content: /*
   * Copyright (C) 2014 The Android Open Source Project
   * Copyright (c) 2000, 2013, Oracle and/or its affiliates. All rights reserved.
   * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   *
   * This code is free software; you can redistribute it and/or modify it
   * under the terms of the GNU General Public License version 2 only, as
   * published by the Free Software Foundation.  Oracle designates this
   * particular file as subject to the "Classpath" exception as provided
   * by Oracle in the LICENSE file that accompanied this code.
   *
   * This code is distributed in the hope that it will be useful, but WITHOUT
   * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
   * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
   * version 2 for more details (a copy is included in the LICENSE file that
   * accompanied this code).
   *
   * You should have received a copy of the GNU General Public License version
   * 2 along with this work; if not, write to the Free Software Foundation,
   * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
   *
   * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
   * or visit www.oracle.com if you need additional information or have any
   * questions.
   */
  
  package java.nio;
  
  import java.io.FileDescriptor;
  import sun.misc.Unsafe;
  
  
  /**
   * A direct byte buffer whose content is a memory-mapped region of a file.
   *
   * <p> Mapped byte buffers are created via the {@link
   * java.nio.channels.FileChannel#map FileChannel.map} method.  This class
   * extends the {@link ByteBuffer} class with operations that are specific to
   * memory-mapped file regions.
   *
   * <p> A mapped byte buffer and the file mapping that it represents remain
   * valid until the buffer itself is garbage-collected.
   *
   * <p> The content of a mapped byte buffer can change at any time, for example
   * if the content of the corresponding region of the mapped file is changed by
   * this program or another.  Whether or not such changes occur, and when they
   * occur, is operating-system dependent and therefore unspecified.
   *
   * <a name="inaccess"></a><p> All or part of a mapped byte buffer may become
   * inaccessible at any time, for example if the mapped file is truncated.  An
   * attempt to access an inaccessible region of a mapped byte buffer will not
   * change the buffer's content and will cause an unspecified exception to be
   * thrown either at the time of the access or at some later time.  It is
   * therefore strongly recommended that appropriate precautions be taken to
   * avoid the manipulation of a mapped file by this program, or by a
   * concurrently running program, except to read or write the file's content.
   *
   * <p> Mapped byte buffers otherwise behave no differently than ordinary direct
   * byte buffers. </p>
   *
   *
   * @author Mark Reinhold
   * @author JSR-51 Expert Group
   * @since 1.4
   */
  
  public abstract class MappedByteBuffer
      extends ByteBuffer
  {
  
      // This is a little bit backwards: By rights MappedByteBuffer should be a
      // subclass of DirectByteBuffer, but to keep the spec clear and simple, and
      // for optimization purposes, it's easier to do it the other way around.
      // This works because DirectByteBuffer is a package-private class.
  
      // For mapped buffers, a FileDescriptor that may be used for mapping
      // operations if valid; null if the buffer is not mapped.
      private final FileDescriptor fd;
  
      // This should only be invoked by the DirectByteBuffer constructors
      //
      MappedByteBuffer(int mark, int pos, int lim, int cap, // package-private
                       FileDescriptor fd)
      {
          super(mark, pos, lim, cap);
          this.fd = fd;
      }
  
      // Android-added: Additional constructor for use by Android's DirectByteBuffer.
      MappedByteBuffer(int mark, int pos, int lim, int cap, byte[] buf, int offset) {
          super(mark, pos, lim, cap, buf, offset);
          this.fd = null;
      }
  
      MappedByteBuffer(int mark, int pos, int lim, int cap) { // package-private
          super(mark, pos, lim, cap);
          this.fd = null;
      }
  
      private void checkMapped() {
          if (fd == null)
              // Can only happen if a luser explicitly casts a direct byte buffer
              throw new UnsupportedOperationException();
      }
  
      // Returns the distance (in bytes) of the buffer from the page aligned address
      // of the mapping. Computed each time to avoid storing in every direct buffer.
      private long mappingOffset() {
          int ps = Bits.pageSize();
          long offset = address % ps;
          return (offset >= 0) ? offset : (ps + offset);
      }
  
      private long mappingAddress(long mappingOffset) {
          return address - mappingOffset;
      }
  
      private long mappingLength(long mappingOffset) {
          return (long)capacity() + mappingOffset;
      }
  
      /**
       * Tells whether or not this buffer's content is resident in physical
       * memory.
       *
       * <p> A return value of <tt>true</tt> implies that it is highly likely
       * that all of the data in this buffer is resident in physical memory and
       * may therefore be accessed without incurring any virtual-memory page
       * faults or I/O operations.  A return value of <tt>false</tt> does not
       * necessarily imply that the buffer's content is not resident in physical
       * memory.
       *
       * <p> The returned value is a hint, rather than a guarantee, because the
       * underlying operating system may have paged out some of the buffer's data
       * by the time that an invocation of this method returns.  </p>
       *
       * @return  <tt>true</tt> if it is likely that this buffer's content
       *          is resident in physical memory
       */
      public final boolean isLoaded() {
          checkMapped();
          if ((address == 0) || (capacity() == 0))
              return true;
          long offset = mappingOffset();
          long length = mappingLength(offset);
          return isLoaded0(mappingAddress(offset), length, Bits.pageCount(length));
      }
  
      // not used, but a potential target for a store, see load() for details.
      private static byte unused;
  
      /**
       * Loads this buffer's content into physical memory.
       *
       * <p> This method makes a best effort to ensure that, when it returns,
       * this buffer's content is resident in physical memory.  Invoking this
       * method may cause some number of page faults and I/O operations to
       * occur. </p>
       *
       * @return  This buffer
       */
      public final MappedByteBuffer load() {
          checkMapped();
          if ((address == 0) || (capacity() == 0))
              return this;
          long offset = mappingOffset();
          long length = mappingLength(offset);
          load0(mappingAddress(offset), length);
  
          // Read a byte from each page to bring it into memory. A checksum
          // is computed as we go along to prevent the compiler from otherwise
          // considering the loop as dead code.
          Unsafe unsafe = Unsafe.getUnsafe();
          int ps = Bits.pageSize();
          int count = Bits.pageCount(length);
          long a = mappingAddress(offset);
          byte x = 0;
          for (int i=0; i<count; i++) {
              x ^= unsafe.getByte(a);
              a += ps;
          }
          if (unused != 0)
              unused = x;
  
          return this;
      }
  
      /**
       * Forces any changes made to this buffer's content to be written to the
       * storage device containing the mapped file.
       *
       * <p> If the file mapped into this buffer resides on a local storage
       * device then when this method returns it is guaranteed that all changes
       * made to the buffer since it was created, or since this method was last
       * invoked, will have been written to that device.
       *
       * <p> If the file does not reside on a local device then no such guarantee
       * is made.
       *
       * <p> If this buffer was not mapped in read/write mode ({@link
       * java.nio.channels.FileChannel.MapMode#READ_WRITE}) then invoking this
       * method has no effect. </p>
       *
       * @return  This buffer
       */
      public final MappedByteBuffer force() {
          checkMapped();
          if ((address != 0) && (capacity() != 0)) {
              long offset = mappingOffset();
              force0(fd, mappingAddress(offset), mappingLength(offset));
          }
          return this;
      }
  
      private native boolean isLoaded0(long address, long length, int pageCount);
      private native void load0(long address, long length);
      private native void force0(FileDescriptor fd, long address, long length);
  }
  