从PHP中的Java DataOutputStream读取二进制数据时的整数格式是什么？

Question

I'm aware that this is probably not the best idea but I've been playing around trying to read a file in PHP that was encoded using Java's DataOutputStream.

Specifically, in Java I use:

dataOutputStream.writeInt(number);

Then in PHP I read the file using:

$data = fread($handle, 4);
$number = unpack('N', $data);

The strange thing is that the only format character in PHP that gives the correct value is 'N', which is supposed to represent "unsigned long (always 32 bit, big endian byte order)". I thought that int in java was always signed?

Is it possible to reliably read data encoded in Java in this way or not? In this case the integer will only ever need to be positive. It may also need to be quite large so writeShort() is not possible. Otherwise of course I could use XML or JSON or something.

Answer 1

This is fine, as long as you don't need that extra bit. l (instead of N ) would work on a big endian machine.

Note, however, that the maximum number that you can store is 2,147,483,647 unless you want to do some math on the Java side to get the proper negative integer to represent the desired unsigned integer.

Note that a signed Java integer uses the two's complement method to represent a negative number, so it's not as easy as flipping a bit.

Answer 2

DataOutputStream.writeInt :

Writes an int to the underlying output stream as four bytes, high byte first.

The formats available for the unpack function for signed integers all use machine dependent byte order. My guess is that your machine uses a different byte order than Java. If that is true, the DataOutputStream + unpack combination will not work for any signed primitive.