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/*
* 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.Executor;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;
/**
* An object with an operational state, plus asynchronous {@link #startAsync()} and
* {@link #stopAsync()} lifecycle methods to transition between states. Example services include
* webservers, RPC servers and timers.
*
* The normal lifecycle of a service is:
*
* - {@linkplain State#NEW NEW} ->
*
- {@linkplain State#STARTING STARTING} ->
*
- {@linkplain State#RUNNING RUNNING} ->
*
- {@linkplain State#STOPPING STOPPING} ->
*
- {@linkplain State#TERMINATED TERMINATED}
*
*
* There are deviations from this if there are failures or if {@link Service#stopAsync} is called
* before the {@link Service} reaches the {@linkplain State#RUNNING RUNNING} state. The set of legal
* transitions form a DAG,
* 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.
*
*
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. A stopped service may not be restarted.
*
* @return this
* @throws IllegalStateException if the service is not {@link State#NEW}
*
* @since 15.0
*/
Service startAsync();
/**
* 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 this
* @since 15.0
*/
Service stopAsync();
/**
* Waits for the {@link Service} to reach the {@linkplain State#RUNNING running state}.
*
* @throws IllegalStateException if the service reaches a state from which it is not possible to
* enter the {@link State#RUNNING} state. e.g. if the {@code state} is
* {@code State#TERMINATED} when this method is called then this will throw an
* IllegalStateException.
*
* @since 15.0
*/
void awaitRunning();
/**
* Waits for the {@link Service} to reach the {@linkplain State#RUNNING running state} for no
* more than the given time.
*
* @param timeout the maximum time to wait
* @param unit the time unit of the timeout argument
* @throws TimeoutException if the service has not reached the given state within the deadline
* @throws IllegalStateException if the service reaches a state from which it is not possible to
* enter the {@link State#RUNNING RUNNING} state. e.g. if the {@code state} is
* {@code State#TERMINATED} when this method is called then this will throw an
* IllegalStateException.
*
* @since 15.0
*/
void awaitRunning(long timeout, TimeUnit unit) throws TimeoutException;
/**
* Waits for the {@link Service} to reach the {@linkplain State#TERMINATED terminated state}.
*
* @throws IllegalStateException if the service {@linkplain State#FAILED fails}.
*
* @since 15.0
*/
void awaitTerminated();
/**
* Waits for the {@link Service} to reach a terminal state (either
* {@link Service.State#TERMINATED terminated} or {@link Service.State#FAILED failed}) for no
* more than the given time.
*
* @param timeout the maximum time to wait
* @param unit the time unit of the timeout argument
* @throws TimeoutException if the service has not reached the given state within the deadline
* @throws IllegalStateException if the service {@linkplain State#FAILED fails}.
* @since 15.0
*/
void awaitTerminated(long timeout, TimeUnit unit) throws TimeoutException;
/**
* Returns the {@link Throwable} that caused this service to fail.
*
* @throws IllegalStateException if this service's state isn't {@linkplain State#FAILED FAILED}.
*
* @since 14.0
*/
Throwable failureCause();
/**
* 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.
*
*
{@code addListener} guarantees execution ordering across calls to a given listener but not
* across calls to multiple listeners. Specifically, a given listener will have its callbacks
* invoked in the same order as the underlying service enters those states. Additionally, at most
* one of the listener's callbacks will execute at once. However, multiple listeners' callbacks
* may execute concurrently, and listeners may execute in an order different from the one in which
* they were registered.
*
*
RuntimeExceptions thrown by a listener will be caught and logged. Any exception thrown
* during {@code Executor.execute} (e.g., a {@code RejectedExecutionException}) 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 listeners callback methods will be run. For fast,
* lightweight listeners that would be safe to execute in any thread, consider
* {@link MoreExecutors#directExecutor}.
* @since 13.0
*/
void addListener(Listener listener, Executor executor);
/**
* The lifecycle states of a service.
*
*
The ordering of the {@link State} enum is defined such that if there is a state transition
* from {@code A -> B} then {@code A.compareTo(B} < 0}. N.B. The converse is not true, i.e. if
* {@code A.compareTo(B} < 0} then there is not guaranteed to be a valid state transition
* {@code A -> B}.
*
* @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 {
@Override boolean isTerminal() {
return false;
}
},
/**
* A service in this state is transitioning to {@link #RUNNING}.
*/
STARTING {
@Override boolean isTerminal() {
return false;
}
},
/**
* A service in this state is operational.
*/
RUNNING {
@Override boolean isTerminal() {
return false;
}
},
/**
* A service in this state is transitioning to {@link #TERMINATED}.
*/
STOPPING {
@Override boolean isTerminal() {
return false;
}
},
/**
* A service in this state has completed execution normally. It does minimal work and consumes
* minimal resources.
*/
TERMINATED {
@Override boolean isTerminal() {
return true;
}
},
/**
* A service in this state has encountered a problem and may not be operational. It cannot be
* started nor stopped.
*/
FAILED {
@Override boolean isTerminal() {
return true;
}
};
/** Returns true if this state is terminal. */
abstract boolean isTerminal();
}
/**
* A listener for the various state changes that a {@link Service} goes through in its lifecycle.
*
*
All methods are no-ops by default, implementors should override the ones they care about.
*
* @author Luke Sandberg
* @since 15.0 (present as an interface in 13.0)
*/
@Beta // should come out of Beta when Service does
abstract class Listener {
/**
* Called when the service transitions from {@linkplain State#NEW NEW} to
* {@linkplain State#STARTING STARTING}. This occurs when {@link Service#startAsync} is called
* the first time.
*/
public 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.
*/
public 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#stopAsync} is called.
*
* @param from The previous state that is being transitioned from.
*/
public 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}.
*/
public 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.
*/
public void failed(State from, Throwable failure) {}
}
}