heron/common/src/java/com/twitter/heron/common/basics/SingletonRegistry.java

// Copyright 2016 Twitter. All rights reserved.
//
// 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.twitter.heron.common.basics;

import java.util.HashMap;
import java.util.Map;

public enum SingletonRegistry {
  INSTANCE;

  /**
   * Cache of singleton objects: bean name --> instance
   * We use HashMap and synchronized all the operations on it.
   * The reasons are:
   * 1. The register methods need to be atomic, so make these methods synchronized.
   * 2. We need synchronized to guarantee the thread safe of singletonObjects (HashMap itself
   * is not thread safe).
   * 3. ConcurrentHashMap doesn't support the register atomic operations and thus we didn't use
   * it here
   */
  private final Map<String, Object> singletonObjects = new HashMap<String, Object>();

  public boolean containsSingleton(String beanName) {
    synchronized (this.singletonObjects) {
      return singletonObjects.containsKey(beanName);
    }
  }

  public Object getSingleton(String beanName) {
    synchronized (this.singletonObjects) {
      return this.singletonObjects.get(beanName);
    }
  }

  public int getSingletonCount() {
    synchronized (this.singletonObjects) {
      return singletonObjects.size();
    }
  }

  // Typically invoked during registry configuration, but can also be used for runtime registration
  // of singletons. As a consequence, a registry implementation should synchronize singleton access.
  public void registerSingleton(String beanName, Object singletonObject) {
    assert beanName != null && singletonObject != null;

    synchronized (this.singletonObjects) {
      Object oldObject = this.singletonObjects.get(beanName);
      if (oldObject != null) {
        throw new IllegalStateException("Could not register object [" + singletonObject
            + "] under bean name '" + beanName + "': there is already object [" + oldObject
            + "] bound");
      }
      this.singletonObjects.put(beanName, singletonObject);
    }
  }

  // Typically invoked during registry configuration, but can also be used for runtime registration
  // of singletons. As a consequence, a registry implementation should synchronize singleton access.
  // Singleton has to be registered first or otherwise IllegalStateException would throw
  public void updateSingleton(String beanName, Object singletonObject) {
    assert beanName != null && singletonObject != null;

    synchronized (this.singletonObjects) {
      if (!this.singletonObjects.containsKey(beanName)) {
        throw new IllegalStateException("Could not update object [" + singletonObject
            + "] under bean name '" + beanName + "': it have not been registered yet.");

      }
      this.singletonObjects.put(beanName, singletonObject);
    }
  }
}