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	/*
	* 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;
	}

	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);
	}

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