processing/src/main/java/org/apache/druid/guice/LifecycleModule.java

/*
 * 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.druid.guice;

import com.google.inject.Binder;
import com.google.inject.Injector;
import com.google.inject.Key;
import com.google.inject.Module;
import com.google.inject.Provides;
import com.google.inject.TypeLiteral;
import com.google.inject.multibindings.Multibinder;
import com.google.inject.name.Names;
import org.apache.druid.java.util.common.lifecycle.Lifecycle;

import java.lang.annotation.Annotation;
import java.util.Set;

/**
 * A Module to add lifecycle management to the injector.  {@link DruidGuiceExtensions} must also be included.
 */
public class LifecycleModule implements Module
{
  /**
   * This scope includes final logging shutdown, so all other handlers in this lifecycle scope should avoid logging in
   * their stop() method, either failing silently or failing violently and throwing an exception causing an ungraceful
   * exit.
   */
  private final LifecycleScope initScope = new LifecycleScope(Lifecycle.Stage.INIT);
  private final LifecycleScope normalScope = new LifecycleScope(Lifecycle.Stage.NORMAL);
  private final LifecycleScope serverScope = new LifecycleScope(Lifecycle.Stage.SERVER);
  private final LifecycleScope annoucementsScope = new LifecycleScope(Lifecycle.Stage.ANNOUNCEMENTS);

  /**
   * Registers a class to instantiate eagerly.  Classes mentioned here will be pulled out of
   * the injector with an injector.getInstance() call when the lifecycle is created.
   *
   * Eagerly loaded classes will *not* be automatically added to the Lifecycle unless they are bound to the proper
   * scope.  That is, they are generally eagerly loaded because the loading operation will produce some beneficial
   * side-effect even if nothing actually directly depends on the instance.
   *
   * This mechanism exists to allow the {@link Lifecycle} to be the primary entry point from the injector, not to
   * auto-register things with the {@link Lifecycle}.  It is also possible to just bind things eagerly with Guice,
   * but that is almost never the correct option.  Guice eager bindings are pre-instantiated before the object graph
   * is materialized and injected, meaning that objects are not actually instantiated in dependency order.
   * Registering with the LifecyceModule, on the other hand, will instantiate the objects after the normal object
   * graph has already been instantiated, meaning that objects will be created in dependency order and this will
   * only actually instantiate something that wasn't actually depended upon.
   *
   * @param clazz the class to instantiate
   * @return this, for chaining.
   */
  public static void register(Binder binder, Class<?> clazz)
  {
    registerKey(binder, Key.get(clazz));
  }

  /**
   * Registers a class/annotation combination to instantiate eagerly.  Classes mentioned here will be pulled out of
   * the injector with an injector.getInstance() call when the lifecycle is created.
   *
   * Eagerly loaded classes will *not* be automatically added to the Lifecycle unless they are bound to the proper
   * scope.  That is, they are generally eagerly loaded because the loading operation will produce some beneficial
   * side-effect even if nothing actually directly depends on the instance.
   *
   * This mechanism exists to allow the {@link Lifecycle} to be the primary entry point from the injector, not to
   * auto-register things with the {@link Lifecycle}.  It is also possible to just bind things eagerly with Guice,
   * but that is almost never the correct option.  Guice eager bindings are pre-instantiated before the object graph
   * is materialized and injected, meaning that objects are not actually instantiated in dependency order.
   * Registering with the LifecyceModule, on the other hand, will instantiate the objects after the normal object
   * graph has already been instantiated, meaning that objects will be created in dependency order and this will
   * only actually instantiate something that wasn't actually dependend upon.
   *
   * @param clazz the class to instantiate
   * @param annotation The annotation class to register with Guice
   * @return this, for chaining
   */
  public static void register(Binder binder, Class<?> clazz, Class<? extends Annotation> annotation)
  {
    registerKey(binder, Key.get(clazz, annotation));
  }

  /**
   * Registers a key to instantiate eagerly.  {@link Key}s mentioned here will be pulled out of
   * the injector with an injector.getInstance() call when the lifecycle is created.
   *
   * Eagerly loaded classes will *not* be automatically added to the Lifecycle unless they are bound to the proper
   * scope.  That is, they are generally eagerly loaded because the loading operation will produce some beneficial
   * side-effect even if nothing actually directly depends on the instance.
   *
   * This mechanism exists to allow the {@link Lifecycle} to be the primary entry point from the injector, not to
   * auto-register things with the {@link Lifecycle}.  It is also possible to just bind things eagerly with Guice,
   * but that is almost never the correct option.  Guice eager bindings are pre-instantiated before the object graph
   * is materialized and injected, meaning that objects are not actually instantiated in dependency order.
   * Registering with the LifecyceModule, on the other hand, will instantiate the objects after the normal object
   * graph has already been instantiated, meaning that objects will be created in dependency order and this will
   * only actually instantiate something that wasn't actually dependend upon.
   *
   * @param key The key to use in finding the DruidNode instance
   */
  public static void registerKey(Binder binder, Key<?> key)
  {
    getEagerBinder(binder).addBinding().toInstance(new KeyHolder<Object>(key));
  }

  private static Multibinder<KeyHolder> getEagerBinder(Binder binder)
  {
    return Multibinder.newSetBinder(binder, KeyHolder.class, Names.named("lifecycle"));
  }

  @Override
  public void configure(Binder binder)
  {
    getEagerBinder(binder); // Load up the eager binder so that it will inject the empty set at a minimum.

    binder.bindScope(ManageLifecycleInit.class, initScope);
    binder.bindScope(ManageLifecycle.class, normalScope);
    binder.bindScope(ManageLifecycleServer.class, serverScope);
    binder.bindScope(ManageLifecycleAnnouncements.class, annoucementsScope);
  }

  @Provides @LazySingleton
  public Lifecycle getLifecycle(final Injector injector)
  {
    final Key<Set<KeyHolder>> keyHolderKey = Key.get(new TypeLiteral<Set<KeyHolder>>(){}, Names.named("lifecycle"));
    final Set<KeyHolder> eagerClasses = injector.getInstance(keyHolderKey);

    Lifecycle lifecycle = new Lifecycle("module")
    {
      @Override
      public void start() throws Exception
      {
        for (KeyHolder<?> holder : eagerClasses) {
          injector.getInstance(holder.getKey()); // Pull the key so as to "eagerly" load up the class.
        }
        super.start();
      }
    };
    initScope.setLifecycle(lifecycle);
    normalScope.setLifecycle(lifecycle);
    serverScope.setLifecycle(lifecycle);
    annoucementsScope.setLifecycle(lifecycle);

    return lifecycle;
  }
}