/**
    * 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.lang.reflect.Field;
  import java.lang.reflect.Method;
  import java.security.AccessController;
  import java.security.PrivilegedAction;
  
  import org.apache.commons.logging.Log;
  import org.apache.commons.logging.LogFactory;
  import org.apache.hadoop.hbase.classification.InterfaceAudience;
  
  @InterfaceAudience.Private
  public class UnsafeAvailChecker {
  
    private static final String CLASS_NAME = "sun.misc.Unsafe";
    private static final Log LOG = LogFactory.getLog(UnsafeAvailChecker.class);
    private static boolean avail = false;
    private static boolean unaligned = false;
  
    static {
      avail = AccessController.doPrivileged(new PrivilegedAction<Boolean>() {
        @Override
        public Boolean run() {
          try {
            Class<?> clazz = Class.forName(CLASS_NAME);
            Field f = clazz.getDeclaredField("theUnsafe");
            f.setAccessible(true);
            return f.get(null) != null;
          } catch (Throwable e) {
            LOG.warn("sun.misc.Unsafe is not available/accessible", e);
          }
          return false;
        }
      });
      // When Unsafe itself is not available/accessible consider unaligned as false.
      if (avail) {
        try {
          // Using java.nio.Bits#unaligned() to check for unaligned-access capability
          Class<?> clazz = Class.forName("java.nio.Bits");
          Method m = clazz.getDeclaredMethod("unaligned");
          m.setAccessible(true);
          unaligned = (Boolean) m.invoke(null);
        } catch (Exception e) {
          LOG.warn("java.nio.Bits#unaligned() check failed."
              + "Unsafe based read/write of primitive types won't be used", e);
        }
      }
    }
  
    /**
     * @return true when running JVM is having sun's Unsafe package available in it and it is
     *         accessible.
     */
    public static boolean isAvailable() {
      return avail;
    }
  
    /**
     * @return true when running JVM is having sun's Unsafe package available in it and underlying
     *         system having unaligned-access capability.
     */
    public static boolean unaligned() {
      return unaligned;
    }
  
    private UnsafeAvailChecker() {
      // private constructor to avoid instantiation
    }
  }