java/11 : java.base/java/nio/MappedByteBuffer.java

MappedByteBuffer
https://openjdk.java.net/
GPLv2 + Classpath Exception
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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.
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 * 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).
 *
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 * 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.
 Mapped byte buffers are created via the FileChannel.map method. This class extends the ByteBuffer class with operations that are specific to memory-mapped file regions. 
 A mapped byte buffer and the file mapping that it represents remain
valid until the buffer itself is garbage-collected.
 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.
 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.
 Mapped byte buffers otherwise behave no differently than ordinary direct
byte buffers. 
Author: Mark Reinhold, JSR-51 Expert Group
Since: 1.4/**
 * 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.
 A return value of 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 false does not necessarily imply that the buffer's content is not resident in physical memory. 
 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.  
Returns:  true if it is likely that this buffer's content is resident in physical memory/**
     * 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.
 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. 
Returns:  This buffer/**
     * 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.
 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.
 If the file does not reside on a local device then no such guarantee
is made.
 If this buffer was not mapped in read/write mode (MapMode.READ_WRITE) then invoking this method has no effect. 
Returns:  This buffer/**
     * 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}
/**
     * {@inheritDoc}
     */
    @Override
    public final MappedByteBuffer position(int newPosition) {
        super.position(newPosition);
        return this;
    }

    {@inheritDoc}
/**
     * {@inheritDoc}
     */
    @Override
    public final MappedByteBuffer limit(int newLimit) {
        super.limit(newLimit);
        return this;
    }

    {@inheritDoc}
/**
     * {@inheritDoc}
     */
    @Override
    public final MappedByteBuffer mark() {
        super.mark();
        return this;
    }

    {@inheritDoc}
/**
     * {@inheritDoc}
     */
    @Override
    public final MappedByteBuffer reset() {
        super.reset();
        return this;
    }

    {@inheritDoc}
/**
     * {@inheritDoc}
     */
    @Override
    public final MappedByteBuffer clear() {
        super.clear();
        return this;
    }

    {@inheritDoc}
/**
     * {@inheritDoc}
     */
    @Override
    public final MappedByteBuffer flip() {
        super.flip();
        return this;
    }

    {@inheritDoc}
/**
     * {@inheritDoc}
     */
    @Override
    public final MappedByteBuffer rewind() {
        super.rewind();
        return this;
    }
}
/

java/ 11/ java.base/java/nio/MappedByteBuffer.java
