# Class AbstractSelector

java.lang.Object
java.nio.channels.Selector
java.nio.channels.spi.AbstractSelector

All Implemented Interfaces:
  `Closeable, AutoCloseable`

public abstract class AbstractSelector extends Selector
Base implementation class for selectors.

This class encapsulates the low-level machinery required to implement the interruption of selection operations. A concrete selector class must invoke the `begin` and `end` methods before and after, respectively, invoking an I/O operation that might block indefinitely. In order to ensure that the `end` method is always invoked, these methods should be used within a `try` ... `finally` block:

        try {
            begin();
            // Perform blocking I/O operation here
            ...
        } finally {
            end();
        }

This class also defines methods for maintaining a selector's cancelled-key set and for removing a key from its channel's key set, and declares the abstract `register` method that is invoked by a selectable channel's `register` method in order to perform the actual work of registering a channel.

Since:
  1.4

- ## Constructor Summary

  Constructors
  Modifier
  Constructor
  Description
  `protected`
  `AbstractSelector(SelectorProvider provider)`
  Initializes a new instance of this class.

- ## Method Summary

  Modifier and Type
  Method
  Description
  `protected final void`
  `begin()`
  Marks the beginning of an I/O operation that might block indefinitely.
  `protected final Set<SelectionKey>`
  `cancelledKeys()`
  Retrieves this selector's cancelled-key set.
  `final void`
  `close()`
  Closes this selector.
  `protected final void`
  `deregister(AbstractSelectionKey key)`
  Removes the given key from its channel's key set.
  `protected final void`
  `end()`
  Marks the end of an I/O operation that might block indefinitely.
  `protected abstract void`
  `implCloseSelector()`
  Closes this selector.
  `final boolean`
  `isOpen()`
  Tells whether or not this selector is open.
  `final SelectorProvider`
  `provider()`
  Returns the provider that created this channel.
  `protected abstract SelectionKey`
  `register(AbstractSelectableChannel ch, int ops, Object att)`
  Registers the given channel with this selector.

  ### Methods declared in class Selector

  `keys, open, select, select, select, select, selectedKeys, selectNow, selectNow, wakeup`

  ### Methods declared in class Object

  `clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

- ## Constructor Details

  - ### AbstractSelector

    protected AbstractSelector(SelectorProvider provider)
    Initializes a new instance of this class.
    Parameters:
      `provider` - The provider that created this selector

- ## Method Details

  - ### close

    public final void close() throws IOException
    Closes this selector.

    If the selector has already been closed then this method returns immediately. Otherwise it marks the selector as closed and then invokes the `implCloseSelector` method in order to complete the close operation.

    Specified by:
      `close` in interface `AutoCloseable`
    Specified by:
      `close` in interface `Closeable`
    Specified by:
      `close` in class `Selector`
    Throws:
      `IOException` - If an I/O error occurs

  - ### implCloseSelector

    protected abstract void implCloseSelector() throws IOException
    Closes this selector.

    This method is invoked by the `close` method in order to perform the actual work of closing the selector. This method is only invoked if the selector has not yet been closed, and it is never invoked more than once.

    An implementation of this method must arrange for any other thread that is blocked in a selection operation upon this selector to return immediately as if by invoking the `wakeup` method.

    Throws:
      `IOException` - If an I/O error occurs while closing the selector

  - ### isOpen

    public final boolean isOpen()
    Description copied from class: `Selector`
    Tells whether or not this selector is open.
    Specified by:
      `isOpen` in class `Selector`
    Returns:
      `true` if, and only if, this selector is open

  - ### provider

    public final SelectorProvider provider()
    Returns the provider that created this channel.
    Specified by:
      `provider` in class `Selector`
    Returns:
      The provider that created this channel

  - ### cancelledKeys

    protected final Set<SelectionKey> cancelledKeys()
    Retrieves this selector's cancelled-key set.

    This set should only be used while synchronized upon it.

    Returns:
      The cancelled-key set

  - ### register

    protected abstract SelectionKey register(AbstractSelectableChannel ch, int ops, Object att)
    Registers the given channel with this selector.

    This method is invoked by a channel's `register` method in order to perform the actual work of registering the channel with this selector.

    Parameters:
      `ch` - The channel to be registered
      `ops` - The initial interest set, which must be valid
      `att` - The initial attachment for the resulting key
    Returns:
      A new key representing the registration of the given channel with this selector

  - ### deregister

    protected final void deregister(AbstractSelectionKey key)
    Removes the given key from its channel's key set.

    This method must be invoked by the selector for each channel that it deregisters.

    Parameters:
      `key` - The selection key to be removed

  - ### begin

    protected final void begin()
    Marks the beginning of an I/O operation that might block indefinitely.

    This method should be invoked in tandem with the `end` method, using a `try` ... `finally` block as shown above, in order to implement interruption for this selector.

    Invoking this method arranges for the selector's `wakeup` method to be invoked if a thread's `interrupt` method is invoked while the thread is blocked in an I/O operation upon the selector.

  - ### end

    protected final void end()
    Marks the end of an I/O operation that might block indefinitely.

    This method should be invoked in tandem with the `begin` method, using a `try` ... `finally` block as shown above, in order to implement interruption for this selector.