com.google.inject.service.AsyncService.java Source code

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Here is the source code for com.google.inject.service.AsyncService.java

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/**
 * 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 com.google.common.util.concurrent.Runnables;

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 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>(Runnables.doNothing(), 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>(Runnables.doNothing(), 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;
    }
}