Package java.nio.channels

Class Selector

  • All Implemented Interfaces:
    Closeable, AutoCloseable
    Direct Known Subclasses:
    AbstractSelector
    public abstract class Selector
extends Object
implements Closeable
    A controller for the selection of SelectableChannel objects. Selectable channels can be registered with a selector and get a SelectionKey that represents the registration. The keys are also added to the selector's key set. Selection keys can be canceled so that the corresponding channel is no longer registered with the selector.

    By invoking the select method, the key set is checked and all keys that have been canceled since last select operation are moved to the set of canceled keys. During the select operation, the channels registered with this selector are checked to see whether they are ready for operation according to their interest set.

    • Constructor Detail

      • Selector

        protected Selector()
        Constructs a new Selector.

    • Method Detail

      • close

        public abstract void close()
                    throws IOException
        Closes this selector. Ongoing calls to the select methods of this selector will get interrupted. This interruption behaves as if the wakeup() method of this selector is called. After this, all keys that are still valid are invalidated and their channels are unregistered. All resources held by this selector are released.

        Any further attempt of using this selector after this method has been called (except calling close() or wakeup()) results in a ClosedSelectorException being thrown.

        Specified by:
        close in interface AutoCloseable
        Specified by:
        close in interface Closeable
        Throws:
        IOException - if an I/O error occurs.

      • isOpen

        public abstract boolean isOpen()
        Indicates whether this selector is open.
        Returns:
        true if this selector is not closed, false otherwise.

      • keys

        public abstract Set<SelectionKey> keys()
        Gets the set of registered keys.

        The returned set cannot be changed directly but can be modified indirectly by operations on the Selector. It should therefore not be treated as thread-safe.

        Returns:
        the set of registered keys.

      • provider

        public abstract SelectorProvider provider()
        Gets the provider of this selector.
        Returns:
        the provider of this selector.

      • select

        public abstract int select()
                    throws IOException
        Detects if any of the registered channels is ready for I/O operations according to its interest set. This method does not return until at least one channel is ready, wakeup() is invoked or the calling thread is interrupted.
        Returns:
        the number of channels that are ready for operation.
        Throws:
        IOException - if an I/O error occurs.
        ClosedSelectorException - if the selector is closed.

      • select

        public abstract int select​(long timeout)
                    throws IOException
        Detects if any of the registered channels is ready for I/O operations according to its interest set. This method does not return until at least one channel is ready, wakeup() is invoked, the calling thread is interrupted or the specified timeout expires.
        Parameters:
        timeout - the non-negative timeout in millisecond; 0 will block forever if no channels get ready.
        Returns:
        the number of channels that are ready for operation.
        Throws:
        ClosedSelectorException - if the selector is closed.
        IllegalArgumentException - if the given timeout argument is less than zero.
        IOException - if an I/O error occurs.

      • selectedKeys

        public abstract Set<SelectionKey> selectedKeys()
        Gets the selection keys whose channels are ready for operation.

        Keys cannot be added to the set directly. Keys can be removed. The set can be modified indirectly by operations on the Selector. It should therefore not be treated as thread-safe.

        Returns:
        the selection keys whose channels are ready for operation.
        Throws:
        ClosedSelectorException - if the selector is closed.

      • selectNow

        public abstract int selectNow()
                       throws IOException
        Detects if any of the registered channels is ready for I/O operations according to its interest set. This operation will return immediately.
        Returns:
        the number of channels that are ready for operation, 0 if none is ready.
        Throws:
        IOException - if an I/O error occurrs.
        ClosedSelectorException - if the selector is closed.

      • wakeup

        public abstract Selector wakeup()
        Forces blocked select operations to return immediately.

        If no select operation is blocked when wakeup() is called then the next select operation will return immediately. This can be undone by a call to selectNow(); after calling selectNow(), a subsequent call of select can block again.

        Returns:
        this selector.
        Throws:
        ClosedSelectorException - if the selector is closed.