Module java.base
Package java.nio

Class MappedByteBuffer

All Implemented Interfaces:
Comparable<ByteBuffer>

public abstract class MappedByteBuffer extends ByteBuffer
A direct byte buffer whose content is a memory-mapped region of a file.

Mapped byte buffers are created via the FileChannel.map method. This class extends the 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.

Since:
1.4
  • Method Details

    • isLoaded

      public final boolean isLoaded()
      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.

      Returns:
      true if it is likely that this buffer's content is resident in physical memory
    • load

      public final MappedByteBuffer load()
      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.

      Returns:
      This buffer
    • force

      public final MappedByteBuffer force()
      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 (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.

      Returns:
      This buffer
      Throws:
      UncheckedIOException - If an I/O error occurs writing the buffer's content to the storage device containing the mapped file
    • force

      public final MappedByteBuffer force(int index, int length)
      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 index in this buffer and is 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 (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.

      Parameters:
      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 capacity()
      length - The length of the region in bytes; must be non-negative and no larger than capacity() - index
      Returns:
      This buffer
      Throws:
      IndexOutOfBoundsException - if the preconditions on the index and length do not hold.
      UncheckedIOException - If an I/O error occurs writing the buffer's content to the storage device containing the mapped file
      Since:
      13
    • position

      public final MappedByteBuffer position(int newPosition)
      Sets this buffer's position. If the mark is defined and larger than the new position then it is discarded.
      Overrides:
      position in class Buffer
      Parameters:
      newPosition - The new position value; must be non-negative and no larger than the current limit
      Returns:
      This buffer
    • limit

      public final MappedByteBuffer limit(int newLimit)
      Sets this buffer's limit. If the position is larger than the new limit then it is set to the new limit. If the mark is defined and larger than the new limit then it is discarded.
      Overrides:
      limit in class Buffer
      Parameters:
      newLimit - The new limit value; must be non-negative and no larger than this buffer's capacity
      Returns:
      This buffer
    • mark

      public final MappedByteBuffer mark()
      Sets this buffer's mark at its position.
      Overrides:
      mark in class Buffer
      Returns:
      This buffer
    • reset

      public final MappedByteBuffer reset()
      Resets this buffer's position to the previously-marked position.

      Invoking this method neither changes nor discards the mark's value.

      Overrides:
      reset in class Buffer
      Returns:
      This buffer
    • clear

      public final MappedByteBuffer clear()
      Clears this buffer. The position is set to zero, the limit is set to the capacity, and the mark is discarded.

      Invoke this method before using a sequence of channel-read or put operations to fill this buffer. For example:

       buf.clear();     // Prepare buffer for reading
       in.read(buf);    // Read data

      This method does not actually erase the data in the buffer, but it is named as if it did because it will most often be used in situations in which that might as well be the case.

      Overrides:
      clear in class Buffer
      Returns:
      This buffer
    • flip

      public final MappedByteBuffer flip()
      Flips this buffer. The limit is set to the current position and then the position is set to zero. If the mark is defined then it is discarded.

      After a sequence of channel-read or put operations, invoke this method to prepare for a sequence of channel-write or relative get operations. For example:

       buf.put(magic);    // Prepend header
       in.read(buf);      // Read data into rest of buffer
       buf.flip();        // Flip buffer
       out.write(buf);    // Write header + data to channel

      This method is often used in conjunction with the compact method when transferring data from one place to another.

      Overrides:
      flip in class Buffer
      Returns:
      This buffer
    • rewind

      public final MappedByteBuffer rewind()
      Rewinds this buffer. The position is set to zero and the mark is discarded.

      Invoke this method before a sequence of channel-write or get operations, assuming that the limit has already been set appropriately. For example:

       out.write(buf);    // Write remaining data
       buf.rewind();      // Rewind buffer
       buf.get(array);    // Copy data into array
      Overrides:
      rewind in class Buffer
      Returns:
      This buffer