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 sealed class MappedByteBuffer extends ByteBuffer permits DirectByteBuffer { // 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, MemorySegment 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, MemorySegment segment) { super(mark, pos, lim, cap, segment); this.fd = null; this.isSync = isSync; } MappedByteBuffer(int mark, int pos, int lim, int cap, MemorySegment segment) { // package-private super(mark, pos, lim, cap, segment); this.fd = null; this.isSync = false; } UnmapperProxy unmapper() { return fd == null ? null : new UnmapperProxy() { // Ensure safe publication as MappedByteBuffer.this.address is not final private final long addr = address; @Override public long address() { return addr; } @Override public FileDescriptor fileDescriptor() { return fd; } @Override public boolean isSync() { return isSync; } @Override public void unmap() { Unsafe.getUnsafe().invokeCleaner(MappedByteBuffer.this); } }; } /** * 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(session(), 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(session(), 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(session(), fd, address, isSync, index, length); } return this; } // -- Covariant return type overrides /** * {@inheritDoc} * @since 9 */ @Override public final MappedByteBuffer position(int newPosition) { super.position(newPosition); return this; } /** * {@inheritDoc} * @since 9 */ @Override public final MappedByteBuffer limit(int newLimit) { super.limit(newLimit); return this; } /** * {@inheritDoc} * @since 9 */ @Override public final MappedByteBuffer mark() { super.mark(); return this; } /** * {@inheritDoc} * @since 9 */ @Override public final MappedByteBuffer reset() { super.reset(); return this; } /** * {@inheritDoc} * @since 9 */ @Override public final MappedByteBuffer clear() { super.clear(); return this; } /** * {@inheritDoc} * @since 9 */ @Override public final MappedByteBuffer flip() { super.flip(); return this; } /** * {@inheritDoc} * @since 9 */ @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())}. * * @since 17 */ @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. * * @since 17 */ @Override public abstract MappedByteBuffer slice(int index, int length); /** * {@inheritDoc} * * @since 17 */ @Override public abstract MappedByteBuffer duplicate(); /** * {@inheritDoc} * @throws ReadOnlyBufferException {@inheritDoc} * * @since 17 */ @Override public abstract MappedByteBuffer compact(); }