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   *  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
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   *    https://www.apache.org/licenses/LICENSE-2.0
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  package org.apache.cayenne.di.spi;
  
  import org.apache.cayenne.di.DIRuntimeException;
  import org.apache.cayenne.di.Provider;
  
  /**
   * A provider that provides scoping for other providers.
   * 
   * @since 3.1
   */
  public class DefaultScopeProvider<T> implements Provider<T> {
  
      private Provider<T> delegate;
      private DefaultScope scope;
  
      // presumably "volatile" works in Java 5 and newer to prevent double-checked locking
      private volatile T instance;
  
      public DefaultScopeProvider(DefaultScope scope, Provider<T> delegate) {
          this.scope = scope;
          this.delegate = delegate;
  
          scope.addScopeEventListener(this);
      }
  
      @Override
      public T get() {
  
          if (instance == null) {
              synchronized (this) {
                  if (instance == null) {
                      instance = delegate.get();
  
                      if (instance == null) {
                          throw new DIRuntimeException(
                                  "Underlying provider (%s) returned NULL instance",
                                  delegate.getClass().getName());
                      }
  
                      scope.addScopeEventListener(instance);
                  }
              }
          }
  
          return instance;
      }
  
      @AfterScopeEnd
      public void afterScopeEnd() throws Exception {
          Object localInstance = instance;
  
          if (localInstance != null) {
              instance = null;
              scope.removeScopeEventListener(localInstance);
          }
      }
  }
  