An asynchronous channel group encapsulates the mechanics required to * handle the completion of I/O operations initiated by {@link AsynchronousChannel * 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 * {@link CompletionHandler 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 {@link * #withFixedThreadPool withFixedThreadPool} or {@link #withCachedThreadPool * 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 {@link java.util.concurrent.ThreadFactory ThreadFactory} for the * default group is not configured then the pooled threads of the default group * are {@link Thread#isDaemon daemon} threads. * *
| System property | *Description | *
|---|---|
| {@code java.nio.channels.DefaultThreadPool.threadFactory} | *The value of this property is taken to be the fully-qualified name * of a concrete {@link java.util.concurrent.ThreadFactory ThreadFactory} * class. The class is loaded using the system class loader and instantiated. * The factory's {@link java.util.concurrent.ThreadFactory#newThread * 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. | *
| {@code java.nio.channels.DefaultThreadPool.initialSize} | *The value of the {@code initialSize} parameter for the default * group (see {@link #withCachedThreadPool withCachedThreadPool}). * The value of the property is taken to be the {@code String} * representation of an {@code Integer} that is the initial size parameter. * If the value cannot be parsed as an {@code 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 {@link
* AsynchronousServerSocketChannel#accept(Object,CompletionHandler) accept}).
*
* Shutdown and Termination
*
*
The {@link #shutdown() 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 * {@link ShutdownChannelGroupException}. Whether or not a group is shutdown can * be tested using the {@link #isShutdown() 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 * {@link #isTerminated() isTerminated} method is used to test if the group has * terminated, and the {@link #awaitTermination awaitTermination} method can be * used to block until the group has terminated. * *
The {@link #shutdownNow() shutdownNow} method can be used to initiate a * forceful shutdown of the group. In addition to the actions performed * by an orderly shutdown, the {@code shutdownNow} method closes all open channels * in the group as if by invoking the {@link AsynchronousChannel#close close} * method. * * @since 1.7 * * @see AsynchronousSocketChannel#open(AsynchronousChannelGroup) * @see AsynchronousServerSocketChannel#open(AsynchronousChannelGroup) */ public abstract class AsynchronousChannelGroup { private final AsynchronousChannelProvider provider; /** * Initialize a new instance of this class. * * @param provider * The asynchronous channel provider for this group */ protected AsynchronousChannelGroup(AsynchronousChannelProvider provider) { this.provider = provider; } /** * Returns the provider that created this channel group. * * @return The provider that created this channel group */ public final AsynchronousChannelProvider provider() { return provider; } /** * 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 {@code 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 {@link * AsynchronousChannelProvider#openAsynchronousChannelGroup(int,ThreadFactory) * openAsynchronousChannelGroup(int,ThreadFactory)} method of the system-wide * default {@link AsynchronousChannelProvider} object. * * @param nThreads * The number of threads in the pool * @param threadFactory * The factory to use when creating new threads * * @return A new asynchronous channel group * * @throws IllegalArgumentException * If {@code nThreads <= 0} * @throws IOException * If an I/O error occurs */ public static AsynchronousChannelGroup withFixedThreadPool(int nThreads, ThreadFactory threadFactory) throws IOException { return AsynchronousChannelProvider.provider() .openAsynchronousChannelGroup(nThreads, threadFactory); } /** * Creates an asynchronous channel group with a given thread pool that * creates new threads as needed. * *
The {@code executor} parameter is an {@code 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 {@code 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 indictae 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 {@link ExecutorService#shutdown shutdown} of the executor * service. Shutting down the executor service by other means results in * unspecified behavior. * *
The group is created by invoking the {@link * AsynchronousChannelProvider#openAsynchronousChannelGroup(ExecutorService,int) * openAsynchronousChannelGroup(ExecutorService,int)} method of the system-wide * default {@link AsynchronousChannelProvider} object. * * @param executor * The thread pool for the resulting group * @param initialSize * A value {@code >=0} or a negative value for implementation * specific default * * @return A new asynchronous channel group * * @throws IOException * If an I/O error occurs * * @see java.util.concurrent.Executors#newCachedThreadPool */ public static AsynchronousChannelGroup withCachedThreadPool(ExecutorService executor, int initialSize) throws IOException { return AsynchronousChannelProvider.provider() .openAsynchronousChannelGroup(executor, initialSize); } /** * Creates an asynchronous channel group with a given thread pool. * *
The {@code executor} parameter is an {@code 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 {@link * ExecutorService#execute 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 {@link ExecutorService#shutdown shutdown} of the executor * service. Shutting down the executor service by other means results in * unspecified behavior. * *
The group is created by invoking the {@link * AsynchronousChannelProvider#openAsynchronousChannelGroup(ExecutorService,int) * openAsynchronousChannelGroup(ExecutorService,int)} method of the system-wide * default {@link AsynchronousChannelProvider} object with an {@code * initialSize} of {@code 0}. * * @param executor * The thread pool for the resulting group * * @return A new asynchronous channel group * * @throws IOException * If an I/O error occurs */ public static AsynchronousChannelGroup withThreadPool(ExecutorService executor) throws IOException { return AsynchronousChannelProvider.provider() .openAsynchronousChannelGroup(executor, 0); } /** * Tells whether or not this asynchronous channel group is shutdown. * * @return {@code true} if this asynchronous channel group is shutdown or * has been marked for shutdown. */ public abstract boolean isShutdown(); /** * Tells whether or not this group has terminated. * *
Where this method returns {@code true}, then the associated thread * pool has also {@link ExecutorService#isTerminated terminated}. * * @return {@code true} if this group has terminated */ public abstract boolean isTerminated(); /** * 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 {@link 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. */ public abstract void shutdown(); /** * Shuts down the group and closes all open channels in the group. * *
In addition to the actions performed by the {@link #shutdown() shutdown} * method, this method invokes the {@link AsynchronousChannel#close 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. * * @throws IOException * If an I/O error occurs */ public abstract void shutdownNow() throws IOException; /** * 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. * * @param timeout * The maximum time to wait, or zero or less to not wait * @param unit * The time unit of the timeout argument * * @return {@code true} if the group has terminated; {@code false} if the * timeout elapsed before termination * * @throws InterruptedException * If interrupted while waiting */ public abstract boolean awaitTermination(long timeout, TimeUnit unit) throws InterruptedException; }