heron/api/src/java/com/twitter/heron/classification/InterfaceAudience.java

// Copyright 2016 Twitter. All rights reserved.
//
// 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.
package com.twitter.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
}