hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/security/ssl/FileMonitoringTimerTask.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.hadoop.security.ssl;

import org.apache.hadoop.thirdparty.com.google.common.annotations.VisibleForTesting;
import org.apache.hadoop.thirdparty.com.google.common.base.Preconditions;
import org.apache.hadoop.classification.InterfaceAudience;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.nio.file.Path;
import java.util.TimerTask;
import java.util.function.Consumer;

/**
 * Implements basic logic to track when a file changes on disk and call the action
 * passed to the constructor when it does. An exception handler can optionally also be specified
 * in the constructor, otherwise any exception occurring during process will be logged
 * using this class' logger.
 */
@InterfaceAudience.Private
public class FileMonitoringTimerTask extends TimerTask {

  static final Logger LOG = LoggerFactory.getLogger(FileMonitoringTimerTask.class);

  @VisibleForTesting
  static final String PROCESS_ERROR_MESSAGE =
      "Could not process file change : ";

  final private Path filePath;
  final private Consumer<Path> onFileChange;
  final Consumer<Throwable> onChangeFailure;
  private long lastProcessed;

  /**
   * Create file monitoring task to be scheduled using a standard Java {@link java.util.Timer}
   * instance.
   *
   * @param filePath The path to the file to monitor.
   * @param onFileChange The function to call when the file has changed.
   * @param onChangeFailure The function to call when an exception is thrown during the
   *                       file change processing.
   */
  public FileMonitoringTimerTask(Path filePath, Consumer<Path> onFileChange,
      Consumer<Throwable> onChangeFailure) {
    Preconditions.checkNotNull(filePath, "path to monitor disk file is not set");
    Preconditions.checkNotNull(onFileChange, "action to monitor disk file is not set");

    this.filePath = filePath;
    this.lastProcessed = filePath.toFile().lastModified();
    this.onFileChange = onFileChange;
    this.onChangeFailure = onChangeFailure;
  }

  @Override
  public void run() {
    if (lastProcessed != filePath.toFile().lastModified()) {
      try {
        onFileChange.accept(filePath);
      } catch (Throwable t) {
        if (onChangeFailure  != null) {
          onChangeFailure.accept(t);
        } else {
          LOG.error(PROCESS_ERROR_MESSAGE + filePath.toString(), t);
        }
      }
      lastProcessed = filePath.toFile().lastModified();
    }
  }
}