View Javadoc

1   /**
2    * Licensed to the Apache Software Foundation (ASF) under one
3    * or more contributor license agreements.  See the NOTICE file
4    * distributed with this work for additional information
5    * regarding copyright ownership.  The ASF licenses this file
6    * to you under the Apache License, Version 2.0 (the
7    * "License"); you may not use this file except in compliance
8    * with the License.  You may obtain a copy of the License at
9    *
10   *     http://www.apache.org/licenses/LICENSE-2.0
11   *
12   * Unless required by applicable law or agreed to in writing, software
13   * distributed under the License is distributed on an "AS IS" BASIS,
14   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
15   * See the License for the specific language governing permissions and
16   * limitations under the License.
17   */
18  package org.apache.hadoop.hbase.util;
19  
20  import java.net.URL;
21  import java.net.URLClassLoader;
22  import java.util.concurrent.ConcurrentHashMap;
23  
24  import org.apache.hadoop.classification.InterfaceAudience;
25  
26  import com.google.common.base.Preconditions;
27  
28  /**
29   * Base class loader that defines couple shared constants used
30   * by sub-classes. It also defined method getClassLoadingLock for parallel
31   * class loading and JDK 1.6 support. This method (getClassLoadingLock)
32   * is similar to the same method in the base class Java ClassLoader
33   * introduced in JDK 1.7, but not in JDK 1.6.
34   */
35  @InterfaceAudience.Private
36  public class ClassLoaderBase extends URLClassLoader {
37  
38    // Maps class name to the corresponding lock object
39    private final ConcurrentHashMap<String, Object> parallelLockMap
40      = new ConcurrentHashMap<String, Object>();
41  
42    protected static final String DEFAULT_LOCAL_DIR = "/tmp/hbase-local-dir";
43    protected static final String LOCAL_DIR_KEY = "hbase.local.dir";
44  
45    /**
46     * Parent class loader.
47     */
48    protected final ClassLoader parent;
49  
50    /**
51     * Creates a DynamicClassLoader that can load classes dynamically
52     * from jar files under a specific folder.
53     *
54     * @param parent the parent ClassLoader to set.
55     */
56    public ClassLoaderBase(final ClassLoader parent) {
57      super(new URL[]{}, parent);
58      Preconditions.checkNotNull(parent, "No parent classloader!");
59      this.parent = parent;
60    }
61  
62    /**
63     * Returns the lock object for class loading operations.
64     */
65    protected Object getClassLoadingLock(String className) {
66      Object lock = parallelLockMap.get(className);
67      if (lock != null) {
68        return lock;
69      }
70  
71      Object newLock = new Object();
72      lock = parallelLockMap.putIfAbsent(className, newLock);
73      if (lock == null) {
74        lock = newLock;
75      }
76      return lock;
77    }
78  }