content: /*
   * 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 com.datastax.oss.driver.internal.core.util.collection;
  
  import com.datastax.oss.driver.api.core.metadata.Node;
  import net.jcip.annotations.ThreadSafe;
  
  /**
   * A query plan where nodes are computed lazily, when the plan is consumed for the first time.
   *
   * <p>This class can be useful when a query plan computation is heavy but the plan has a low chance
   * of ever being consumed, e.g. the last query plan in a {@link CompositeQueryPlan}.
   */
  @ThreadSafe
  public abstract class LazyQueryPlan extends QueryPlanBase {
  
    private volatile Object[] nodes;
  
    /**
     * Computes and returns the nodes to use for this query plan.
     *
     * <p>For efficiency, the declared return type is {@code Object[]} but all elements must be
     * instances of {@link Node}. See {@link #getNodes()} for details.
     *
     * <p>This method is guaranteed to be invoked only once, at the first call to {@link #poll()}.
     *
     * <p>Implementors must avoid blocking calls in this method as it will be invoked on the driver's
     * hot path.
     */
    protected abstract Object[] computeNodes();
  
    @Override
    protected Object[] getNodes() {
      if (nodes == null) {
        synchronized (this) {
          if (nodes == null) {
            nodes = computeNodes();
          }
        }
      }
      return nodes;
    }
  }
  