/*-
 * #%L
 * LmdbJava
 * %%
 * Copyright (C) 2016 - 2023 The LmdbJava Open Source Project
 * %%
 * 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.
 * #L%
 */

package org.lmdbjava;

import edu.umd.cs.findbugs.annotations.SuppressFBWarnings;

/**
 * Supports creating strong references in manner compatible with Java 8.
 */
public final class ReferenceUtil {

  private ReferenceUtil() {
  }

  /**
   * Ensures that the object referenced by the given reference remains
   * <em>strongly reachable</em>, regardless of any prior actions of the program
   * that might otherwise cause the object to become unreachable. Thus, the
   * referenced object is not reclaimable by garbage collection at least until
   * after the invocation of this method.
   *
   * <p>
   * Recent versions of the JDK have a nasty habit of prematurely deciding
   * objects are unreachable (eg
   * <a href="https://tinyurl.com/so26642153">StackOverflow question
   * 26642153</a>.
   *
   * <p>
   * <code>java.lang.ref.Reference.reachabilityFence</code> offers a solution to
   * this problem, but it was only introduced in Java 9. LmdbJava presently
   * supports Java 8 and therefore this method provides an alternative.
   *
   * <p>
   * This method is always implemented as a synchronization on {@code ref}.
   * <b>It is the caller's responsibility to ensure that this synchronization
   * will not cause deadlock.</b>
   *
   * @param ref the reference (null is acceptable but has no effect)
   * @see <a href="https://github.com/netty/netty/pull/8410">Netty PR 8410</a>
   */
  @SuppressFBWarnings({"ESync_EMPTY_SYNC", "UC_USELESS_VOID_METHOD"})
  public static void reachabilityFence0(final Object ref) {
    if (ref != null) {
      synchronized (ref) {
        // Empty synchronized is ok: https://stackoverflow.com/a/31933260/1151521
      }
    }
  }
}
