MapFilepublic class MapFile extends Object | Test behavior of Memory mapped buffer types. Create a file, write
some data to it, then create three different types of mappings
to it. Observe the effects of changes through the buffer APIs
and updating the file directly. The data spans page boundaries
to illustrate the page-oriented nature of Copy-On-Write mappings.
Created: April 2002 |
| Methods Summary |
|---|
| public static void | dumpBuffer(java.lang.String prefix, java.nio.ByteBuffer buffer)
System.out.print (prefix + ": '");
int nulls = 0;
int limit = buffer.limit();
for (int i = 0; i < limit; i++) {
char c = (char) buffer.get (i);
if (c == '\u0000") {
nulls++;
continue;
}
if (nulls != 0) {
System.out.print ("|[" + nulls
+ " nulls]|");
nulls = 0;
}
System.out.print (c);
}
System.out.println ("'");
| | public static void | main(java.lang.String[] argv)
// create a temp file and get a channel connected to it
File tempFile = File.createTempFile ("mmaptest", null);
RandomAccessFile file = new RandomAccessFile (tempFile, "rw");
FileChannel channel = file.getChannel();
ByteBuffer temp = ByteBuffer.allocate (100);
// put something in the file, starting at location 0
temp.put ("This is the file content".getBytes());
temp.flip();
channel.write (temp, 0);
// Put something else in the file, starting at location 8192
// 8192 is 8k, almost certainly a different memory/FS page.
// This may cause a file hole, depending on the
// filesystem page size.
temp.clear();
temp.put ("This is more file content".getBytes());
temp.flip();
channel.write (temp, 8192);
// create three types of mappings to the same file
MappedByteBuffer ro = channel.map (
FileChannel.MapMode.READ_ONLY, 0, channel.size());
MappedByteBuffer rw = channel.map (
FileChannel.MapMode.READ_WRITE, 0, channel.size());
MappedByteBuffer cow = channel.map (
FileChannel.MapMode.PRIVATE, 0, channel.size());
// The buffer states before any modifications
System.out.println ("Begin");
showBuffers (ro, rw, cow);
// modify the Copy-On-Write buffer
cow.position (8);
cow.put ("COW".getBytes());
System.out.println ("Change to COW buffer");
showBuffers (ro, rw, cow);
// Modify the Read/Write buffer
rw.position (9);
rw.put (" R/W ".getBytes());
rw.position (8194);
rw.put (" R/W ".getBytes());
rw.force();
System.out.println ("Change to R/W buffer");
showBuffers (ro, rw, cow);
// Write to the file through the channel, hit both pages
temp.clear();
temp.put ("Channel write ".getBytes());
temp.flip();
channel.write (temp, 0);
temp.rewind();
channel.write (temp, 8202);
System.out.println ("Write on channel");
showBuffers (ro, rw, cow);
// modify the Copy-On-Write buffer again
cow.position (8207);
cow.put (" COW2 ".getBytes());
System.out.println ("Second change to COW buffer");
showBuffers (ro, rw, cow);
// Modify the Read/Write buffer
rw.position (0);
rw.put (" R/W2 ".getBytes());
rw.position (8210);
rw.put (" R/W2 ".getBytes());
rw.force();
System.out.println ("Second change to R/W buffer");
showBuffers (ro, rw, cow);
// cleanup
channel.close();
file.close();
tempFile.delete();
| | public static void | showBuffers(java.nio.ByteBuffer ro, java.nio.ByteBuffer rw, java.nio.ByteBuffer cow)
dumpBuffer ("R/O", ro);
dumpBuffer ("R/W", rw);
dumpBuffer ("COW", cow);
System.out.println ("");
|
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