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/**
 * 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.hadoop.security.alias;

import java.io.IOException;
import java.net.URI;
import java.net.URISyntaxException;
import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
import java.util.ServiceLoader;

import org.apache.hadoop.classification.InterfaceAudience;
import org.apache.hadoop.classification.InterfaceStability;
import org.apache.hadoop.conf.Configuration;

/**
 * A factory to create a list of CredentialProvider based on the path given in a
 * Configuration. It uses a service loader interface to find the available
 * CredentialProviders and create them based on the list of URIs.
 */
@InterfaceAudience.Public
@InterfaceStability.Unstable
public abstract class CredentialProviderFactory {
  public static final String CREDENTIAL_PROVIDER_PATH =
      "hadoop.security.credential.provider.path";

  public abstract CredentialProvider createProvider(URI providerName,
                                             Configuration conf
                                             ) throws IOException;

  private static final ServiceLoader serviceLoader =
      ServiceLoader.load(CredentialProviderFactory.class);

  // Iterate through the serviceLoader to avoid lazy loading.
  // Lazy loading would require synchronization in concurrent use cases.
  static {
    Iterator iterServices = serviceLoader.iterator();
    while (iterServices.hasNext()) {
      iterServices.next();
    }
  }

  public static List getProviders(Configuration conf
                                               ) throws IOException {
    List result = new ArrayList();
    for(String path: conf.getStringCollection(CREDENTIAL_PROVIDER_PATH)) {
      try {
        URI uri = new URI(path);
        boolean found = false;
        // Iterate serviceLoader in a synchronized block since
        // serviceLoader iterator is not thread-safe.
        synchronized (serviceLoader) {
          for (CredentialProviderFactory factory : serviceLoader) {
            CredentialProvider kp = factory.createProvider(uri, conf);
            if (kp != null) {
              result.add(kp);
              found = true;
              break;
            }
          }
        }
        if (!found) {
          throw new IOException("No CredentialProviderFactory for " + uri + " in " +
              CREDENTIAL_PROVIDER_PATH);
        }
      } catch (URISyntaxException error) {
        throw new IOException("Bad configuration of " + CREDENTIAL_PROVIDER_PATH +
            " at " + path, error);
      }
    }
    return result;
  }
}