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package java.nio;
import java.io.FileDescriptor;
import java.io.UncheckedIOException;
import java.lang.ref.Reference;
import java.util.Objects;
import jdk.internal.access.foreign.MemorySegmentProxy;
import jdk.internal.access.foreign.UnmapperProxy;
import jdk.internal.misc.ScopedMemoryAccess;
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 {@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.
*
*
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
* @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;
// A flag true if this buffer is mapped against non-volatile
// memory using one of the extended FileChannel.MapMode modes,
// MapMode.READ_ONLY_SYNC or MapMode.READ_WRITE_SYNC and false if
// it is mapped using any of the other modes. This flag only
// determines the behavior of force operations.
private final boolean isSync;
static final ScopedMemoryAccess SCOPED_MEMORY_ACCESS = ScopedMemoryAccess.getScopedMemoryAccess();
// This should only be invoked by the DirectByteBuffer constructors
//
MappedByteBuffer(int mark, int pos, int lim, int cap, // package-private
FileDescriptor fd, boolean isSync, MemorySegmentProxy segment) {
super(mark, pos, lim, cap, segment);
this.fd = fd;
this.isSync = isSync;
}
MappedByteBuffer(int mark, int pos, int lim, int cap, // package-private
boolean isSync, MemorySegmentProxy segment) {
super(mark, pos, lim, cap, segment);
this.fd = null;
this.isSync = isSync;
}
MappedByteBuffer(int mark, int pos, int lim, int cap, MemorySegmentProxy segment) { // package-private
super(mark, pos, lim, cap, segment);
this.fd = null;
this.isSync = false;
}
UnmapperProxy unmapper() {
return fd != null ?
new UnmapperProxy() {
@Override
public long address() {
return address;
}
@Override
public FileDescriptor fileDescriptor() {
return fd;
}
@Override
public boolean isSync() {
return isSync;
}
@Override
public void unmap() {
Unsafe.getUnsafe().invokeCleaner(MappedByteBuffer.this);
}
} : null;
}
/**
* Tells whether this buffer was mapped against a non-volatile
* memory device by passing one of the sync map modes {@link
* jdk.nio.mapmode.ExtendedMapMode#READ_ONLY_SYNC
* ExtendedMapModeMapMode#READ_ONLY_SYNC} or {@link
* jdk.nio.mapmode.ExtendedMapMode#READ_ONLY_SYNC
* ExtendedMapMode#READ_WRITE_SYNC} in the call to {@link
* java.nio.channels.FileChannel#map FileChannel.map} or was
* mapped by passing one of the other map modes.
*
* @return true if the file was mapped using one of the sync map
* modes, otherwise false.
*/
final boolean isSync() { // package-private
return isSync;
}
/**
* Returns the {@code FileDescriptor} associated with this
* {@code MappedByteBuffer}.
*
* @return the buffer's file descriptor; may be {@code null}
*/
final FileDescriptor fileDescriptor() { // package-private
return fd;
}
/**
* Tells whether or not this buffer's content is resident in physical
* memory.
*
* 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.
*
*
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.
*
* @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;
}
return SCOPED_MEMORY_ACCESS.isLoaded(scope(), address, isSync, capacity());
}
/**
* 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.
*
* @return This buffer
*/
public final MappedByteBuffer load() {
if (fd == null) {
return this;
}
try {
SCOPED_MEMORY_ACCESS.load(scope(), address, isSync, capacity());
} finally {
Reference.reachabilityFence(this);
}
return this;
}
/**
* Forces any changes made to this buffer's content to be written to the
* storage device containing the mapped file. The region starts at index
* zero in this buffer and is {@code capacity()} bytes. An invocation of
* this method behaves in exactly the same way as the invocation
* {@link force(int,int) force(0,capacity())}.
*
* 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 ({@link
* java.nio.channels.FileChannel.MapMode#READ_WRITE}) then
* invoking this method may have no effect. In particular, the
* method has no effect for buffers mapped in read-only or private
* mapping modes. This method may or may not have an effect for
* implementation-specific mapping modes.
*
* @throws UncheckedIOException
* If an I/O error occurs writing the buffer's content to the
* storage device containing the mapped file
*
* @return This buffer
*/
public final MappedByteBuffer force() {
if (fd == null) {
return this;
}
int capacity = capacity();
if (isSync || ((address != 0) && (capacity != 0))) {
return force(0, capacity);
}
return this;
}
/**
* Forces any changes made to a region of this buffer's content to
* be written to the storage device containing the mapped
* file. The region starts at the given {@code index} in this
* buffer and is {@code length} bytes.
*
* 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 selected region buffer since it
* was created, or since this method was last invoked, will have
* been written to that device. The force operation is free to
* write bytes that lie outside the specified region, for example
* to ensure that data blocks of some device-specific granularity
* are transferred in their entirety.
*
*
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 ({@link
* java.nio.channels.FileChannel.MapMode#READ_WRITE}) then
* invoking this method may have no effect. In particular, the
* method has no effect for buffers mapped in read-only or private
* mapping modes. This method may or may not have an effect for
* implementation-specific mapping modes.
*
* @param index
* The index of the first byte in the buffer region that is
* to be written back to storage; must be non-negative
* and less than {@code capacity()}
*
* @param length
* The length of the region in bytes; must be non-negative
* and no larger than {@code capacity() - index}
*
* @throws IndexOutOfBoundsException
* if the preconditions on the index and length do not
* hold.
*
* @throws UncheckedIOException
* If an I/O error occurs writing the buffer's content to the
* storage device containing the mapped file
*
* @return This buffer
*
* @since 13
*/
public final MappedByteBuffer force(int index, int length) {
if (fd == null) {
return this;
}
int capacity = capacity();
if ((address != 0) && (capacity != 0)) {
// check inputs
Objects.checkFromIndexSize(index, length, capacity);
SCOPED_MEMORY_ACCESS.force(scope(), fd, address, isSync, index, length);
}
return this;
}
// -- 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;
}
/**
* {@inheritDoc}
*
* Reading bytes into physical memory by invoking {@code load()} on the
* returned buffer, or writing bytes to the storage device by invoking
* {@code force()} on the returned buffer, will only act on the sub-range
* of this buffer that the returned buffer represents, namely
* {@code [position(),limit())}.
*/
@Override
public abstract MappedByteBuffer slice();
/**
* {@inheritDoc}
*
*
Reading bytes into physical memory by invoking {@code load()} on the
* returned buffer, or writing bytes to the storage device by invoking
* {@code force()} on the returned buffer, will only act on the sub-range
* of this buffer that the returned buffer represents, namely
* {@code [index,index+length)}, where {@code index} and {@code length} are
* assumed to satisfy the preconditions.
*/
@Override
public abstract MappedByteBuffer slice(int index, int length);
/**
* {@inheritDoc}
*/
@Override
public abstract MappedByteBuffer duplicate();
/**
* {@inheritDoc}
*/
@Override
public abstract MappedByteBuffer compact();
}