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This abstract class defines the API for channels that can be used with a Selector object to allow a thread to block while waiting for activity on any of a group of channels. All channel classes in the java.nio.channels package except for FileChannel are subclasses of SelectableChannel.

A selectable channel may only be registered with a Selector if it is nonblocking, so this class defines the configureBlocking( ) method. Pass false to this method to put a channel into nonblocking mode, or pass true to make calls to its read( ) and/or write( ) methods block. Use isBlocking( ) to determine the current blocking mode of a selectable channel.

Register a SelectableChannel with a Selector by calling the register( ) method of the channel (not of the selector). There are two versions of this method: both take a Selector object and a bitmask that specifies the set of channel operations that are to be "selected" on that channel. (see SelectionKey for the constants that can be OR-ed together to form this bitmask). Both methods return a SelectionKey object that represents the registration of the channel with the selector. One version of the register( ) method also takes an arbitrary object argument which serves as an "attachment" to the SelectionKey and allows you to associate arbitrary data with it. The validOps( ) method returns a bitmask that specifies the set of operations that a particular channel object allows to be selected. The bitmask passed to register( ) may only contain bits that are set in this validOps( ) value.

Note that SelectableChannel does not define a deregister( ) method. Instead, to remove a channel from the set of channels being monitored by a Selector, you must call the cancel( ) method of the SelectionKey returned by register( ).

Call isRegistered( ) to determine whether a SelectableChannel is registered with any Selector. (Note that a single channel may be registered with more than one Selector.) If you did not keep track of the SelectionKey returned by a call to register( ), you can query it with the keyFor( ) method.

See Selector and SelectionKey for further details on multiplexing selectable channels.

Figure 13-37. java.nio.channels.SelectableChannel

public abstract class SelectableChannel extends java.nio.channels.spi.
AbstractInterruptibleChannel implements Channel {
// Protected Constructors
     protected SelectableChannel( );  
// Public Instance Methods
     public abstract Object blockingLock( );  
     public abstract SelectableChannel configureBlocking(boolean block) 
     public abstract boolean isBlocking( );  
     public abstract boolean isRegistered( );  
     public abstract SelectionKey keyFor(Selector sel);  
     public abstract java.nio.channels.spi.SelectorProvider provider( );  
     public final SelectionKey register(Selector sel, int ops) 
        throws ClosedChannelException;  
     public abstract SelectionKey register(Selector sel, int ops, Object att) 
        throws ClosedChannelException;  
     public abstract int validOps( );  



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