/**
 * 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.hbase.util;

import java.net.URL;
import java.net.URLClassLoader;
import java.util.concurrent.ConcurrentHashMap;

import org.apache.hadoop.classification.InterfaceAudience;

import com.google.common.base.Preconditions;

/**
 * Base class loader that defines couple shared constants used
 * by sub-classes. It also defined method getClassLoadingLock for parallel
 * class loading and JDK 1.6 support. This method (getClassLoadingLock)
 * is similar to the same method in the base class Java ClassLoader
 * introduced in JDK 1.7, but not in JDK 1.6.
 */
@InterfaceAudience.Private
public class ClassLoaderBase extends URLClassLoader {

  // Maps class name to the corresponding lock object
  private final ConcurrentHashMap<String, Object> parallelLockMap
    = new ConcurrentHashMap<String, Object>();

  protected static final String DEFAULT_LOCAL_DIR = "/tmp/hbase-local-dir";
  protected static final String LOCAL_DIR_KEY = "hbase.local.dir";

  /**
   * Parent class loader.
   */
  protected final ClassLoader parent;

  /**
   * Creates a DynamicClassLoader that can load classes dynamically
   * from jar files under a specific folder.
   *
   * @param parent the parent ClassLoader to set.
   */
  public ClassLoaderBase(final ClassLoader parent) {
    super(new URL[]{}, parent);
    Preconditions.checkNotNull(parent, "No parent classloader!");
    this.parent = parent;
  }

  /**
   * Returns the lock object for class loading operations.
   */
  protected Object getClassLoadingLock(String className) {
    Object lock = parallelLockMap.get(className);
    if (lock != null) {
      return lock;
    }

    Object newLock = new Object();
    lock = parallelLockMap.putIfAbsent(className, newLock);
    if (lock == null) {
      lock = newLock;
    }
    return lock;
  }
}
