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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;
 
 
An object with an operational state, plus asynchronous start() and stop() lifecycle methods to transition between states. Example services include webservers, RPC servers and timers.

The normal lifecycle of a service is:

  • NEW ->
  • STARTING ->
  • RUNNING ->
  • STOPPING ->
  • TERMINATED

There are deviations from this if there are failures or if stop() is called before the Service reaches the 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 Service.State.FAILED and Service.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(s):
Jesse Wilson
Luke Sandberg
Since:
9.0 (in 1.0 as com.google.common.base.Service)
 
 public interface Service {
  
If the service state is Service.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.

Returns:
a future for the startup result, regardless of whether this call initiated startup. Calling java.util.concurrent.Future.get() will block until the service has finished starting, and returns one of Service.State.RUNNING, Service.State.STOPPING or Service.State.TERMINATED. If the service fails to start, java.util.concurrent.Future.get() will throw an java.util.concurrent.ExecutionException, and the service's state will be Service.State.FAILED. If it has already finished starting, java.util.concurrent.Future.get() returns immediately. Cancelling this future has no effect on the service.
 
Initiates service startup (if necessary), returning once the service has finished starting. Unlike calling start().get(), this method throws no checked exceptions, and it cannot be interrupted.

Returns:
the state of the service when startup finished.
Throws:
com.google.common.util.concurrent.UncheckedExecutionException if startup failed
 
   State startAndWait();

  
Returns true if this service is running.
 
   boolean isRunning();

  
Returns the lifecycle state of the service.
 
   State state();

  
If the service is starting or running, this initiates service shutdown and returns immediately. If the service is new, it is terminated without having been started nor stopped. If the service has already been stopped, this method returns immediately without taking action.

Returns:
a future for the shutdown result, regardless of whether this call initiated shutdown. Calling java.util.concurrent.Future.get() will block until the service has finished shutting down, and either returns Service.State.TERMINATED or throws an java.util.concurrent.ExecutionException. If it has already finished stopping, java.util.concurrent.Future.get() returns immediately. Cancelling this future has no effect on the service.
Initiates service shutdown (if necessary), returning once the service has finished stopping. If this is Service.State.STARTING, startup will be cancelled. If this is Service.State.NEW, it is terminated without having been started nor stopped. Unlike calling stop().get(), this method throws no checked exceptions.

Returns:
the state of the service when shutdown finished.
Throws:
com.google.common.util.concurrent.UncheckedExecutionException if the service has failed or fails during shutdown
  State stopAndWait();

  
Registers a Service.Listener to be 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.

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.

Exceptions thrown by a listener will be propagated up to the executor. Any exception thrown during Executor.execute (e.g., a RejectedExecutionException or an exception thrown by inline execution) will be caught and logged.

Parameters:
listener the listener to run when the service changes state is complete
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 MoreExecutors.sameThreadExecutor().
Since:
13.0
  void addListener(Listener listenerExecutor executor);

  
The lifecycle states of a service.

Since:
9.0 (in 1.0 as 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 RUNNING.
    STARTING,

    
A service in this state is operational.
    RUNNING,

    
A service in this state is transitioning to 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 Service goes through in its lifecycle.

Author(s):
Luke Sandberg
Since:
13.0
  @Beta // should come out of Beta when Service does
  interface Listener {
    
Called when the service transitions from NEW to STARTING. This occurs when Service.start() or Service.startAndWait() is called the first time.
    void starting();

    
Called when the service transitions from STARTING to RUNNING. This occurs when a service has successfully started.
    void running();

    
Called when the service transitions to the STOPPING state. The only valid values for from are STARTING or RUNNING. This occurs when Service.stop() is called.

Parameters:
from The previous state that is being transitioned from.
    void stopping(State from);

    
Called when the service transitions to the TERMINATED state. The TERMINATED state is a terminal state in the transition diagram. Therefore, if this method is called, no other methods will be called on the Service.Listener.

Parameters:
from The previous state that is being transitioned from. The only valid values for this are NEW, RUNNING or STOPPING.
    void terminated(State from);

    
Called when the service transitions to the FAILED state. The FAILED state is a terminal state in the transition diagram. Therefore, if this method is called, no other methods will be called on the Service.Listener.

Parameters:
from The previous state that is being transitioned from. Failure can occur in any state with the exception of NEW or TERMINATED.
failure The exception that caused the failure.
    void failed(State fromThrowable failure);
  }
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