datasketches-memory-java11/src/main/java/org/apache/datasketches/memory/internal/VirtualMachineMemory.java

/*
 * 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.datasketches.memory.internal;

import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;

/**
 * Extracts a version-dependent reference to the `jdk.internal.misc.VM` into
 * a standalone class. The package name for VM has changed in
 * later versions. The appropriate class will be loaded by the class loader
 * depending on the Java version that is used.
 * For more information, see: https://openjdk.java.net/jeps/238
 */
public final class VirtualMachineMemory {

  private static final Class<?> VM_CLASS;
  private static final Method VM_MAX_DIRECT_MEMORY_METHOD;
  private static final Method VM_IS_DIRECT_MEMORY_PAGE_ALIGNED_METHOD;
  private static final long maxDBBMemory;
  private static final boolean isPageAligned;

  static {
    try {
      VM_CLASS = Class.forName("jdk.internal.misc.VM");
      VM_MAX_DIRECT_MEMORY_METHOD = VM_CLASS.getDeclaredMethod("maxDirectMemory");
      VM_MAX_DIRECT_MEMORY_METHOD.setAccessible(true);
      maxDBBMemory = (long) VM_MAX_DIRECT_MEMORY_METHOD.invoke(null); //static method

      VM_IS_DIRECT_MEMORY_PAGE_ALIGNED_METHOD =
          VM_CLASS.getDeclaredMethod("isDirectMemoryPageAligned");
      VM_IS_DIRECT_MEMORY_PAGE_ALIGNED_METHOD.setAccessible(true);
      isPageAligned = (boolean) VM_IS_DIRECT_MEMORY_PAGE_ALIGNED_METHOD
          .invoke(null); //static method
    } catch (final ClassNotFoundException | NoSuchMethodException |  IllegalAccessException
        | IllegalArgumentException | InvocationTargetException | SecurityException e) {
      throw new RuntimeException("Could not acquire jdk.internal.misc.VM: " + e.getClass());
    }
  }

  /**
   * Returns the maximum amount of allocatable direct buffer memory.
   * The directMemory variable is initialized during system initialization.
   * @return the maximum amount of allocatable direct buffer memory.
   */
  public static long getMaxDBBMemory() {
    return maxDBBMemory;
  }

  /**
   * Returns true if the direct buffers should be page aligned.
   * @return flag that determines whether direct buffers should be page aligned.
   */
  public static boolean getIsPageAligned() {
    return isPageAligned;
  }
}