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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

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 * 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.

 *

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package java.nio;

import java.io.FileDescriptor;

import java.lang.ref.Reference;

import jdk.internal.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 id="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;

    }

    MappedByteBuffer(int mark, int pos, int lim, int cap) { // package-private

        super(mark, pos, lim, cap);

        this.fd = null;

    }

    // 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 {@code true} 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 {@code false} 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  {@code true} if it is likely that this buffer's content

     *          is resident in physical memory

     */

    public final boolean isLoaded() {

        if (fd == null) {

            return true;

        }

        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() {

        if (fd == null) {

            return this;

        }

        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;

        try {

            for (int i=0; i<count; i++) {

                // TODO consider changing to getByteOpaque thus avoiding

                // dead code elimination and the need to calculate a checksum

                x ^= unsafe.getByte(a);

                a += ps;

            }

        } finally {

            Reference.reachabilityFence(this);

        }

        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() {

        if (fd == null) {

            return this;

        }

        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);

    // -- Covariant return type overrides

    /**

     * {@inheritDoc}

     */

    @Override

    public final MappedByteBuffer position(int newPosition) {

        super.position(newPosition);

        return this;

    }

    /**

     * {@inheritDoc}

     */

    @Override

    public final MappedByteBuffer limit(int newLimit) {

        super.limit(newLimit);

        return this;

    }

    /**

     * {@inheritDoc}

     */

    @Override

    public final MappedByteBuffer mark() {

        super.mark();

        return this;

    }

    /**

     * {@inheritDoc}

     */

    @Override

    public final MappedByteBuffer reset() {

        super.reset();

        return this;

    }

    /**

     * {@inheritDoc}

     */

    @Override

    public final MappedByteBuffer clear() {

        super.clear();

        return this;

    }

    /**

     * {@inheritDoc}

     */

    @Override

    public final MappedByteBuffer flip() {

        super.flip();

        return this;

    }

    /**

     * {@inheritDoc}

     */

    @Override

    public final MappedByteBuffer rewind() {

        super.rewind();

        return this;

    }

}