NIO之坑：完全理解NIO Selector

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Selector是什麼

• Selector是一個或多個SelectableChannel對象的多路複用器.

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如何創建一個Selector對象

• 一個selector對象可以通過調用Selector.open()來創建，這個工廠方法會使用系統默認的selector provider來創建一個新的selector對象。或者我們還可以通過實現抽象類SelectorProvider自定義一個selector provider，然後調用它的openSelector()來創建，
  例如：new SelectorProviderImpl().openSelector()
• 除非調用selector.close()，否則該selector將會一直保持打開狀態。

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如何將selectable channel註冊到selector中

   SelectionKey key = channel.register(selector,Selectionkey.XXX); 

• 通過channel的register方法，將channel註冊到給定的selector中，並返回一個表示註冊關係的SelectionKey 對象。

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selector如何維護selection keys

一個selector維護着三個selection keys集合:

• key set 包含着所有selectionKeys，當前所有註冊到selector中的channel返回的註冊關係SelectionKey都包含在內，這個集合可以通過selector.keys() 方法返回。
• selected-key set 包含着一部分selectionKeys，其中的每個selectionKey所關聯的channel在selection operation期間被檢測出至少 準備好 了一個可以在興趣集中匹配到的操作。這個集合可以通過調用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selectionKeys，其中的每個selectionKey都已經被取消，但是所關聯channel還沒有被撤銷登記。cancelled-key set 不能夠被直接返回，但也一定是 key set 的子集。

對於一個新創建的selector其中這三個集合都是空着的。

通過channel的register方法，一個selectionKey被增加到selector的 key set 中。

無論通過channel.close()還是通過selectionKey.cancel()來取消一個selectionKey ，這個selectionKey都會被立即添加到selector的 cancelled-key set 中，但是所關聯的channel並沒有立即被撤銷登記，直到發生下次 selection operations, 這些channel才被從selector中撤銷登記，與此同時這些Cancelled keys纔會被從這個selector的所有selectionKey set（可能是_key set_、selected-key set、cancelled-key set）中移除，但是不會影響這些集合本身。

在 selection operations 期間，一些selectionKey會被選中添加到 selected-key set 中。其中的每個key可以通過selectiedKeys.remove()或selectiedKeys.iterator().remove()直接從 selected-key set 中移除，除此之外不能夠通過任何方式被直接移除。特殊的，selected-key set 中的keys還可以在 selection operations 期間被間接移除。但是是不可以直接向 selected-key set 添加key的。

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selector如何選擇就緒channel

• 每次 selection operation 期間， keys都可以添加到或從selector’s selected-key set 被移除，同時也可以從它的 key 和 cancelled-key sets 被移除。 selection operation 可以被觸發通過執行selector.select()，selector.select(long)，和selector.selectNow() 方法，並且這些方法涉及到以下三個步驟：

1. 首先每個位於 cancelled-key set 中的key會從每個包含它的key集合中被移除，並且對應的channel會被撤銷登記。這個步驟使得 cancelled-key set 變爲空。

2. 查詢底層操作系統來獲得關於selector中剩餘channel的就續事件從 selection operation 開始截止到此刻的更新情況，只要哪個channel的就續事件的更新部分有至少一個與興趣集中的操作匹配上，那麼將會執行以下兩個動作：

   1. 如果這個channel's key 沒有存在 於 selected-key set 那麼將它添加到這個集合中，並將它的就緒操作集(ready-operation set)修改成 只包含使得channel被報告就緒的操作，任何先前記錄在就緒操作集中的就緒信息都會被丟棄。

   2. 否則，如果這個channel's key 存在 於 selected-key set ，那麼就保留就緒操作集中先前的就緒信息，並將這些 使得channel被報告就緒的操作 寫入進去；總而言之，系統底層會通過按位與&操作更新當前就緒集。

   如果這些Key的興趣集爲空，那麼 selected-key set 和 keys’的就續集(ready-operation sets)都不會被更新。

3. 如果在步驟（2）正在進行時將任何key添加到 cancelled-key set，則按步驟（1）處理它們。

• selection operations 是否會阻塞等待一個或多個通道準備就緒，以及等待多長時間，這是三種選擇方法之間唯一的本質區別。

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selector線程安全嗎

多線程併發情況下Selectors本身是線程安全的，但是他們所持有的key sets不是線程安全的。

selection operations 按順序在selector本身，key set 和 selected-key set 上同步。 它們還在上面的步驟（1）和（3）期間在 canceled-key set 上同步。

在 selection operations 期間改變key的興趣集，對於本次操作將不會產生任何影響；它們的影響將會在下次 selection operations 期間發生。

selectionKey可能會被取消，channel可能隨時關閉。 因此，在一個或多個選擇器的key集中存在並不意味着selectionKey有效或其channel是開放的。有可能另一個線程取消selectionKey或關閉一個channel，應用程序代碼應該小心地同步並檢查這些條件。

一個線程通過selector.select()或selector.select(long)方法產生的阻塞可以被其他線程用以下三種方式的任意一種來中斷：

• 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 期間會順序的同步selectorand all three key sets 。

一個selector的 key set 和 selected-key set 通常情況下是線程不安全的。如果一個線程想要修改這個集合，需要同步控制它。通過key集合的iterator()方法返回的Iterators提供了快速失敗（fail-fast）：如果在創建迭代器之後修改了set，除了通過調用迭代器自己的remove() 方法之外，將拋出ConcurrentModificationException 。

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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.