/*
* Copyright (C) 2009 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.checkArgument;
import static com.google.common.base.Preconditions.checkNotNull;
import static com.google.common.base.Preconditions.checkState;
import static com.google.common.util.concurrent.Service.State.FAILED;
import static com.google.common.util.concurrent.Service.State.NEW;
import static com.google.common.util.concurrent.Service.State.RUNNING;
import static com.google.common.util.concurrent.Service.State.STARTING;
import static com.google.common.util.concurrent.Service.State.STOPPING;
import static com.google.common.util.concurrent.Service.State.TERMINATED;
import com.google.common.annotations.Beta;
import com.google.common.util.concurrent.ListenerCallQueue.Callback;
import com.google.common.util.concurrent.Monitor.Guard;
import com.google.common.util.concurrent.Service.State; // javadoc needs this
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.concurrent.Executor;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;
import javax.annotation.Nullable;
import javax.annotation.concurrent.GuardedBy;
import javax.annotation.concurrent.Immutable;
/**
* 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
* @author Luke Sandberg
* @since 1.0
*/
@Beta
public abstract class AbstractService implements Service {
private static final Callback<Listener> STARTING_CALLBACK =
new Callback<Listener>("starting()") {
@Override void call(Listener listener) {
listener.starting();
}
};
private static final Callback<Listener> RUNNING_CALLBACK =
new Callback<Listener>("running()") {
@Override void call(Listener listener) {
listener.running();
}
};
private static final Callback<Listener> STOPPING_FROM_STARTING_CALLBACK =
stoppingCallback(STARTING);
private static final Callback<Listener> STOPPING_FROM_RUNNING_CALLBACK =
stoppingCallback(RUNNING);
private static final Callback<Listener> TERMINATED_FROM_NEW_CALLBACK =
terminatedCallback(NEW);
private static final Callback<Listener> TERMINATED_FROM_RUNNING_CALLBACK =
terminatedCallback(RUNNING);
private static final Callback<Listener> TERMINATED_FROM_STOPPING_CALLBACK =
terminatedCallback(STOPPING);
private static Callback<Listener> terminatedCallback(final State from) {
return new Callback<Listener>("terminated({from = " + from + "})") {
@Override void call(Listener listener) {
listener.terminated(from);
}
};
}
private static Callback<Listener> stoppingCallback(final State from) {
return new Callback<Listener>("stopping({from = " + from + "})") {
@Override void call(Listener listener) {
listener.stopping(from);
}
};
}
private final Monitor monitor = new Monitor();
private final Guard isStartable = new Guard(monitor) {
@Override public boolean isSatisfied() {
return state() == NEW;
}
};
private final Guard isStoppable = new Guard(monitor) {
@Override public boolean isSatisfied() {
return state().compareTo(RUNNING) <= 0;
}
};
private final Guard hasReachedRunning = new Guard(monitor) {
@Override public boolean isSatisfied() {
return state().compareTo(RUNNING) >= 0;
}
};
private final Guard isStopped = new Guard(monitor) {
@Override public boolean isSatisfied() {
return state().isTerminal();
}
};
/**
* The listeners to notify during a state transition.
*/
@GuardedBy("monitor")
private final List<ListenerCallQueue<Listener>> listeners =
Collections.synchronizedList(new ArrayList<ListenerCallQueue<Listener>>());
/**
* The current state of the service. This should be written with the lock held but can be read
* without it because it is an immutable object in a volatile field. This is desirable so that
* methods like {@link #state}, {@link #failureCause} and notably {@link #toString} can be run
* without grabbing the lock.
*
* <p>To update this field correctly the lock must be held to guarantee that the state is
* consistent.
*/
@GuardedBy("monitor")
private volatile StateSnapshot snapshot = new StateSnapshot(NEW);
/** Constructor for use by subclasses. */
protected AbstractService() {}
/**
* This method is called by {@link #startAsync} 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 #startAsync} 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 #stopAsync} is
* called multiple times.
