Module java.base

Class AsynchronousChannelGroup


public abstract class AsynchronousChannelGroup extends Object
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.

System properties
System property Description
java.nio.channels.DefaultThreadPool.threadFactory The value of this property is taken to be the fully-qualified name of a concrete ThreadFactory class. The class is loaded using the system class loader and instantiated. The factory's newThread method is invoked to create each thread for the default group's thread pool. If the process to load and instantiate the value of the property fails then an unspecified error is thrown during the construction of the default group.
java.nio.channels.DefaultThreadPool.initialSize The value of the initialSize parameter for the default group (see withCachedThreadPool). The value of the property is taken to be the String representation of an Integer that is the initial size parameter. If the value cannot be parsed as an Integer it causes an unspecified error to be thrown during the construction of the default group.


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.

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