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25
26package java.nio.channels;
27
28import java.nio.channels.spi.AsynchronousChannelProvider;
29import java.io.IOException;
30import java.util.concurrent.ExecutorService;
31import java.util.concurrent.ThreadFactory;
32import java.util.concurrent.TimeUnit;
33
34/**
35 * A grouping of asynchronous channels for the purpose of resource sharing.
36 *
37 * <p> An asynchronous channel group encapsulates the mechanics required to
38 * handle the completion of I/O operations initiated by {@link AsynchronousChannel
39 * asynchronous channels} that are bound to the group. A group has an associated
40 * thread pool to which tasks are submitted to handle I/O events and dispatch to
41 * {@link CompletionHandler completion-handlers} that consume the result of
42 * asynchronous operations performed on channels in the group. In addition to
43 * handling I/O events, the pooled threads may also execute other tasks required
44 * to support the execution of asynchronous I/O operations.
45 *
46 * <p> An asynchronous channel group is created by invoking the {@link
47 * #withFixedThreadPool withFixedThreadPool} or {@link #withCachedThreadPool
48 * withCachedThreadPool} methods defined here. Channels are bound to a group by
49 * specifying the group when constructing the channel. The associated thread
50 * pool is <em>owned</em> by the group; termination of the group results in the
51 * shutdown of the associated thread pool.
52 *
53 * <p> In addition to groups created explicitly, the Java virtual machine
54 * maintains a system-wide <em>default group</em> that is constructed
55 * automatically. Asynchronous channels that do not specify a group at
56 * construction time are bound to the default group. The default group has an
57 * associated thread pool that creates new threads as needed. The default group
58 * may be configured by means of system properties defined in the table below.
59 * Where the {@link java.util.concurrent.ThreadFactory ThreadFactory} for the
60 * default group is not configured then the pooled threads of the default group
61 * are {@link Thread#isDaemon daemon} threads.
62 *
63 * <table border summary="System properties">
64 *   <tr>
65 *     <th>System property</th>
66 *     <th>Description</th>
67 *   </tr>
68 *   <tr>
69 *     <td> {@code java.nio.channels.DefaultThreadPool.threadFactory} </td>
70 *     <td> The value of this property is taken to be the fully-qualified name
71 *     of a concrete {@link java.util.concurrent.ThreadFactory ThreadFactory}
72 *     class. The class is loaded using the system class loader and instantiated.
73 *     The factory's {@link java.util.concurrent.ThreadFactory#newThread
74 *     newThread} method is invoked to create each thread for the default
75 *     group's thread pool. If the process to load and instantiate the value
76 *     of the property fails then an unspecified error is thrown during the
77 *     construction of the default group. </td>
78 *   </tr>
79 *   <tr>
80 *     <td> {@code java.nio.channels.DefaultThreadPool.initialSize} </td>
81 *     <td> The value of the {@code initialSize} parameter for the default
82 *     group (see {@link #withCachedThreadPool withCachedThreadPool}).
83 *     The value of the property is taken to be the {@code String}
84 *     representation of an {@code Integer} that is the initial size parameter.
85 *     If the value cannot be parsed as an {@code Integer} it causes an
86 *     unspecified error to be thrown during the construction of the default
87 *     group. </td>
88 *   </tr>
89 * </table>
90 *
91 * <a name="threading"></a><h2>Threading</h2>
92 *
93 * <p> The completion handler for an I/O operation initiated on a channel bound
94 * to a group is guaranteed to be invoked by one of the pooled threads in the
95 * group. This ensures that the completion handler is run by a thread with the
96 * expected <em>identity</em>.
97 *
98 * <p> Where an I/O operation completes immediately, and the initiating thread
99 * is one of the pooled threads in the group then the completion handler may
100 * be invoked directly by the initiating thread. To avoid stack overflow, an
101 * implementation may impose a limit as to the number of activations on the
102 * thread stack. Some I/O operations may prohibit invoking the completion
103 * handler directly by the initiating thread (see {@link
104 * AsynchronousServerSocketChannel#accept(Object,CompletionHandler) accept}).