*/
protected abstract void doStop();
@Override public final Service startAsync() {
if (monitor.enterIf(isStartable)) {
try {
snapshot = new StateSnapshot(STARTING);
starting();
doStart();
// TODO(user): justify why we are catching Throwable and not RuntimeException
} catch (Throwable startupFailure) {
notifyFailed(startupFailure);
} finally {
monitor.leave();
executeListeners();
}
} else {
throw new IllegalStateException("Service " + this + " has already been started");
}
return this;
}
@Override public final Service stopAsync() {
if (monitor.enterIf(isStoppable)) {
try {
State previous = state();
switch (previous) {
case NEW:
snapshot = new StateSnapshot(TERMINATED);
terminated(NEW);
break;
case STARTING:
snapshot = new StateSnapshot(STARTING, true, null);
stopping(STARTING);
break;
case RUNNING:
snapshot = new StateSnapshot(STOPPING);
stopping(RUNNING);
doStop();
break;
case STOPPING:
case TERMINATED:
case FAILED:
// These cases are impossible due to the if statement above.
throw new AssertionError("isStoppable is incorrectly implemented, saw: " + previous);
default:
throw new AssertionError("Unexpected state: " + previous);
}
// TODO(user): justify why we are catching Throwable and not RuntimeException. Also, we
// may inadvertently catch our AssertionErrors.
} catch (Throwable shutdownFailure) {
notifyFailed(shutdownFailure);
} finally {
monitor.leave();
executeListeners();
}
}
return this;
}
@Override public final void awaitRunning() {
monitor.enterWhenUninterruptibly(hasReachedRunning);
try {
checkCurrentState(RUNNING);
} finally {
monitor.leave();
}
}
@Override public final void awaitRunning(long timeout, TimeUnit unit) throws TimeoutException {
if (monitor.enterWhenUninterruptibly(hasReachedRunning, timeout, unit)) {
try {
checkCurrentState(RUNNING);
} finally {
monitor.leave();
}
} else {
// It is possible due to races the we are currently in the expected state even though we
// timed out. e.g. if we weren't event able to grab the lock within the timeout we would never
// even check the guard. I don't think we care too much about this use case but it could lead
// to a confusing error message.
throw new TimeoutException("Timed out waiting for " + this + " to reach the RUNNING state. "
+ "Current state: " + state());
}
}
@Override public final void awaitTerminated() {
monitor.enterWhenUninterruptibly(isStopped);
try {
checkCurrentState(TERMINATED);
} finally {
monitor.leave();
}
}
@Override public final void awaitTerminated(long timeout, TimeUnit unit) throws TimeoutException {
if (monitor.enterWhenUninterruptibly(isStopped, timeout, unit)) {
try {
checkCurrentState(TERMINATED);
} finally {
monitor.leave();
}
} else {
// It is possible due to races the we are currently in the expected state even though we
// timed out. e.g. if we weren't event able to grab the lock within the timeout we would never
// even check the guard. I don't think we care too much about this use case but it could lead
// to a confusing error message.
throw new TimeoutException("Timed out waiting for " + this + " to reach a terminal state. "
+ "Current state: " + state());
}
}
/** Checks that the current state is equal to the expected state. */
@GuardedBy("monitor")
private void checkCurrentState(State expected) {
State actual = state();
if (actual != expected) {
if (actual == FAILED) {
// Handle this specially so that we can include the failureCause, if there is one.
throw new IllegalStateException("Expected the service to be " + expected
+ ", but the service has FAILED", failureCause());
}
throw new IllegalStateException("Expected the service to be " + expected + ", but was "
+ actual);
}
}
/**
* 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() {
monitor.enter();
try {
// We have to examine the internal state of the snapshot here to properly handle the stop
// while starting case.
if (snapshot.state != STARTING) {
IllegalStateException failure = new IllegalStateException(
"Cannot notifyStarted() when the service is " + snapshot.state);
notifyFailed(failure);
throw failure;
}
if (snapshot.shutdownWhenStartupFinishes) {
snapshot = new StateSnapshot(STOPPING);
// We don't call listeners here because we already did that when we set the
// shutdownWhenStartupFinishes flag.
doStop();
} else {
snapshot = new StateSnapshot(RUNNING);
running();
}
} finally {
monitor.leave();
executeListeners();
}
}
/**
* 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() {
monitor.enter();
try {
// We check the internal state of the snapshot instead of state() directly so we don't allow
// notifyStopped() to be called while STARTING, even if stop() has already been called.
