write file in memory with java.nio?

Question

With nio it is possible to map an existing file in memory. But is it possible to create it only in memory without file on the hard drive ?

I want to mimic the CreateFileMapping windows functions which allow you to write in memory.

Is there an equivalent system in Java ?

The goal is to write in memory in order for another program ( c ) to read it.

You can tell Windows to create a memory mapping of the pagefile rather than a named file. Is that what you're looking for? — Gabe, Mar 22 '12 at 17:38
Indeed, the goal is to write in memory in order for another program ( c ) to read it. — Foobyto, Mar 23 '12 at 10:37

score 3 · Accepted Answer · answered Mar 22 '12 at 21:36

3

Have a look at the following. A file is created but this might be as close as your going to get.

MappedByteBuffer
MappedByteBuffer.load()
FileChannel
FileChannel.map()

Here is a snippet to try and get you started.

    filePipe = new File(tempDirectory, namedPipe.getName() + ".pipe");
    try {
        int pipeSize = 4096;
        randomAccessFile = new RandomAccessFile(filePipe, "rw");
        fileChannel = randomAccessFile.getChannel();
        mappedByteBuffer = fileChannel.map(FileChannel.MapMode.READ_WRITE, 0, pipeSize);
        mappedByteBuffer.load();
    } catch (Exception e) {
    ...

answered Mar 22 '12 at 21:36

Java42

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This work and indeed, everything is in RAM (i check with some utilities, no hdd i/o) But, do you know how to retrieve the mapped file with c code ? using window function ? – Foobyto Mar 23 '12 at 14:56
@Foobyto - No I don't. Worth adding a new question to get it answered. – Java42 Mar 23 '12 at 16:57
Actually, after research, you can't write in RAM with java, you need to do it through the system api. This thing for exemple: http://msdn.microsoft.com/en-us/library/windows/desktop/aa366537 Java just let you to put a file in RAM in order to modify it faster, that's all. – Foobyto Apr 05 '12 at 14:37

score 0 · Answer 2 · answered Mar 22 '12 at 22:53

Most libraries in Java deal with input and output streams as opposed to java.io.File objects.
Examples: image reading, XML, audio, zip

Where possible, when dealing with I/O, use streams.

This may not be what you want, however, if you need random access to the data.

When using memory mapped files, and you get a MappedByteBuffer from a FileChannel using FileChannel.map(), if you don't need a file just use a ByteBuffer instead, which exists totally in memory. Create one of these using ByteBuffer.allocate() or ByteBuffer.allocateDirect().

write file in memory with java.nio?

2 Answers2

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