1 /*

     2  * Copyright (c) 2016, Oracle and/or its affiliates. All rights reserved.

     3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

     4  *

     5  * This code is free software; you can redistribute it and/or modify it

     6  * under the terms of the GNU General  License version 2 only, as

     7  * published by the Free Software Foundation.  Oracle designates this

     8  * particular file as subject to the "Classpath" exception as provided

     9  * by Oracle in the LICENSE file that accompanied this code.

    10  *

    11  * This code is distributed in the hope that it will be useful, but WITHOUT

    12  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

    13  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General  License

    14  * version 2 for more details (a copy is included in the LICENSE file that

    15  * accompanied this code).

    16  *

    17  * You should have received a copy of the GNU General  License version

    18  * 2 along with this work; if not, write to the Free Software Foundation,

    19  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

    20  *

    21  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

    22  * or visit www.oracle.com if you need additional information or have any

    23  * questions.

    24  */

    25 package java.net.http;

    26 

    27 import java.util.concurrent.Executor;

    28 import java.util.concurrent.RejectedExecutionException;

    29 import java.util.concurrent.atomic.AtomicInteger;

    30 

    31 import static java.util.Objects.requireNonNull;

    32 

    33 //

    34 // The problem:

    35 // ------------

    36 //   1. For every invocation of 'signal()' there must be at least

    37 //      1 invocation of 'handler.run()' that goes after

    38 //   2. There must be no more than 1 thread running the 'handler.run()'

    39 //      at any given time

    40 //

    41 // For example, imagine each signal increments (+1) some number. Then the

    42 // handler responds (eventually) the way that makes the number 0.

    43 //

    44 // For each signal there's a response. Several signals may be handled by a

    45 // single response.

    46 //

    47 final class WSSignalHandler {

    48 

    49     // In this state the task is neither submitted nor running.

    50     // No one is handling signals. If a new signal has been received, the task

    51     // has to be submitted to the executor in order to handle this signal.

    52     private static final int DONE    = 0;

    53 

    54     // In this state the task is running.

    55     // * If the signaller has found the task in this state it will try to change

    56     //   the state to RERUN in order to make the already running task to handle

    57     //   the new signal before exiting.

    58     // * If the task has found itself in this state it will exit.

    59     private static final int RUNNING = 1;

    60 

    61     // A signal to the task, that it must rerun on the spot (without being

    62     // resubmitted to the executor).

    63     // If the task has found itself in this state it resets the state to

    64     // RUNNING and repeats the pass.

    65     private static final int RERUN   = 2;

    66 

    67     private final AtomicInteger state = new AtomicInteger(DONE);

    68 

    69     private final Executor executor;

    70     private final Runnable task;

    71 

    72     WSSignalHandler(Executor executor, Runnable handler) {

    73         this.executor = requireNonNull(executor);

    74         requireNonNull(handler);

    75 

    76         task = () -> {

    77             while (!Thread.currentThread().isInterrupted()) {

    78 

    79                 try {

    80                     handler.run();

    81                 } catch (Exception e) {

    82                     // Sorry, the task won't be automatically retried;

    83                     // hope next signals (if any) will kick off the handling

    84                     state.set(DONE);

    85                     throw e;

    86                 }

    87 

    88                 int prev = state.getAndUpdate(s -> {

    89                     if (s == RUNNING) {

    90                         return DONE;

    91                     } else {

    92                         return RUNNING;

    93                     }

    94                 });

    95 

    96                 // Can't be DONE, since only the task itself may transit state

    97                 // into DONE (with one exception: RejectedExecution in signal();

    98                 // but in that case we couldn't be here at all)

    99                 assert prev == RUNNING || prev == RERUN;

   100 

   101                 if (prev == RUNNING) {

   102                     break;

   103                 }

   104             }

   105         };

   106     }

   107 

   108     // Invoked by outer code to signal

   109     void signal() {

   110 

   111         int prev = state.getAndUpdate(s -> {

   112             switch (s) {

   113                 case RUNNING:

   114                     return RERUN;

   115                 case DONE:

   116                     return RUNNING;

   117                 case RERUN:

   118                     return RERUN;

   119                 default:

   120                     throw new InternalError(String.valueOf(s));

   121             }

   122         });

   123 

   124         if (prev != DONE) {

   125             // Nothing to do! piggybacking on previous signal

   126             return;

   127         }

   128         try {

   129             executor.execute(task);

   130         } catch (RejectedExecutionException e) {

   131             // Sorry some signal() invocations may have been accepted, but won't

   132             // be done, since the 'task' couldn't be submitted

   133             state.set(DONE);

   134             throw e;

   135         }

   136     }

   137 }