State previous = snapshot.state;
if (previous != STOPPING && previous != RUNNING) {
IllegalStateException failure = new IllegalStateException(
"Cannot notifyStopped() when the service is " + previous);
notifyFailed(failure);
throw failure;
}
snapshot = new StateSnapshot(TERMINATED);
terminated(previous);
} finally {
monitor.leave();
executeListeners();
}
}
/**
* 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);
monitor.enter();
try {
State previous = state();
switch (previous) {
case NEW:
case TERMINATED:
throw new IllegalStateException("Failed while in state:" + previous, cause);
case RUNNING:
case STARTING:
case STOPPING:
snapshot = new StateSnapshot(FAILED, false, cause);
failed(previous, cause);
break;
case FAILED:
// Do nothing
break;
default:
throw new AssertionError("Unexpected state: " + previous);
}
} finally {
monitor.leave();
executeListeners();
}
}
@Override
public final boolean isRunning() {
return state() == RUNNING;
}
@Override
public final State state() {
return snapshot.externalState();
}
/**
* @since 14.0
*/
@Override
public final Throwable failureCause() {
return snapshot.failureCause();
}
/**
* @since 13.0
*/
@Override
public final void addListener(Listener listener, Executor executor) {
checkNotNull(listener, "listener");
checkNotNull(executor, "executor");
monitor.enter();
try {
if (!state().isTerminal()) {
listeners.add(new ListenerCallQueue<Listener>(listener, executor));
}
} finally {
monitor.leave();
}
}
@Override public String toString() {
return getClass().getSimpleName() + " [" + state() + "]";
}
/**
* Attempts to execute all the listeners in {@link #listeners} while not holding the
* {@link #monitor}.
*/
private void executeListeners() {
if (!monitor.isOccupiedByCurrentThread()) {
// iterate by index to avoid concurrent modification exceptions
for (int i = 0; i < listeners.size(); i++) {
listeners.get(i).execute();
}
}
}
@GuardedBy("monitor")
private void starting() {
STARTING_CALLBACK.enqueueOn(listeners);
}
@GuardedBy("monitor")
private void running() {
RUNNING_CALLBACK.enqueueOn(listeners);
}
@GuardedBy("monitor")
private void stopping(final State from) {
if (from == State.STARTING) {
STOPPING_FROM_STARTING_CALLBACK.enqueueOn(listeners);
} else if (from == State.RUNNING) {
STOPPING_FROM_RUNNING_CALLBACK.enqueueOn(listeners);
} else {
throw new AssertionError();
}
}
@GuardedBy("monitor")
private void terminated(final State from) {
switch(from) {
case NEW:
TERMINATED_FROM_NEW_CALLBACK.enqueueOn(listeners);
break;
case RUNNING:
TERMINATED_FROM_RUNNING_CALLBACK.enqueueOn(listeners);
break;
case STOPPING:
TERMINATED_FROM_STOPPING_CALLBACK.enqueueOn(listeners);
break;
case STARTING:
case TERMINATED:
case FAILED:
default:
throw new AssertionError();
}
}
@GuardedBy("monitor")
private void failed(final State from, final Throwable cause) {
// can't memoize this one due to the exception
new Callback<Listener>("failed({from = " + from + ", cause = " + cause + "})") {
@Override void call(Listener listener) {
listener.failed(from, cause);
}
}.enqueueOn(listeners);
}
/**
* An immutable snapshot of the current state of the service. This class represents a consistent
* snapshot of the state and therefore it can be used to answer simple queries without needing to
* grab a lock.
*/
@Immutable
private static final class StateSnapshot {
/**
* The internal state, which equals external state unless
* shutdownWhenStartupFinishes is true.
*/
final State state;
/**
* If true, the user requested a shutdown while the service was still starting
* up.
*/
final boolean shutdownWhenStartupFinishes;
/**
* The exception that caused this service to fail. This will be {@code null}
* unless the service has failed.
*/
@Nullable
final Throwable failure;
StateSnapshot(State internalState) {
this(internalState, false, null);
}
StateSnapshot(
State internalState, boolean shutdownWhenStartupFinishes, @Nullable Throwable failure) {
checkArgument(!shutdownWhenStartupFinishes || internalState == STARTING,
"shudownWhenStartupFinishes can only be set if state is STARTING. Got %s instead.",
internalState);
checkArgument(!(failure != null ^ internalState == FAILED),
"A failure cause should be set if and only if the state is failed. Got %s and %s "
+ "instead.", internalState, failure);
this.state = internalState;
this.shutdownWhenStartupFinishes = shutdownWhenStartupFinishes;
this.failure = failure;
}
/** @see Service#state() */
State externalState() {
if (shutdownWhenStartupFinishes && state == STARTING) {
return STOPPING;
} else {
return state;
}
}
/** @see Service#failureCause() */
Throwable failureCause() {
checkState(state == FAILED,
"failureCause() is only valid if the service has failed, service is %s", state);
return failure;
}
}
}