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Java example source code file (ListenerCallQueue.java)

This example Java source code file (ListenerCallQueue.java) is included in the alvinalexander.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" TM.

Learn more about this Java project at its project page.

Java - Java tags/keywords

callback, exception, executor, guardedby, gwtincompatible, listenercallqueue, log, logger, logging, override, runnable, runtimeexception, string, threading, threads, util

The ListenerCallQueue.java Java example source code

/*
 * Copyright (C) 2014 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
 * in compliance with the License. You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software distributed under the License
 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
 * or implied. See the License for the specific language governing permissions and limitations under
 * the License.
 */

package com.google.common.util.concurrent;

import static com.google.common.base.Preconditions.checkNotNull;

import com.google.common.annotations.GwtIncompatible;
import com.google.common.base.Preconditions;
import com.google.common.collect.Queues;

import java.util.Queue;
import java.util.concurrent.Executor;
import java.util.logging.Level;
import java.util.logging.Logger;

import javax.annotation.concurrent.GuardedBy;

/**
 * A special purpose queue/executor that executes listener callbacks serially on a configured
 * executor. Each callback task can be enqueued and executed as separate phases.
 *
 * <p>This class is very similar to {@link SerializingExecutor} with the exception that tasks can be
 * enqueued without necessarily executing immediately.
 */
@GwtIncompatible
final class ListenerCallQueue<L> implements Runnable {
  // TODO(cpovirk): consider using the logger associated with listener.getClass().
  private static final Logger logger = Logger.getLogger(ListenerCallQueue.class.getName());

  abstract static class Callback<L> {
    private final String methodCall;

    Callback(String methodCall) {
      this.methodCall = methodCall;
    }

    abstract void call(L listener);

    /** Helper method to add this callback to all the queues. */
    void enqueueOn(Iterable<ListenerCallQueue queues) {
      for (ListenerCallQueue<L> queue : queues) {
        queue.add(this);
      }
    }
  }

  private final L listener;
  private final Executor executor;

  @GuardedBy("this")
  private final Queue<Callback waitQueue = Queues.newArrayDeque();

  @GuardedBy("this")
  private boolean isThreadScheduled;

  ListenerCallQueue(L listener, Executor executor) {
    this.listener = checkNotNull(listener);
    this.executor = checkNotNull(executor);
  }

  /** Enqueues a task to be run. */
  synchronized void add(Callback<L> callback) {
    waitQueue.add(callback);
  }

  /** Executes all listeners {@linkplain #add added} prior to this call, serially and in order. */
  void execute() {
    boolean scheduleTaskRunner = false;
    synchronized (this) {
      if (!isThreadScheduled) {
        isThreadScheduled = true;
        scheduleTaskRunner = true;
      }
    }
    if (scheduleTaskRunner) {
      try {
        executor.execute(this);
      } catch (RuntimeException e) {
        // reset state in case of an error so that later calls to execute will actually do something
        synchronized (this) {
          isThreadScheduled = false;
        }
        // Log it and keep going.
        logger.log(
            Level.SEVERE,
            "Exception while running callbacks for " + listener + " on " + executor,
            e);
        throw e;
      }
    }
  }

  @Override
  public void run() {
    boolean stillRunning = true;
    try {
      while (true) {
        Callback<L> nextToRun;
        synchronized (ListenerCallQueue.this) {
          Preconditions.checkState(isThreadScheduled);
          nextToRun = waitQueue.poll();
          if (nextToRun == null) {
            isThreadScheduled = false;
            stillRunning = false;
            break;
          }
        }

        // Always run while _not_ holding the lock, to avoid deadlocks.
        try {
          nextToRun.call(listener);
        } catch (RuntimeException e) {
          // Log it and keep going.
          logger.log(
              Level.SEVERE,
              "Exception while executing callback: " + listener + "." + nextToRun.methodCall,
              e);
        }
      }
    } finally {
      if (stillRunning) {
        // An Error is bubbling up. We should mark ourselves as no longer running. That way, if
        // anyone tries to keep using us, we won't be corrupted.
        synchronized (ListenerCallQueue.this) {
          isThreadScheduled = false;
        }
      }
    }
  }
}

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