/*
* 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 com.google.common.annotations.Beta;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.Executor;
/**
* An object with an operational state, plus asynchronous {@link #start()} and {@link #stop()}
* lifecycle methods to transition between states. Example services include webservers, RPC servers
* and timers.
*
* <p>The normal lifecycle of a service is:
* <ul>
* <li>{@linkplain State#NEW NEW} ->
* <li>{@linkplain State#STARTING STARTING} ->
* <li>{@linkplain State#RUNNING RUNNING} ->
* <li>{@linkplain State#STOPPING STOPPING} ->
* <li>{@linkplain State#TERMINATED TERMINATED}
* </ul>
*
* <p>There are deviations from this if there are failures or if {@link Service#stop} is called
* before the {@link Service} reaches the {@linkplain State#RUNNING RUNNING} state. The set of legal
* transitions form a <a href="http://en.wikipedia.org/wiki/Directed_acyclic_graph">DAG</a>,
* therefore every method of the listener will be called at most once. N.B. The {@link State#FAILED}
* and {@link State#TERMINATED} states are terminal states, once a service enters either of these
* states it cannot ever leave them.
*
* <p>Implementors of this interface are strongly encouraged to extend one of the abstract classes
* in this package which implement this interface and make the threading and state management
* easier.
*
* @author Jesse Wilson
* @author Luke Sandberg
* @since 9.0 (in 1.0 as {@code com.google.common.base.Service})
*/
@Beta
public interface Service {
/**
* If the service state is {@link State#NEW}, this initiates service startup and returns
* immediately. If the service has already been started, this method returns immediately without
* taking action. A stopped service may not be restarted.
*
* @return a future for the startup result, regardless of whether this call initiated startup.
* Calling {@link ListenableFuture#get} will block until the service has finished
* starting, and returns one of {@link State#RUNNING}, {@link State#STOPPING} or
* {@link State#TERMINATED}. If the service fails to start, {@link ListenableFuture#get}
* will throw an {@link ExecutionException}, and the service's state will be
* {@link State#FAILED}. If it has already finished starting, {@link ListenableFuture#get}
* returns immediately. Cancelling this future has no effect on the service.
*/
ListenableFuture<State> start();
/**
* Initiates service startup (if necessary), returning once the service has finished starting.
* Unlike calling {@code start().get()}, this method throws no checked exceptions, and it cannot
* be {@linkplain Thread#interrupt interrupted}.
*
* @throws UncheckedExecutionException if startup failed
* @return the state of the service when startup finished.
*/
State startAndWait();
/**
* Returns {@code true} if this service is {@linkplain State#RUNNING running}.
*/
boolean isRunning();
/**
* Returns the lifecycle state of the service.
*/
State state();
/**
* If the service is {@linkplain State#STARTING starting} or {@linkplain State#RUNNING running},
* this initiates service shutdown and returns immediately. If the service is
* {@linkplain State#NEW new}, it is {@linkplain State#TERMINATED terminated} without having been
* started nor stopped. If the service has already been stopped, this method returns immediately
* without taking action.
*
* @return a future for the shutdown result, regardless of whether this call initiated shutdown.
* Calling {@link ListenableFuture#get} will block until the service has finished shutting
* down, and either returns {@link State#TERMINATED} or throws an
* {@link ExecutionException}. If it has already finished stopping,
* {@link ListenableFuture#get} returns immediately. Cancelling this future has no effect
* on the service.
*/
ListenableFuture<State> stop();
/**
* Initiates service shutdown (if necessary), returning once the service has finished stopping. If
* this is {@link State#STARTING}, startup will be cancelled. If this is {@link State#NEW}, it is
* {@link State#TERMINATED terminated} without having been started nor stopped. Unlike calling
* {@code stop().get()}, this method throws no checked exceptions.
*
* @throws UncheckedExecutionException if the service has failed or fails during shutdown
* @return the state of the service when shutdown finished.
