The MappedByteBuffer.java Java example source code
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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"> 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 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 <tt>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 <tt>true 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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