/*
 * Copyright 2020 Google LLC
 *
 * 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.cloud.spanner;

/**
 * Generic {@link AbstractLazyInitializer} for any heavy-weight object that might throw an exception
 * during initialization. The underlying object is initialized at most once.
 */
public abstract class AbstractLazyInitializer<T> {
  private final Object lock = new Object();
  private volatile boolean initialized;
  private volatile T object;
  private volatile Exception error;

  /** Returns an initialized instance of T. */
  T get() throws Exception {
    // First check without a lock to improve performance.
    if (!initialized) {
      synchronized (lock) {
        if (!initialized) {
          try {
            object = initialize();
          } catch (Exception e) {
            error = e;
          }
          initialized = true;
        }
      }
    }
    if (error != null) {
      throw error;
    }
    return object;
  }

  /**
   * Initializes the actual object that should be returned. Is called once the first time an
   * instance of T is required.
   */
  public abstract T initialize() throws Exception;
}

/*
 * Copyright 2020 Google LLC
 *
 * 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.cloud.spanner;

/**
 * Generic {@link AbstractLazyInitializer} for any heavy-weight object that might throw an exception
 * during initialization. The underlying object is initialized at most once.
 */
public abstract class AbstractLazyInitializer<T> {
  private final Object lock = new Object();
  private volatile boolean initialized;
  private volatile T object;
  private volatile Exception error;

  /** Returns an initialized instance of T. */
  public T get() throws Exception {
    // First check without a lock to improve performance.
    if (!initialized) {
      synchronized (lock) {
        if (!initialized) {
          try {
            object = initialize();
          } catch (Exception e) {
            error = e;
          }
          initialized = true;
        }
      }
    }
    if (error != null) {
      throw error;
    }
    return object;
  }

  /**
   * Initializes the actual object that should be returned. Is called once the first time an
   * instance of T is required.
   */
  public abstract T initialize() throws Exception;
}

increase visibility of #get() (#486)