*/
State stopAndWait();
/**
* Registers a {@link Listener} to be {@linkplain Executor#execute executed} on the given
* executor. The listener will have the corresponding transition method called whenever the
* service changes state. The listener will not have previous state changes replayed, so it is
* suggested that listeners are added before the service starts.
*
* <p>There is no guaranteed ordering of execution of listeners, but any listener added through
* this method is guaranteed to be called whenever there is a state change.
*
* <p>Exceptions thrown by a listener will be propagated up to the executor. Any exception thrown
* during {@code Executor.execute} (e.g., a {@code RejectedExecutionException} or an exception
* thrown by {@linkplain MoreExecutors#sameThreadExecutor inline execution}) will be caught and
* logged.
*
* @param listener the listener to run when the service changes state is complete
* @param executor the executor in which the the listeners callback methods will be run. For fast,
* lightweight listeners that would be safe to execute in any thread, consider
* {@link MoreExecutors#sameThreadExecutor}.
* @since 13.0
*/
void addListener(Listener listener, Executor executor);
/**
* The lifecycle states of a service.
*
* @since 9.0 (in 1.0 as {@code com.google.common.base.Service.State})
*/
@Beta // should come out of Beta when Service does
enum State {
/**
* A service in this state is inactive. It does minimal work and consumes
* minimal resources.
*/
NEW,
/**
* A service in this state is transitioning to {@link #RUNNING}.
*/
STARTING,
/**
* A service in this state is operational.
*/
RUNNING,
/**
* A service in this state is transitioning to {@link #TERMINATED}.
*/
STOPPING,
/**
* A service in this state has completed execution normally. It does minimal work and consumes
* minimal resources.
*/
TERMINATED,
/**
* A service in this state has encountered a problem and may not be operational. It cannot be
* started nor stopped.
*/
FAILED
}
/**
* A listener for the various state changes that a {@link Service} goes through in its lifecycle.
*
* @author Luke Sandberg
* @since 13.0
*/
@Beta // should come out of Beta when Service does
interface Listener {
/**
* Called when the service transitions from {@linkplain State#NEW NEW} to
* {@linkplain State#STARTING STARTING}. This occurs when {@link Service#start} or
* {@link Service#startAndWait} is called the first time.
*/
void starting();
/**
* Called when the service transitions from {@linkplain State#STARTING STARTING} to
* {@linkplain State#RUNNING RUNNING}. This occurs when a service has successfully started.
*/
void running();
/**
* Called when the service transitions to the {@linkplain State#STOPPING STOPPING} state. The
* only valid values for {@code from} are {@linkplain State#STARTING STARTING} or
* {@linkplain State#RUNNING RUNNING}. This occurs when {@link Service#stop} is called.
*
* @param from The previous state that is being transitioned from.
*/
void stopping(State from);
/**
* Called when the service transitions to the {@linkplain State#TERMINATED TERMINATED} state.
* The {@linkplain State#TERMINATED TERMINATED} state is a terminal state in the transition
* diagram. Therefore, if this method is called, no other methods will be called on the
* {@link Listener}.
*
* @param from The previous state that is being transitioned from. The only valid values for
* this are {@linkplain State#NEW NEW}, {@linkplain State#RUNNING RUNNING} or
* {@linkplain State#STOPPING STOPPING}.
*/
void terminated(State from);
/**
* Called when the service transitions to the {@linkplain State#FAILED FAILED} state. The
* {@linkplain State#FAILED FAILED} state is a terminal state in the transition diagram.
* Therefore, if this method is called, no other methods will be called on the {@link Listener}.
*
* @param from The previous state that is being transitioned from. Failure can occur in any
* state with the exception of {@linkplain State#NEW NEW} or
* {@linkplain State#TERMINATED TERMINATED}.
* @param failure The exception that caused the failure.
*/
void failed(State from, Throwable failure);
}
}