MappedByteBuffer


1   /*
2    * @(#)MappedByteBuffer.java    1.23 03/12/19
3    *
4    * Copyright 2004 Sun Microsystems, Inc. All rights reserved.
5    * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
6    */
7   
8   package java.nio;
9   
10  
11  /**
12   * A direct byte buffer whose content is a memory-mapped region of a file.
13   *
14   * <p> Mapped byte buffers are created via the {@link
15   * java.nio.channels.FileChannel#map FileChannel.map} method.  This class
16   * extends the {@link ByteBuffer} class with operations that are specific to
17   * memory-mapped file regions.
18   * 
19   * <p> A mapped byte buffer and the file mapping that it represents remain
20   * valid until the buffer itself is garbage-collected.
21   *
22   * <p> The content of a mapped byte buffer can change at any time, for example
23   * if the content of the corresponding region of the mapped file is changed by
24   * this program or another.  Whether or not such changes occur, and when they
25   * occur, is operating-system dependent and therefore unspecified.
26   * 
27   * <a name="inaccess"><p> All or part of a mapped byte buffer may become
28   * inaccessible at any time, for example if the mapped file is truncated.  An
29   * attempt to access an inaccessible region of a mapped byte buffer will not
30   * change the buffer's content and will cause an unspecified exception to be
31   * thrown either at the time of the access or at some later time.  It is
32   * therefore strongly recommended that appropriate precautions be taken to
33   * avoid the manipulation of a mapped file by this program, or by a
34   * concurrently running program, except to read or write the file's content.
35   *
36   * <p> Mapped byte buffers otherwise behave no differently than ordinary direct
37   * byte buffers. </p>
38   *
39   *
40   * @author Mark Reinhold
41   * @author JSR-51 Expert Group
42   * @version 1.23, 03/12/19
43   * @since 1.4
44   */
45  
46  public abstract class MappedByteBuffer
47      extends ByteBuffer  
48  {
49  
50      // This is a little bit backwards: By rights MappedByteBuffer should be a
51      // subclass of DirectByteBuffer, but to keep the spec clear and simple, and
52      // for optimization purposes, it's easier to do it the other way around.
53      // This works because DirectByteBuffer is a package-private class.
54  
55      // Volatile to make sure that the finalization thread sees the current
56      // value of this so that a region is not accidentally unmapped again later.
57      volatile boolean isAMappedBuffer;           // package-private
58  
59      // This should only be invoked by the DirectByteBuffer constructors
60      //
61      MappedByteBuffer(int mark, int pos, int lim, int cap, // package-private
62               boolean mapped)
63      {
64      super(mark, pos, lim, cap);
65      isAMappedBuffer = mapped;
66      }
67  
68      MappedByteBuffer(int mark, int pos, int lim, int cap) { // package-private
69      super(mark, pos, lim, cap);
70      isAMappedBuffer = false;
71      }
72  
73      private void checkMapped() {
74      if (!isAMappedBuffer)
75          // Can only happen if a luser explicitly casts a direct byte buffer
76          throw new UnsupportedOperationException  ();
77      }
78  
79      /**
80       * Tells whether or not this buffer's content is resident in physical
81       * memory.
82       *
83       * <p> A return value of <tt>true</tt> implies that it is highly likely
84       * that all of the data in this buffer is resident in physical memory and
85       * may therefore be accessed without incurring any virtual-memory page
86       * faults or I/O operations.  A return value of <tt>false</tt> does not
87       * necessarily imply that the buffer's content is not resident in physical
88       * memory.
89       *
90       * <p> The returned value is a hint, rather than a guarantee, because the
91       * underlying operating system may have paged out some of the buffer's data
92       * by the time that an invocation of this method returns.  </p>
93       *
94       * @return  <tt>true</tt> if it is likely that this buffer's content
95       *          is resident in physical memory
96       */
97      public final boolean isLoaded() {
98      checkMapped();
99          if ((address == 0) || (capacity() == 0))
100             return true;
101     return isLoaded0(((DirectByteBuffer  )this).address(), capacity());
102     }
103 
104     /**
105      * Loads this buffer's content into physical memory.
106      *
107      * <p> This method makes a best effort to ensure that, when it returns,
108      * this buffer's content is resident in physical memory.  Invoking this
109      * method may cause some number of page faults and I/O operations to
110      * occur. </p>
111      *
112      * @return  This buffer
113      */
114     public final MappedByteBuffer   load() {
115     checkMapped();
116         if ((address == 0) || (capacity() == 0))
117             return this;
118     load0(((DirectByteBuffer  )this).address(), capacity(), Bits.pageSize());
119     return this;
120     }
121 
122     /**
123      * Forces any changes made to this buffer's content to be written to the
124      * storage device containing the mapped file.
125      *
126      * <p> If the file mapped into this buffer resides on a local storage
127      * device then when this method returns it is guaranteed that all changes
128      * made to the buffer since it was created, or since this method was last
129      * invoked, will have been written to that device.
130      *
131      * <p> If the file does not reside on a local device then no such guarantee
132      * is made.
133      *
134      * <p> If this buffer was not mapped in read/write mode ({@link
135      * java.nio.channels.FileChannel.MapMode#READ_WRITE}) then invoking this
136      * method has no effect. </p>
137      *
138      * @return  This buffer
139      */
140     public final MappedByteBuffer   force() {
141     checkMapped();
142         if ((address == 0) || (capacity() == 0))
143             return this;
144     force0(((DirectByteBuffer  )this).address(), capacity());
145     return this;
146     }
147 
148     private native boolean isLoaded0(long address, long length);
149     private native int load0(long address, long length, int pageSize);
150     private native void force0(long address, long length);
151 
152 }
153
A to Z: JavaDoc & Examples Daily Java News & Articles Open Source Projects Open Source Codes Free Computer Books Remove Frame
Popular Tags