A grouping of asynchronous channels for the purpose of resource sharing.

An asynchronous channel group encapsulates the mechanics required to handle the completion of I/O operations initiated by asynchronous channels that are bound to the group. A group has an associated thread pool to which tasks are submitted to handle I/O events and dispatch to completion-handlers that consume the result of asynchronous operations performed on channels in the group. In addition to handling I/O events, the pooled threads may also execute other tasks required to support the execution of asynchronous I/O operations.

An asynchronous channel group is created by invoking the withFixedThreadPool or withCachedThreadPool methods defined here. Channels are bound to a group by specifying the group when constructing the channel. The associated thread pool is owned by the group; termination of the group results in the shutdown of the associated thread pool.

In addition to groups created explicitly, the Java virtual machine maintains a system-wide default group that is constructed automatically. Asynchronous channels that do not specify a group at construction time are bound to the default group. The default group has an associated thread pool that creates new threads as needed. The default group may be configured by means of system properties defined in the table below. Where the ThreadFactory for the default group is not configured then the pooled threads of the default group are daemon threads.

Threading

The completion handler for an I/O operation initiated on a channel bound to a group is guaranteed to be invoked by one of the pooled threads in the group. This ensures that the completion handler is run by a thread with the expected identity.

Where an I/O operation completes immediately, and the initiating thread is one of the pooled threads in the group then the completion handler may be invoked directly by the initiating thread. To avoid stack overflow, an implementation may impose a limit as to the number of activations on the thread stack. Some I/O operations may prohibit invoking the completion handler directly by the initiating thread (see accept).

Shutdown and Termination

The shutdown method is used to initiate an orderly shutdown of a group. An orderly shutdown marks the group as shutdown; further attempts to construct a channel that binds to the group will throw ShutdownChannelGroupException. Whether or not a group is shutdown can be tested using the isShutdown method. Once shutdown, the group terminates when all asynchronous channels that are bound to the group are closed, all actively executing completion handlers have run to completion, and resources used by the group are released. No attempt is made to stop or interrupt threads that are executing completion handlers. The isTerminated method is used to test if the group has terminated, and the awaitTermination method can be used to block until the group has terminated.

The shutdownNow method can be used to initiate a forceful shutdown of the group. In addition to the actions performed by an orderly shutdown, the shutdownNow method closes all open channels in the group as if by invoking the close method.

Creates an asynchronous channel group with a fixed thread pool.

The resulting asynchronous channel group reuses a fixed number of threads. At any point, at most nThreads threads will be active processing tasks that are submitted to handle I/O events and dispatch completion results for operations initiated on asynchronous channels in the group.

The group is created by invoking the openAsynchronousChannelGroup(int,ThreadFactory) method of the system-wide default AsynchronousChannelProvider object.

Creates an asynchronous channel group with a given thread pool that creates new threads as needed.

The executor parameter is an ExecutorService that creates new threads as needed to execute tasks that are submitted to handle I/O events and dispatch completion results for operations initiated on asynchronous channels in the group. It may reuse previously constructed threads when they are available.

The initialSize parameter may be used by the implementation as a hint as to the initial number of tasks it may submit. For example, it may be used to indicate the initial number of threads that wait on I/O events.

The executor is intended to be used exclusively by the resulting asynchronous channel group. Termination of the group results in the orderly shutdown of the executor service. Shutting down the executor service by other means results in unspecified behavior.

The group is created by invoking the openAsynchronousChannelGroup(ExecutorService,int) method of the system-wide default AsynchronousChannelProvider object.

Creates an asynchronous channel group with a given thread pool.

The executor parameter is an ExecutorService that executes tasks submitted to dispatch completion results for operations initiated on asynchronous channels in the group.

Care should be taken when configuring the executor service. It should support direct handoff or unbounded queuing of submitted tasks, and the thread that invokes the execute method should never invoke the task directly. An implementation may mandate additional constraints.

The executor is intended to be used exclusively by the resulting asynchronous channel group. Termination of the group results in the orderly shutdown of the executor service. Shutting down the executor service by other means results in unspecified behavior.

The group is created by invoking the openAsynchronousChannelGroup(ExecutorService,int) method of the system-wide default AsynchronousChannelProvider object with an initialSize of 0.

Tells whether or not this group has terminated.

Where this method returns true, then the associated thread pool has also terminated.

Initiates an orderly shutdown of the group.

This method marks the group as shutdown. Further attempts to construct channel that binds to this group will throw ShutdownChannelGroupException. The group terminates when all asynchronous channels in the group are closed, all actively executing completion handlers have run to completion, and all resources have been released. This method has no effect if the group is already shutdown.

Shuts down the group and closes all open channels in the group.

In addition to the actions performed by the shutdown method, this method invokes the close method on all open channels in the group. This method does not attempt to stop or interrupt threads that are executing completion handlers. The group terminates when all actively executing completion handlers have run to completion and all resources have been released. This method may be invoked at any time. If some other thread has already invoked it, then another invocation will block until the first invocation is complete, after which it will return without effect.

Awaits termination of the group.

This method blocks until the group has terminated, or the timeout occurs, or the current thread is interrupted, whichever happens first.