/*
    * Licensed to the Apache Software Foundation (ASF) under one or more
    * contributor license agreements. See the NOTICE file distributed with
    * this work for additional information regarding copyright ownership.
    * The ASF licenses this file to You 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 org.apache.logging.log4j.core.util;
  
  import java.io.Serializable;
  import java.lang.ref.Reference;
  import java.lang.ref.SoftReference;
  import java.lang.ref.WeakReference;
  import java.util.Collection;
  import java.util.concurrent.CopyOnWriteArrayList;
  import java.util.concurrent.Executors;
  import java.util.concurrent.ThreadFactory;
  import java.util.concurrent.atomic.AtomicReference;
  
  import org.apache.logging.log4j.Logger;
  import org.apache.logging.log4j.core.LifeCycle;
  import org.apache.logging.log4j.core.LifeCycle.State;
  import org.apache.logging.log4j.status.StatusLogger;
  
  /**
   * ShutdownRegistrationStrategy that simply uses {@link Runtime#addShutdownHook(Thread)}. If no strategy is specified,
   * this one is used for shutdown hook registration.
   *
   * @since 2.1
   */
  public class DefaultShutdownCallbackRegistry implements ShutdownCallbackRegistry, LifeCycle, Runnable, Serializable {
  
      private static final long serialVersionUID = 1L;
      protected static final Logger LOGGER = StatusLogger.getLogger();
  
      private final AtomicReference<State> state = new AtomicReference<State>(State.INITIALIZED);
      private final ThreadFactory threadFactory;
      private final Collection<Cancellable> hooks = new CopyOnWriteArrayList<Cancellable>();
      private Reference<Thread> shutdownHookRef;
  
      /**
       * Constructs a DefaultShutdownRegistrationStrategy.
       */
      public DefaultShutdownCallbackRegistry() {
          this(Executors.defaultThreadFactory());
      }
  
      /**
       * Constructs a DefaultShutdownRegistrationStrategy using the given {@link ThreadFactory}.
       *
       * @param threadFactory the ThreadFactory to use to create a {@link Runtime} shutdown hook thread
       */
      protected DefaultShutdownCallbackRegistry(final ThreadFactory threadFactory) {
          this.threadFactory = threadFactory;
      }
  
      /**
       * Executes the registered shutdown callbacks.
       */
      @Override
      public void run() {
          if (state.compareAndSet(State.STARTED, State.STOPPING)) {
              for (final Runnable hook : hooks) {
                  try {
                      hook.run();
                  } catch (final Throwable t) {
                      LOGGER.error(SHUTDOWN_HOOK_MARKER, "Caught exception executing shutdown hook {}", hook, t);
                  }
              }
              state.set(State.STOPPED);
          }
      }
  
      @Override
      public Cancellable addShutdownCallback(final Runnable callback) {
          if (isStarted()) {
              final Cancellable receipt = new Cancellable() {
                  // use a reference to prevent memory leaks
                  private final Reference<Runnable> hook = new SoftReference<Runnable>(callback);
  
                  @Override
                  public void cancel() {
                      hook.clear();
                      hooks.remove(this);
                  }
  
                  @Override
                  public void run() {
                      final Runnable hook = this.hook.get();
                     if (hook != null) {
                         hook.run();
                         this.hook.clear();
                     }
                 }
 
                 @Override
                 public String toString() {
                     return String.valueOf(hook.get());
                 }
             };
             hooks.add(receipt);
             return receipt;
         }
         throw new IllegalStateException("Cannot add new shutdown hook as this is not started. Current state: " +
             state.get().name());
     }
 
     /**
      * Registers the shutdown thread only if this is initialized.
      */
     @Override
     public void start() {
         if (state.compareAndSet(State.INITIALIZED, State.STARTING)) {
             try {
                 addShutdownHook(threadFactory.newThread(this));
                 state.set(State.STARTED);
             } catch (final Exception e) {
                 LOGGER.catching(e);
                 state.set(State.STOPPED);
             }
         }
     }
 
     private void addShutdownHook(final Thread thread) {
         shutdownHookRef = new WeakReference<Thread>(thread);
         Runtime.getRuntime().addShutdownHook(thread);
     }
 
     /**
      * Cancels the shutdown thread only if this is started.
      */
     @Override
     public void stop() {
         if (state.compareAndSet(State.STARTED, State.STOPPING)) {
             try {
                 removeShutdownHook();
             } finally {
                 state.set(State.STOPPED);
             }
         }
     }
 
     private void removeShutdownHook() {
         final Thread shutdownThread = shutdownHookRef.get();
         if (shutdownThread != null) {
             Runtime.getRuntime().removeShutdownHook(shutdownThread);
             shutdownHookRef.enqueue();
         }
     }
 
     @Override
     public State getState() {
         return state.get();
     }
 
     /**
      * Indicates if this can accept shutdown hooks.
      *
      * @return true if this can accept shutdown hooks
      */
     @Override
     public boolean isStarted() {
         return state.get() == State.STARTED;
     }
 
     @Override
     public boolean isStopped() {
         return state.get() == State.STOPPED;
     }
 
 }