Java example source code file (AsyncService.java)

This example Java source code file (AsyncService.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

asyncservice, callable, cannot, do_nothing, executorservice, futuretask, must, override, runnable, state, threading, threads

The AsyncService.java Java example source code

/**
 * Copyright (C) 2010 Google Inc.
 *
 * 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.inject.service;

import com.google.common.base.Preconditions;

import java.util.concurrent.Callable;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Future;
import java.util.concurrent.FutureTask;

/**
 * An asynchronous implementation of {@link com.google.inject.service.Service}
 * that provides convenience callbacks to create your own services.
 *
 * @author dhanji@gmail.com (Dhanji R. Prasanna)
 */
public abstract class AsyncService implements Service {
  private static final Runnable DO_NOTHING = new Runnable() {
    @Override public void run() {}
  };

  private final ExecutorService executor;

  private volatile State state;

  public AsyncService(ExecutorService executor) {
    this.executor = executor;
  }

  public synchronized final Future<State> start() {
    Preconditions.checkState(state != State.STOPPED,
        "Cannot restart a service that has been stopped");

    // Starts are idempotent.
    if (state == State.STARTED) {
      return new FutureTask<State>(DO_NOTHING, State.STARTED);
    }

    return executor.submit(new Callable<State>() {
      public State call() {
        onStart();
        return state = State.STARTED;
      }
    });
  }

  /**
   * Called back when this service must do its start work. Typically occurs
   * in a background thread. The result of this method is returned to the
   * original caller of {@link Service#start()} and can thus be used to
   * return a status message after start completes (or fails as the case
   * may be).
   */
  protected abstract void onStart();

  public synchronized final Future<State> stop() {
    Preconditions.checkState(state != null, "Must start this service before you stop it!");

    // Likewise, stops are idempotent.
    if (state == State.STOPPED) {
      return new FutureTask<State>(DO_NOTHING, State.STOPPED);
    }

    return executor.submit(new Callable<State>() {
      public State call() {
        onStop();
        return state = State.STOPPED;
      }
    });
  }

  /**
   * Called back when this service must shutdown. Typically occurs
   * in a background thread. The result of this method is returned to the
   * original caller of {@link Service#stop()} and can thus be used to
   * return a status message after stop completes (or fails as the case
   * may be).
   */
  protected abstract void onStop();

  public final State state() {
    return state;
  }
}