heron/api/src/java/org/apache/heron/classification/InterfaceAudience.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.heron.classification;

import java.lang.annotation.Documented;
import java.lang.annotation.Inherited;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;

/**
 * Annotation to inform users of a package, class or method's intended audience.
 * Currently the audience can be {@link Public}, {@link LimitedPrivate} or
 * {@link Private}.<br>
 * All public classes must have InterfaceAudience annotation. <br>
 * <ul>
 * <li>Public classes that are not marked with this annotation must be
 * considered by default as {@link Private}.</li>
 *
 * <li>External applications must only use classes that are marked
 * {@link Public}. Avoid using non public classes as these classes
 * could be removed or change in incompatible ways.</li>
 *
 * <li>Heron topologies must only use classes that are marked {@link Public}. Heron administrators
 * may use classes that are {@link LimitedPrivate} or {@link Public}</li>
 *
 * <li> Methods may have a different annotation that it is more restrictive
 * compared to the audience classification of the class. Example: A class
 * might be {@link Public}, but a method may be {@link LimitedPrivate}
 * </li></ul>
 */
@InterfaceAudience.Public
@InterfaceStability.Evolving
public final class InterfaceAudience {

  /**
   * Intended for use by any project or application (i.e. Topology implementations).
   */
  @Documented
  @Retention(RetentionPolicy.RUNTIME)
  @Inherited
  public @interface Public { }

  /**
   * Intended only for code that extends Heron itself (i.e. spi implementations).
   */
  @Documented
  @Retention(RetentionPolicy.RUNTIME)
  @Inherited
  public @interface LimitedPrivate { }

  /**
   * Intended for use only within Heron itself.
   */
  @Documented
  @Retention(RetentionPolicy.RUNTIME)
  @Inherited
  public @interface Private { }

  private InterfaceAudience() { } // Audience can't exist on its own
}