src/share/classes/java/nio/MappedByteBuffer.java

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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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 * accompanied this code).
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package java.nio;


/**
 * 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"><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.

    // Volatile to make sure that the finalization thread sees the current
    // value of this so that a region is not accidentally unmapped again later.
    volatile boolean isAMappedBuffer;                   // package-private

    // This should only be invoked by the DirectByteBuffer constructors
    //
    MappedByteBuffer(int mark, int pos, int lim, int cap, // package-private
                     boolean mapped)
    {
        super(mark, pos, lim, cap);
        isAMappedBuffer = mapped;
    }

    MappedByteBuffer(int mark, int pos, int lim, int cap) { // package-private
        super(mark, pos, lim, cap);
        isAMappedBuffer = false;
    }

    private void checkMapped() {
        if (!isAMappedBuffer)
            // Can only happen if a luser explicitly casts a direct byte buffer
            throw new UnsupportedOperationException();
    }

    /**
     * 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;
        return isLoaded0(((DirectByteBuffer)this).address(), capacity());
    }

    /**
     * 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;
        load0(((DirectByteBuffer)this).address(), capacity(), Bits.pageSize());
        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))
            return this;
        force0(((DirectByteBuffer)this).address(), capacity());
        return this;
    }

    private native boolean isLoaded0(long address, long length);
    private native int load0(long address, long length, int pageSize);
    private native void force0(long address, long length);

}