/*
* Copyright (C) 2009 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.common.util.concurrent;
import static com.google.common.base.Preconditions.checkNotNull;
import static com.google.common.base.Preconditions.checkState;
import com.google.common.base.Service;
import com.google.common.base.Service.State; // javadoc needs this
import com.google.common.base.Throwables;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.Future;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;
import java.util.concurrent.locks.ReentrantLock;
/**
* Base class for implementing services that can handle {@link #doStart} and
* {@link #doStop} requests, responding to them with {@link #notifyStarted()}
* and {@link #notifyStopped()} callbacks. Its subclasses must manage threads
* manually; consider {@link AbstractExecutionThreadService} if you need only a
* single execution thread.
*
* @author Jesse Wilson
* @since 2009.09.15 <b>tentative</b>
*/
public abstract class AbstractService implements Service {
private final ReentrantLock lock = new ReentrantLock();
private final Transition startup = new Transition();
private final Transition shutdown = new Transition();
/**
* The internal state, which equals external state unless
* shutdownWhenStartupFinishes is true. Guarded by {@code lock}.
*/
private State state = State.NEW;
/**
* If true, the user requested a shutdown while the service was still starting
* up. Guarded by {@code lock}.
*/
private boolean shutdownWhenStartupFinishes = false;
/**
* This method is called by {@link #start} to initiate service startup. The
* invocation of this method should cause a call to {@link #notifyStarted()},
* either during this method's run, or after it has returned. If startup
* fails, the invocation should cause a call to {@link
* #notifyFailed(Throwable)} instead.
*
* <p>This method should return promptly; prefer to do work on a different
* thread where it is convenient. It is invoked exactly once on service
* startup, even when {@link #start} is called multiple times.
*/
protected abstract void doStart();
/**
* This method should be used to initiate service shutdown. The invocation
* of this method should cause a call to {@link #notifyStopped()}, either
* during this method's run, or after it has returned. If shutdown fails, the
* invocation should cause a call to {@link #notifyFailed(Throwable)} instead.
*
* <p>This method should return promptly; prefer to do work on a different
* thread where it is convenient. It is invoked exactly once on service
* shutdown, even when {@link #stop} is called multiple times.
*/
protected abstract void doStop();
public final Future<State> start() {
lock.lock();
try {
if (state == State.NEW) {
state = State.STARTING;
doStart();
}
} catch (Throwable startupFailure) {
// put the exception in the future, the user can get it via Future.get()
notifyFailed(startupFailure);
} finally {
lock.unlock();
}
return startup;
}
public final Future<State> stop() {
lock.lock();
try {
if (state == State.NEW) {
state = State.TERMINATED;
startup.transitionSucceeded(State.TERMINATED);
shutdown.transitionSucceeded(State.TERMINATED);
} else if (state == State.STARTING) {
shutdownWhenStartupFinishes = true;
startup.transitionSucceeded(State.STOPPING);
} else if (state == State.RUNNING) {
state = State.STOPPING;
doStop();
}
} catch (Throwable shutdownFailure) {
// put the exception in the future, the user can get it via Future.get()
notifyFailed(shutdownFailure);
} finally {
lock.unlock();
}
return shutdown;
}
public State startAndWait() {
try {
return start().get();
} catch (InterruptedException e) {
Thread.currentThread().interrupt();
throw new RuntimeException(e);
} catch (ExecutionException e) {
throw Throwables.propagate(e.getCause());
}
}
public State stopAndWait() {
try {
return stop().get();
} catch (ExecutionException e) {
throw Throwables.propagate(e.getCause());
} catch (InterruptedException e) {
Thread.currentThread().interrupt();
throw new RuntimeException(e);
}
}
/**
* Implementing classes should invoke this method once their service has
* started. It will cause the service to transition from {@link
* State#STARTING} to {@link State#RUNNING}.
*
* @throws IllegalStateException if the service is not
* {@link State#STARTING}.
*/
protected final void notifyStarted() {
lock.lock();
try {
if (state != State.STARTING) {
IllegalStateException failure = new IllegalStateException(
"Cannot notifyStarted() when the service is " + state);
notifyFailed(failure);
throw failure;
}
state = State.RUNNING;
if (shutdownWhenStartupFinishes) {
stop();
} else {
startup.transitionSucceeded(State.RUNNING);
}
} finally {
lock.unlock();
}
}
/**
* Implementing classes should invoke this method once their service has
* stopped. It will cause the service to transition from {@link
* State#STOPPING} to {@link State#TERMINATED}.
*
* @throws IllegalStateException if the service is neither {@link
* State#STOPPING} nor {@link State#RUNNING}.
*/
protected final void notifyStopped() {
lock.lock();
try {
if (state != State.STOPPING && state != State.RUNNING) {
IllegalStateException failure = new IllegalStateException(
"Cannot notifyStopped() when the service is " + state);
notifyFailed(failure);
throw failure;
}
state = State.TERMINATED;
shutdown.transitionSucceeded(State.TERMINATED);
} finally {
lock.unlock();
}
}
/**
* Invoke this method to transition the service to the
* {@link State#FAILED}. The service will <b>not be stopped</b> if it
* is running. Invoke this method when a service has failed critically or
* otherwise cannot be started nor stopped.
*/
protected final void notifyFailed(Throwable cause) {
checkNotNull(cause);
lock.lock();
try {
if (state == State.STARTING) {
startup.transitionFailed(cause);
shutdown.transitionFailed(new Exception(
"Service failed to start.", cause));
} else if (state == State.STOPPING) {
shutdown.transitionFailed(cause);
}
state = State.FAILED;
} finally {
lock.unlock();
}
}
public final boolean isRunning() {
return state() == State.RUNNING;
}
public final State state() {
lock.lock();
try {
if (shutdownWhenStartupFinishes && state == State.STARTING) {
return State.STOPPING;
} else {
return state;
}
} finally {
lock.unlock();
}
}
/**
* A change from one service state to another, plus the result of the change.
*
* TODO: could this be renamed to DefaultFuture, with methods
* like setResult(T) and setFailure(T) ?
*/
private static class Transition implements Future<State> {
private final CountDownLatch done = new CountDownLatch(1);
private State result;
private Throwable failureCause;
void transitionSucceeded(State result) {
// guarded by AbstractService.lock
checkState(this.result == null);
this.result = result;
done.countDown();
}
void transitionFailed(Throwable cause) {
// guarded by AbstractService.lock
checkState(result == null);
this.result = State.FAILED;
this.failureCause = cause;
done.countDown();
}
public boolean cancel(boolean mayInterruptIfRunning) {
return false;
}
public boolean isCancelled() {
return false;
}
public boolean isDone() {
return done.getCount() == 0;
}
public State get() throws InterruptedException, ExecutionException {
done.await();
return getImmediately();
}
public State get(long timeout, TimeUnit unit)
throws InterruptedException, ExecutionException, TimeoutException {
if (done.await(timeout, unit)) {
return getImmediately();
}
throw new TimeoutException();
}
private State getImmediately() throws ExecutionException {
if (result == State.FAILED) {
throw new ExecutionException(failureCause);
} else {
return result;
}
}
}
}