105 *
106 * <a name="shutdown"></a><h2>Shutdown and Termination</h2>
107 *
108 * <p> The {@link #shutdown() shutdown} method is used to initiate an <em>orderly
109 * shutdown</em> of a group. An orderly shutdown marks the group as shutdown;
110 * further attempts to construct a channel that binds to the group will throw
111 * {@link ShutdownChannelGroupException}. Whether or not a group is shutdown can
112 * be tested using the {@link #isShutdown() isShutdown} method. Once shutdown,
113 * the group <em>terminates</em> when all asynchronous channels that are bound to
114 * the group are closed, all actively executing completion handlers have run to
115 * completion, and resources used by the group are released. No attempt is made
116 * to stop or interrupt threads that are executing completion handlers. The
117 * {@link #isTerminated() isTerminated} method is used to test if the group has
118 * terminated, and the {@link #awaitTermination awaitTermination} method can be
119 * used to block until the group has terminated.
120 *
121 * <p> The {@link #shutdownNow() shutdownNow} method can be used to initiate a
122 * <em>forceful shutdown</em> of the group. In addition to the actions performed
123 * by an orderly shutdown, the {@code shutdownNow} method closes all open channels
124 * in the group as if by invoking the {@link AsynchronousChannel#close close}
125 * method.
126 *
127 * @since 1.7
128 *
129 * @see AsynchronousSocketChannel#open(AsynchronousChannelGroup)
130 * @see AsynchronousServerSocketChannel#open(AsynchronousChannelGroup)
131 */
132
133public abstract class AsynchronousChannelGroup {
134    private final AsynchronousChannelProvider provider;
135
136    /**
137     * Initialize a new instance of this class.
138     *
139     * @param   provider
140     *          The asynchronous channel provider for this group
141     */
142    protected AsynchronousChannelGroup(AsynchronousChannelProvider provider) {
143        this.provider = provider;
144    }
145
146    /**
147     * Returns the provider that created this channel group.
148     *
149     * @return  The provider that created this channel group
150     */
151    public final AsynchronousChannelProvider provider() {
152        return provider;
153    }
154
155    /**
156     * Creates an asynchronous channel group with a fixed thread pool.
157     *
158     * <p> The resulting asynchronous channel group reuses a fixed number of
159     * threads. At any point, at most {@code nThreads} threads will be active
160     * processing tasks that are submitted to handle I/O events and dispatch
161     * completion results for operations initiated on asynchronous channels in
162     * the group.
163     *
164     * <p> The group is created by invoking the {@link
165     * AsynchronousChannelProvider#openAsynchronousChannelGroup(int,ThreadFactory)
166     * openAsynchronousChannelGroup(int,ThreadFactory)} method of the system-wide
167     * default {@link AsynchronousChannelProvider} object.
168     *
169     * @param   nThreads
170     *          The number of threads in the pool
171     * @param   threadFactory
172     *          The factory to use when creating new threads
173     *
174     * @return  A new asynchronous channel group
175     *
176     * @throws  IllegalArgumentException
177     *          If {@code nThreads <= 0}
178     * @throws  IOException
179     *          If an I/O error occurs
180     */
181    public static AsynchronousChannelGroup withFixedThreadPool(int nThreads,
182                                                               ThreadFactory threadFactory)
183        throws IOException
184    {
185        return AsynchronousChannelProvider.provider()
186            .openAsynchronousChannelGroup(nThreads, threadFactory);
187    }
188
189    /**
190     * Creates an asynchronous channel group with a given thread pool that
191     * creates new threads as needed.
192     *
193     * <p> The {@code executor} parameter is an {@code ExecutorService} that
194     * creates new threads as needed to execute tasks that are submitted to
195     * handle I/O events and dispatch completion results for operations initiated
196     * on asynchronous channels in the group. It may reuse previously constructed
197     * threads when they are available.
198     *
199     * <p> The {@code initialSize} parameter may be used by the implementation
200     * as a <em>hint</em> as to the initial number of tasks it may submit. For
201     * example, it may be used to indicate the initial number of threads that
202     * wait on I/O events.
203     *
204     * <p> The executor is intended to be used exclusively by the resulting
205     * asynchronous channel group. Termination of the group results in the
206     * orderly  {@link ExecutorService#shutdown shutdown} of the executor
207     * service. Shutting down the executor service by other means results in
208     * unspecified behavior.
209     *
210     * <p> The group is created by invoking the {@link
211     * AsynchronousChannelProvider#openAsynchronousChannelGroup(ExecutorService,int)
212     * openAsynchronousChannelGroup(ExecutorService,int)} method of the system-wide
213     * default {@link AsynchronousChannelProvider} object.
