CODEWITHSUNDEEP
pythonjava
N3tMaster
N3tMaster

Reputation: 493

Cannot read integer values saved into binary file with Java

I implemented a procedure for saving 32bit signed integer values, retrieved from PostgreSQL, into a binary file.

I used ByteArrayOutputStream and DataOutputStream

//..


ByteArrayOutputStream baos = new ByteArrayOutputStream();
DataOutputStream out = new DataOutputStream(baos);

//this cycle over my retrieved recordset
while (tdb.next()) {

   out.writeInt((tdb.getInteger(1)));  //getInteger method extract integer value from recordset

}

//convert into byte array
byte[] bytes = baos.toByteArray();

//create file for output
File fileout = new File("./myfilebin.bin");
//write data
Files.write(fileout.toPath(), bytes);

//...

My input data sample contains these values:

0, 0, 42812, 42822, 41483, 0, 0, ...

When I try to read my binary file, I will get these:

0, 0, 1017577472, 1185349632, 195166208, 0, 0, ...

For reading file I wrote a simple python script:

import struct

with open("./myfilebin.bin", "rb") as f:

     for chunk in iter(lambda: f.read(4), ''):
        integer_value = struct.unpack('i', chunk)[0]
        print(integer_value)

Any idea?

Upvotes: 0

Views: 102

Answers (2)

N3tMaster
N3tMaster

Reputation: 493

Ok, the problem was the different behaviour in bit reading order between python, that uses LE, and java.

I need to create a binary file that follows ISOBUS standards and I have to write it using LE.

So I change my code in this way:

FileOutputStream fout = new FileOutputStream("myfileout.bin");

            DataOutputStream out = new DataOutputStream(fout);


            while (tdb.next()) {
                //convert byte endian behaviour for ISOBUS software compatibility
                ByteBuffer bb = ByteBuffer.allocate(4);
                bb.order(ByteOrder.LITTLE_ENDIAN);
                bb.putInt(tdb.getInteger(1));
                //invert significant byte
                bb.flip();

                out.write(bb.array());
            }

            fout.close();
            out.close();

Upvotes: 0

Michail Alexakis
Michail Alexakis

Reputation: 1585

Probably your numbers are Big-Endian (BE) and you try to decode as Little-Endian (LE).

Try: struct.unpack('>i', chunk) to force reading as BE.

See also the docs for struct module on alignment and endianess.

FYI, DataOutputStream always writes in BE as stated in the official javadoc.

Upvotes: 2

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