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; // 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, byte[] buf, int offset) { super(mark, pos, lim, cap, buf, offset); this.fd = null; } 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. * *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 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 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. * *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() { 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. * *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 has no effect.
* * @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); }