214     *
215     * @param   executor
216     *          The thread pool for the resulting group
217     * @param   initialSize
218     *          A value {@code >=0} or a negative value for implementation
219     *          specific default
220     *
221     * @return  A new asynchronous channel group
222     *
223     * @throws  IOException
224     *          If an I/O error occurs
225     *
226     * @see java.util.concurrent.Executors#newCachedThreadPool
227     */
228    public static AsynchronousChannelGroup withCachedThreadPool(ExecutorService executor,
229                                                                int initialSize)
230        throws IOException
231    {
232        return AsynchronousChannelProvider.provider()
233            .openAsynchronousChannelGroup(executor, initialSize);
234    }
235
236    /**
237     * Creates an asynchronous channel group with a given thread pool.
238     *
239     * <p> The {@code executor} parameter is an {@code ExecutorService} that
240     * executes tasks submitted to dispatch completion results for operations
241     * initiated on asynchronous channels in the group.
242     *
243     * <p> Care should be taken when configuring the executor service. It
244     * should support <em>direct handoff</em> or <em>unbounded queuing</em> of
245     * submitted tasks, and the thread that invokes the {@link
246     * ExecutorService#execute execute} method should never invoke the task
247     * directly. An implementation may mandate additional constraints.
248     *
249     * <p> The executor is intended to be used exclusively by the resulting
250     * asynchronous channel group. Termination of the group results in the
251     * orderly  {@link ExecutorService#shutdown shutdown} of the executor
252     * service. Shutting down the executor service by other means results in
253     * unspecified behavior.
254     *
255     * <p> The group is created by invoking the {@link
256     * AsynchronousChannelProvider#openAsynchronousChannelGroup(ExecutorService,int)
257     * openAsynchronousChannelGroup(ExecutorService,int)} method of the system-wide
258     * default {@link AsynchronousChannelProvider} object with an {@code
259     * initialSize} of {@code 0}.
260     *
261     * @param   executor
262     *          The thread pool for the resulting group
263     *
264     * @return  A new asynchronous channel group
265     *
266     * @throws  IOException
267     *          If an I/O error occurs
268     */
269    public static AsynchronousChannelGroup withThreadPool(ExecutorService executor)
270        throws IOException
271    {
272        return AsynchronousChannelProvider.provider()
273            .openAsynchronousChannelGroup(executor, 0);
274    }
275
276    /**
277     * Tells whether or not this asynchronous channel group is shutdown.
278     *
279     * @return  {@code true} if this asynchronous channel group is shutdown or
280     *          has been marked for shutdown.
281     */
282    public abstract boolean isShutdown();
283
284    /**
285     * Tells whether or not this group has terminated.
286     *
287     * <p> Where this method returns {@code true}, then the associated thread
288     * pool has also {@link ExecutorService#isTerminated terminated}.
289     *
290     * @return  {@code true} if this group has terminated
291     */
292    public abstract boolean isTerminated();
293
294    /**
295     * Initiates an orderly shutdown of the group.
296     *
297     * <p> This method marks the group as shutdown. Further attempts to construct
298     * channel that binds to this group will throw {@link ShutdownChannelGroupException}.
299     * The group terminates when all asynchronous channels in the group are
300     * closed, all actively executing completion handlers have run to completion,
301     * and all resources have been released. This method has no effect if the
302     * group is already shutdown.
303     */
304    public abstract void shutdown();
305
306    /**
307     * Shuts down the group and closes all open channels in the group.
308     *
309     * <p> In addition to the actions performed by the {@link #shutdown() shutdown}
310     * method, this method invokes the {@link AsynchronousChannel#close close}
311     * method on all open channels in the group. This method does not attempt to
312     * stop or interrupt threads that are executing completion handlers. The
313     * group terminates when all actively executing completion handlers have run
314     * to completion and all resources have been released. This method may be
315     * invoked at any time. If some other thread has already invoked it, then
316     * another invocation will block until the first invocation is complete,
317     * after which it will return without effect.
318     *
319     * @throws  IOException
320     *          If an I/O error occurs
321     */
322    public abstract void shutdownNow() throws IOException;
323
324    /**
325     * Awaits termination of the group.
326
327     * <p> This method blocks until the group has terminated, or the timeout
328     * occurs, or the current thread is interrupted, whichever happens first.
329     *
330     * @param   timeout
331     *          The maximum time to wait, or zero or less to not wait
332     * @param   unit
333     *          The time unit of the timeout argument
334     *
335     * @return  {@code true} if the group has terminated; {@code false} if the
336     *          timeout elapsed before termination
337     *
338     * @throws  InterruptedException
339     *          If interrupted while waiting
340     */
341    public abstract boolean awaitTermination(long timeout, TimeUnit unit)
342        throws InterruptedException;
343}
344