geode-core/src/main/java/org/apache/geode/CancelCriterion.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.geode;

/**
 * Abstract cancellation proxy for cancelling an operation, esp. a thread.
 *
 * Creators of services or threads should implement a subclass of CancelCriterion, and implement the
 * two methods - cancelInProgress, and generateCancelledException(e).
 *
 * Code inside the service can check to see if the service is cancelled by calling
 * {@link #checkCancelInProgress(Throwable)}. Generally the pattern is to check before performing an
 * operation, check if the service is canceled before propagating an exception further up the stack,
 * and check for cancellation inside a long loop. Eg.
 *
 * <pre>
 * while (true) {
 *   c.checkCancelInProgress(null);
 *   try {
 *     dispatchEvents();
 *   } catch (IOException e) {
 *     c.checkCancelInProgress(e);
 *     throw e;
 *   }
 * }
 * </pre>
 *
 * @see CancelException
 * @since GemFire 5.1
 */
public abstract class CancelCriterion {

  /**
   * Indicate if the service is in the progress of being cancelled. The typical use of this is to
   * indicate, in the case of an {@link InterruptedException}, that the current operation should be
   * cancelled.
   *
   * @return null if the service is not shutting down, or a message that can be used to construct an
   *         exception indicating the service is shut down.
   */
  public abstract String cancelInProgress();

  /**
   * Use this utility function in your implementation of cancelInProgress() and cancelled() to
   * indicate a system failure
   *
   * @return failure string if system failure has occurred
   */
  protected String checkFailure() {
    Throwable tilt = SystemFailure.getFailure();
    if (tilt != null) {
      // Allocate no objects here!
      return SystemFailure.JVM_CORRUPTION;
    }
    return null;
  }

  /**
   * See if the current operation is being cancelled. If so, it either throws a
   * {@link RuntimeException} (usually a {@link CancelException}).
   *
   * @param e an underlying exception or null if there is no exception that triggered this check
   * @see #cancelInProgress()
   */
  public void checkCancelInProgress(Throwable e) {
    SystemFailure.checkFailure();
    String reason = cancelInProgress();
    if (reason == null) {
      return;
    }
    throw generateCancelledException(e);
  }

  /**
   * Template factory method for generating the exception to be thrown by
   * {@link #checkCancelInProgress(Throwable)}. Override this to specify different exception for
   * checkCancelInProgress() to throw.
   *
   * This method should wrap the exception in a service specific CancelationException (eg
   * CacheClosedException). or return null if the service is not being canceled.
   *
   * @param throwable an underlying exception, if any
   * @return RuntimeException to be thrown by checkCancelInProgress(), null if the receiver has not
   *         been cancelled.
   */
  public abstract RuntimeException generateCancelledException(Throwable throwable);

  /**
   * Checks to see if a cancellation is in progress. This is equivalent to the expression
   * (cancelInProgress() != null).
   *
   * @return true if a cancellation is in progress, false if not
   */
  public boolean isCancelInProgress() {
    return cancelInProgress() != null;
  }

}