的

channel

并没有立即被

撤销登记

，直到发生下次

selection operations

, 这些

channel

才被从

selector

中

撤销登记

，与此同时这些Cancelled keys才会被从这个

selector

的所有

selectionKey set

（可能是_key set_、selected-key set、cancelled-key set）中移除，但是不会影响这些集合本身。

在

selection operations

期间，一些

selectionKey

会被选中添加到 selected-key set 中。其中的每个

key

可以通过selectiedKeys.remove()或selectiedKeys.iterator().remove()直接从 selected-key set 中移除，除此之外不能够通过任何方式被直接移除。特殊的，selected-key set 中的

keys

还可以在

selection operations

期间被间接移除。但是是不可以直接向 selected-key set 添加

key

的。

selector如何选择就绪channel

• 每次

  selection operation

  期间， keys都可以添加到或从selector’s selected-key set 被移除，同时也可以从它的 key 和 cancelled-key sets 被移除。

  selection operation

  可以被触发通过执行selector.select()，selector.select(long)，和selector.selectNow() 方法，并且这些方法涉及到以下三个步骤：

1. 首先每个位于 cancelled-key set 中的

   key

   会从每个包含它的

   key

   集合中被移除，并且对应的

   channel

   会被

   撤销登记

   。这个步骤使得 cancelled-key set 变为空。
2. 查询底层操作系统来获得关于

   selector

   中剩余

   channel

   的

   就续事件

   从

   selection operation

   开始截止到此刻的更新情况，只要哪个

   channel

   的

   就续事件的更新部分

   有至少一个与

   兴趣集

   中的操作匹配上，那么将会执行以下两个动作：
   1. 如果这个

      channel's key

      没有存在

      于 selected-key set 那么将它添加到这个集合中，并将它的

      就绪操作集

      (ready-operation set)修改成

      只包含

      使得

      channel

      被报告就绪的操作，任何先前记录在

      就绪操作集

      中的就绪信息都会被丢弃。
   2. 否则，如果这个

      channel's key

      存在

      于 selected-key set ，那么就保留

      就绪操作集

      中先前的就绪信息，并将这些 使得

      channel

      被报告就绪的操作 写入进去；总而言之，系统底层会通过

      按位与&操作

      更新当前就绪集。
   如果这些

   Key

   的

   兴趣集

   为空，那么 selected-key set 和 keys’的

   就续集

   (ready-operation sets)都不会被更新。
3. 如果在步骤（2）正在进行时将任何

   key

   添加到 cancelled-key set，则按步骤（1）处理它们。

• selection operations

  是否会阻塞等待一个或多个通道准备就绪，以及等待多长时间，这是三种选择方法之间唯一的本质区别。

selector线程安全吗

多线程并发情况下

Selectors

本身是线程安全的，但是他们所持有的

key sets

不是线程安全的。

selection operations

按顺序在

selector

本身，key set 和 selected-key set 上同步。 它们还在上面的步骤（1）和（3）期间在 canceled-key set 上同步。

在

selection operations

期间改变

key

的

兴趣集

，对于本次操作将不会产生任何影响；它们的影响将会在下次

selection operations

期间发生。

selectionKey

可能会被取消，

channel

可能随时关闭。 因此，在一个或多个选择器的

key集

中存在并不意味着

selectionKey

有效或其

channel

是开放的。有可能另一个线程取消

selectionKey

或关闭一个

channel

，应用程序代码应该小心地同步并检查这些条件。

一个线程通过selector.select()或selector.select(long)方法产生的阻塞可以被其他线程用以下三种方式的任意一种来中断：

• By invoking the selector’s wakeup() method,
• By invoking the selector’s close() method, or
• By invoking the blocked thread’s interrupt() method, in which case its interrupt status will be set and the selector’s wakeup() method will be invoked.

selector.close() 在

selection operations

期间会顺序的同步

selector

and

all three key sets

。

一个

selector

的 key set 和 selected-key set 通常情况下是线程不安全的。如果一个线程想要修改这个集合，需要同步控制它。通过

key集合

的iterator()方法返回的

Iterators

提供了

快速失败

（fail-fast）：如果在创建迭代器之后修改了set，除了通过调用迭代器自己的remove() 方法之外，将抛出ConcurrentModificationException 。

原文

• A multiplexor of SelectableChannel objects.

  A selector may be created by invoking the

  open

  method of this class, which will use the system’s default

  selector provider

  to create a new selector. A selector may also be created by invoking the

  openSelector

  method of a custom selector provider. A selector remains open until it is closed via its

  close

  method.

  A selectable channel’s registration with a selector is represented by a

  SelectionKey

  object. A selector maintains three sets of selection keys:
  • The key set contains the keys representing the current channel registrations of this selector. This set is returned by the

    keys

    method.
  • 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

    selectedKeys

    method. The selected-key set is always a subset of the key set.
  • 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.

  All three sets are empty in a newly-created selector.

  A key is added to a selector’s key set as a side effect of registering a channel via the channel’s

  register

  method. Cancelled keys are removed from the key set during selection operations. The key set itself is not directly modifiable.

  A key is added to its selector’s cancelled-key set when it is cancelled, whether by closing its channel or by invoking its

  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.

  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

  remove

  method or by invoking the

  remove

  method of an

  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.

  Selection

  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

  select()

  ,

  select(long)

  , and

  selectNow()

  methods, and involves three steps:
  1. 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.
  2. 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:
     1. 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.
     2. 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.

  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.

  3. If any keys were added to the cancelled-key set while step (2) was in progress then they are processed as in step (1).

  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.

  Concurrency

  Selectors are themselves safe for use by multiple concurrent threads; their key sets, however, are not.

  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.

  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.

  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.

  A thread blocked in one of the

  select()

  or

  select(long)

  methods may be interrupted by some other thread in one of three ways:
  • By invoking the selector’s

    wakeup

    method,
  • By invoking the selector’s

    close

    method, or
  • By invoking the blocked thread’s

    interrupt

    method, in which case its interrupt status will be set and the selector’s

    wakeup

    method will be invoked.
  The

  close

  method synchronizes on the selector and all three key sets in the same order as in a selection operation.

  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’

  iterator

  methods are fail-fast: If the set is modified after the iterator is created, in any way except by invoking the iterator’s own

  remove

  method, then a

  ConcurrentModificationException

  will be thrown.