Java tutorial
/* * Copyright (c) 2000, 2019, 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 java.lang.ref.Reference; import java.util.Objects; import jdk.internal.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 id="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; } // Returns the distance (in bytes) of the buffer start from the // largest page aligned address of the mapping less than or equal // to the start address. private long mappingOffset() { return mappingOffset(0); } // Returns the distance (in bytes) of the buffer element // identified by index from the largest page aligned address of // the mapping less than or equal to the element address. Computed // each time to avoid storing in every direct buffer. private long mappingOffset(int index) { int ps = Bits.pageSize(); long indexAddress = address + index; long baseAddress = alignDown(indexAddress, ps); return indexAddress - baseAddress; } // Given an offset previously obtained from calling // mappingOffset() returns the largest page aligned address of the // mapping less than or equal to the buffer start address. private long mappingAddress(long mappingOffset) { return mappingAddress(mappingOffset, 0); } // Given an offset previously otained from calling // mappingOffset(index) returns the largest page aligned address // of the mapping less than or equal to the address of the buffer // element identified by index. private long mappingAddress(long mappingOffset, long index) { long indexAddress = address + index; return indexAddress - mappingOffset; } // given a mappingOffset previously otained from calling // mappingOffset() return that offset added to the buffer // capacity. private long mappingLength(long mappingOffset) { return mappingLength(mappingOffset, (long) capacity()); } // given a mappingOffset previously otained from calling // mappingOffset(index) return that offset added to the supplied // length. private long mappingLength(long mappingOffset, long length) { return length + mappingOffset; } // align address down to page size private static long alignDown(long address, int pageSize) { // pageSize must be a power of 2 return address & ~(pageSize - 1); } /** * Tells whether or not this buffer's content is resident in physical * memory. * * <p> 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. * * <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 {@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; } 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() { if (fd == null) { return this; } 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; try { for (int i = 0; i < count; i++) { // TODO consider changing to getByteOpaque thus avoiding // dead code elimination and the need to calculate a checksum x ^= unsafe.getByte(a); a += ps; } } finally { Reference.reachabilityFence(this); } 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 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. </p> * * @return This buffer */ public final MappedByteBuffer force() { if (fd == null) { return this; } if ((address != 0) && (capacity() != 0)) { long offset = mappingOffset(); force0(fd, mappingAddress(offset), mappingLength(offset)); } 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. * * <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 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. * * <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 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. </p> * * @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 limit() * * @param length * The length of the region in bytes; must be non-negative * and no larger than limit() - index * * @throws IndexOutOfBoundsException * if the preconditions on the index and length do not * hold. * * @return This buffer * * @since 13 */ public final MappedByteBuffer force(int index, int length) { if (fd == null) { return this; } if ((address != 0) && (limit() != 0)) { // check inputs Objects.checkFromIndexSize(index, length, limit()); long offset = mappingOffset(index); force0(fd, mappingAddress(offset, index), mappingLength(offset, length)); } 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); // -- 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; } }