content: package org.apache.lucene.util;
  
  /**
   * 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.
   */
  
  import java.lang.reflect.Method;
  import java.util.Collections;
  import java.util.HashSet;
  import java.util.WeakHashMap;
  import java.util.Set;
  
  /**
   * A utility for keeping backwards compatibility on previously abstract methods
   * (or similar replacements).
   * <p>Before the replacement method can be made abstract, the old method must kept deprecated.
   * If somebody still overrides the deprecated method in a non-final class,
   * you must keep track, of this and maybe delegate to the old method in the subclass.
   * The cost of reflection is minimized by the following usage of this class:</p>
   * <p>Define <strong>static final</strong> fields in the base class ({@code BaseClass}),
   * where the old and new method are declared:</p>
   * <pre>
   *  static final VirtualMethod&lt;BaseClass&gt; newMethod =
   *   new VirtualMethod&lt;BaseClass&gt;(BaseClass.class, "newName", parameters...);
   *  static final VirtualMethod&lt;BaseClass&gt; oldMethod =
   *   new VirtualMethod&lt;BaseClass&gt;(BaseClass.class, "oldName", parameters...);
   * </pre>
   * <p>This enforces the singleton status of these objects, as the maintenance of the cache would be too costly else.
   * If you try to create a second instance of for the same method/{@code baseClass} combination, an exception is thrown.
   * <p>To detect if e.g. the old method was overridden by a more far subclass on the inheritance path to the current
   * instance's class, use a <strong>non-static</strong> field:</p>
   * <pre>
   *  final boolean isDeprecatedMethodOverridden =
   *   oldMethod.getImplementationDistance(this.getClass()) > newMethod.getImplementationDistance(this.getClass());
   *
   *  <em>// alternatively (more readable):</em>
   *  final boolean isDeprecatedMethodOverridden =
   *   VirtualMethod.compareImplementationDistance(this.getClass(), oldMethod, newMethod) > 0
   * </pre> 
   * <p>{@link #getImplementationDistance} returns the distance of the subclass that overrides this method.
   * The one with the larger distance should be used preferable.
   * This way also more complicated method rename scenarios can be handled
   * (think of 2.9 {@code TokenStream} deprecations).</p>
   *
   * @lucene.internal
   */
  public final class VirtualMethod<C> {
  
    private static final Set<Method> singletonSet = Collections.synchronizedSet(new HashSet<Method>());
  
    private final Class<C> baseClass;
    private final String method;
    private final Class<?>[] parameters;
    private final WeakHashMap<Class<? extends C>, Integer> cache =
      new WeakHashMap<Class<? extends C>, Integer>();
  
    /**
     * Creates a new instance for the given {@code baseClass} and method declaration.
     * @throws UnsupportedOperationException if you create a second instance of the same
     *  {@code baseClass} and method declaration combination. This enforces the singleton status.
     * @throws IllegalArgumentException if {@code baseClass} does not declare the given method.
     */
    public VirtualMethod(Class<C> baseClass, String method, Class<?>... parameters) {
      this.baseClass = baseClass;
      this.method = method;
      this.parameters = parameters;
      try {
        if (!singletonSet.add(baseClass.getDeclaredMethod(method, parameters)))
          throw new UnsupportedOperationException(
            "VirtualMethod instances must be singletons and therefore " +
            "assigned to static final members in the same class, they use as baseClass ctor param."
          );
      } catch (NoSuchMethodException nsme) {
        throw new IllegalArgumentException(baseClass.getName() + " has no such method: "+nsme.getMessage());
      }
    }
    
    /**
     * Returns the distance from the {@code baseClass} in which this method is overridden/implemented
     * in the inheritance path between {@code baseClass} and the given subclass {@code subclazz}.
     * @return 0 iff not overridden, else the distance to the base class
     */
    public synchronized int getImplementationDistance(final Class<? extends C> subclazz) {
      Integer distance = cache.get(subclazz);
      if (distance == null) {
        cache.put(subclazz, distance = Integer.valueOf(reflectImplementationDistance(subclazz)));
      }
      return distance.intValue();
    }
    
    /**
     * Returns, if this method is overridden/implemented in the inheritance path between
     * {@code baseClass} and the given subclass {@code subclazz}.
     * <p>You can use this method to detect if a method that should normally be final was overridden
     * by the given instance's class.
     * @return {@code false} iff not overridden
     */
    public boolean isOverriddenAsOf(final Class<? extends C> subclazz) {
      return getImplementationDistance(subclazz) > 0;
    }
    
    private int reflectImplementationDistance(final Class<? extends C> subclazz) {
      if (!baseClass.isAssignableFrom(subclazz))
        throw new IllegalArgumentException(subclazz.getName() + " is not a subclass of " + baseClass.getName());
      boolean overridden = false;
      int distance = 0;
      for (Class<?> clazz = subclazz; clazz != baseClass && clazz != null; clazz = clazz.getSuperclass()) {
        // lookup method, if success mark as overridden
        if (!overridden) {
          try {
            clazz.getDeclaredMethod(method, parameters);
            overridden = true;
          } catch (NoSuchMethodException nsme) {
          }
        }
        
        // increment distance if overridden
        if (overridden) distance++;
      }
      return distance;
    }
    
    /**
     * Utility method that compares the implementation/override distance of two methods.
     * @return <ul>
     *  <li>&gt; 1, iff {@code m1} is overridden/implemented in a subclass of the class overriding/declaring {@code m2}
     *  <li>&lt; 1, iff {@code m2} is overridden in a subclass of the class overriding/declaring {@code m1}
     *  <li>0, iff both methods are overridden in the same class (or are not overridden at all)
     * </ul>
     */
    public static <C> int compareImplementationDistance(final Class<? extends C> clazz,
      final VirtualMethod<C> m1, final VirtualMethod<C> m2)
    {
      return Integer.valueOf(m1.getImplementationDistance(clazz)).compareTo(m2.getImplementationDistance(clazz));
    }
    
  }
  