Java memory Mapped File from Windows system

Question

I'm trying something new, There is an application who send data to a memory-mapped file located at Local\MemFileName

I would like to read it in java,

I tried some tutorials like https://www.baeldung.com/java-mapped-byte-buffer, https://howtodoinjava.com/java7/nio/memory-mapped-files-mappedbytebuffer/

But all seem to read a file in JVM, or I did not understand...

How can I read the content of the file Located in windows system Local\MemFileName

Thanks!

Following: Example code of what i tried

public class Main {

private static final String IRSDKMEM_MAP_FILE_NAME = StringEscapeUtils.unescapeJava("Local\\IRSDKMemMapFileName");
private static final String IRSDKDATA_VALID_EVENT  = StringEscapeUtils.unescapeJava("Local\\IRSDKDataValidEvent");

public static final CharSequence charSequence = "Local\\IRSDKMemMapFileName";

public static void main(String[] args) throws IOException, InterruptedException {

    System.out.println(charSequence);

    try (RandomAccessFile file = new RandomAccessFile(new File(IRSDKMEM_MAP_FILE_NAME), "r")) {
        //Get file channel in read-only mode
        FileChannel fileChannel = file.getChannel();

        //Get direct byte buffer access using channel.map() operation
        MappedByteBuffer buffer = fileChannel.map(FileChannel.MapMode.READ_ONLY, 0, fileChannel.size());

        // the buffer now reads the file as if it were loaded in memory.
        System.out.println("Loaded " + buffer.isLoaded());  //prints false
        System.out.println("capacity" + buffer.capacity());  //Get the size based on content size of file

        //You can read the file from this buffer the way you like.
        for (int i = 0; i < buffer.limit(); i++) {
            System.out.println((char) buffer.get()); //Print the content of file
        }
    }


}

}

Simon G. · Accepted Answer · 2020-06-05 09:05:27Z

1

To read a memory mapped file:

Open a FileChannel on the file using FileChannel.open.

Invoke the map method on the FileChannel to create a MappedByteBuffer covering the area of the file you want to read.

Read the data from the MappedByteBuffer.

answered Jun 5, 2020 at 9:05

Simon G.

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5 Comments

Joffrey Over a year ago

I did it, and I got a NullPointerException. I saw that if i write instead of read, it create the file Local\\MemFileName in the project directory

Simon G. Over a year ago

@JoffreyBonifay Please post the code to show what you tried.

Simon G. Over a year ago

The code you just posted works fine for me! Where do you get a NullPointerException?

Joffrey Over a year ago

Exception in thread "main" java.io.FileNotFoundException: LocalIRSDKMemMapFileName (File not found) at java.io.RandomAccessFile.open0(Native Method) at java.io.RandomAccessFile.open(RandomAccessFile.java:316) at java.io.RandomAccessFile.<init>(RandomAccessFile.java:243) at io.mappedbus.Main.main(Main.java:21)

Joffrey Over a year ago

the file "Local\\IRSDKMemMapFileName" is mapped in memory by an application

Joffrey · Accepted Answer · 2021-01-19 09:48:34Z

1

The solution for me was to use a WindowsService.class implementing methods from JNA library, as you can see: My Library

With this I could open a file mapped in Windows system.

All the previous answers was correct for a file accessible from JVM, but from outside the JVM it was impossible.

Thanks !

answered Jan 19, 2021 at 9:48

Joffrey

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