https://github.com/google/guava/guava: Guava is a suite of core and expanded libraries that include utility classes, google's collections, io classes, and much much more. 
/*
 * 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.util.concurrent.Internal.toNanosSaturated;

import com.google.common.annotations.GwtIncompatible;
import com.google.errorprone.annotations.CanIgnoreReturnValue;
import com.google.errorprone.annotations.DoNotMock;
import java.time.Duration;
import java.util.concurrent.Executor;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;


An object with an operational state, plus asynchronous startAsync() and stopAsync() lifecycle methods to transition between states. Example services include webservers, RPC servers and timers. 
The normal lifecycle of a service is:



There are deviations from this if there are failures or if stopAsync 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 State.FAILED and 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, Luke Sandberg
Since:9.0 (in 1.0 as com.google.common.base.Service)
/**
 * 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.
 *
 * <p>The normal lifecycle of a service is:
 *
 * <ul>
 *   <li>{@linkplain State#NEW NEW} -&gt;
 *   <li>{@linkplain State#STARTING STARTING} -&gt;
 *   <li>{@linkplain State#RUNNING RUNNING} -&gt;
 *   <li>{@linkplain State#STOPPING STOPPING} -&gt;
 *   <li>{@linkplain State#TERMINATED TERMINATED}
 * </ul>
 *
 * <p>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 <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})
 */
@DoNotMock("Create an AbstractIdleService")
@GwtIncompatible
public interface Service {
  
If the service state is State.NEW, this initiates service startup and returns immediately. A stopped service may not be restarted. 
Throws:
Returns:this
Since:15.0
/**
   * 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
   */
  @CanIgnoreReturnValue
  Service startAsync();

  
Returns true if this service is running. 
/** Returns {@code true} if this service is {@linkplain State#RUNNING running}. */
  boolean isRunning();

  
Returns the lifecycle state of the service. 
/** 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:this
Since:15.0
/**
   * 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
   */
  @CanIgnoreReturnValue
  Service stopAsync();

  
Waits for the Service to reach the running state. 
Throws:
  • IllegalStateException – if the service reaches a state from which it is not possible to enter the State.RUNNING state. e.g. if the state is 
    State#TERMINATED when this method is called then this will throw an IllegalStateException.
Since:15.0
/**
   * 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 Service to reach the running state for no more than the given time. 
Params:
  • timeout – the maximum time to wait
Throws:
  • TimeoutException – if the service has not reached the given state within the deadline
  • IllegalStateException – if the service reaches a state from which it is not possible to enter the RUNNING state. e.g. if the state is 
    State#TERMINATED when this method is called then this will throw an IllegalStateException.
Since:28.0
/**
   * 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
   * @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 28.0
   */
  default void awaitRunning(Duration timeout) throws TimeoutException {
    awaitRunning(toNanosSaturated(timeout), TimeUnit.NANOSECONDS);
  }

  
Waits for the Service to reach the running state for no more than the given time. 
Params:
  • timeout – the maximum time to wait
  • unit – the time unit of the timeout argument
Throws:
  • TimeoutException – if the service has not reached the given state within the deadline
  • IllegalStateException – if the service reaches a state from which it is not possible to enter the RUNNING state. e.g. if the state is 
    State#TERMINATED when this method is called then this will throw an IllegalStateException.
Since:15.0
/**
   * 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
   */
  @SuppressWarnings("GoodTime") // should accept a java.time.Duration
  void awaitRunning(long timeout, TimeUnit unit) throws TimeoutException;

  
Waits for the Service to reach the terminated state. 
Throws:
Since:15.0
/**
   * 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 Service to reach a terminal state (either 
terminated or failed) for no more than the given time. 
Params:
  • timeout – the maximum time to wait
Throws:
Since:28.0
/**
   * 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
   * @throws TimeoutException if the service has not reached the given state within the deadline
   * @throws IllegalStateException if the service {@linkplain State#FAILED fails}.
   * @since 28.0
   */
  default void awaitTerminated(Duration timeout) throws TimeoutException {
    awaitTerminated(toNanosSaturated(timeout), TimeUnit.NANOSECONDS);
  }

  
Waits for the Service to reach a terminal state (either 
terminated or failed) for no more than the given time. 
Params:
  • timeout – the maximum time to wait
  • unit – the time unit of the timeout argument
Throws:
Since:15.0
/**
   * 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
   */
  @SuppressWarnings("GoodTime") // should accept a java.time.Duration
  void awaitTerminated(long timeout, TimeUnit unit) throws TimeoutException;

  
Returns the Throwable that caused this service to fail. 
Throws:
Since:14.0
/**
   * 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 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. 
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 Executor.execute (e.g., a RejectedExecutionException) will be caught and logged. 
Params:
  • listener – the listener to run when the service changes state is complete
  • 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 MoreExecutors.directExecutor.
Since:13.0
/**
   * 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>{@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.
   *
   * <p>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 State enum is defined such that if there is a state transition from A -> B then A.compareTo(B) < 0. N.B. The converse is not true, i.e. if A.compareTo(B) < 0 then there is not guaranteed to be a valid state transition A -> B. 
Since:9.0 (in 1.0 as com.google.common.base.Service.State)
/**
   * The lifecycle states of a service.
   *
   * <p>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 <b>not</b> guaranteed to be a valid state transition
   * {@code A -> B}.
   *
   * @since 9.0 (in 1.0 as {@code com.google.common.base.Service.State})
   */
  enum State {
    
A service in this state is inactive. It does minimal work and consumes minimal resources. 
/** 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 RUNNING. 
/** A service in this state is transitioning to {@link #RUNNING}. */
    STARTING {
      @Override
      boolean isTerminal() {
        return false;
      }
    },

    
A service in this state is operational. 
/** A service in this state is operational. */
    RUNNING {
      @Override
      boolean isTerminal() {
        return false;
      }
    },

    
A service in this state is transitioning to TERMINATED. 
/** 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.

/**
     * 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.

/**
     * 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. 
/** Returns true if this state is terminal. */
    abstract boolean isTerminal();
  }

  
A listener for the various state changes that a 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)
/**
   * A listener for the various state changes that a {@link Service} goes through in its lifecycle.
   *
   * <p>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)
   */
  abstract class Listener {
    
Called when the service transitions from NEW to STARTING. This occurs when Service.startAsync is called the first time. 
/**
     * 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 STARTING to RUNNING. This occurs when a service has successfully started. 
/**
     * 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 STOPPING state. The only valid values for from are STARTING or RUNNING. This occurs when Service.stopAsync is called. 
Params:
  • from – The previous state that is being transitioned from.
/**
     * 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 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 Listener. 
Params:
  • from – The previous state that is being transitioned from. Failure can occur in any state with the exception of FAILED and TERMINATED.
/**
     * 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. Failure can occur in any
     *     state with the exception of {@linkplain State#FAILED FAILED} and {@linkplain
     *     State#TERMINATED TERMINATED}.
     */
    public 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 Listener. 
Params:
  • 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.
/**
     * 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) {}
  }
}