Writing Java.class file in NodeJs (preserving magic number)

Question

I have server that replaces some strings in a file. It looks like this:

var stringToBeReplacedWith = "Cool text"; var data = fs.readFileSync(file, 'utf-8'); var RegExp = new RegExp("Stringtobereplaced", 'g'); // global search data = fileContents.replace(RegExp, stringToBeReplacedWith); fs.writeFileSync(file, data);

The code works by the Mime-type/encoding changes.

How can I make sure the Mime-Type is preserved when replacing the string? I've noticed there are plenty of libraries to read the mimetype but (so far) I haven't found a library that does the opposite.

mscdex · Accepted Answer · 2015-02-05 20:11:48Z

2

It's because .class files are binary. You're reading the file in as a UTF-8 string. So when you write it back out, it's writing it out as a malformed UTF-8 string (hence the change in magic number).

As long as you aren't trying to replace multi-byte characters, you could change

var data = fs.readFileSync(file, 'utf-8');

to

var data = fs.readFileSync(file, 'binary');

and

fs.writeFileSync(file, data);

to

fs.writeFileSync(file, data, { encoding: 'binary' });

or

fs.writeFileSync(file, new Buffer(data, 'binary'));

and it should work as you expect.

edited Feb 5, 2015 at 20:11

answered Feb 5, 2015 at 17:19

mscdex

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7 Comments

Mdlc Over a year ago

I'm getting: "Error: Unknown Encoding" when writing to the file. I see there are ways to "encode" the file anyways using a library (stackoverflow.com/questions/14551608/…) but this requires the encoding? Would you perhaps know what encoding to use?

mscdex Over a year ago

What version of node are you using? You can also try the alternative use of fs.writeFileSync that I've now included in my answer.

Mdlc Over a year ago

Just using 'binary' instead of { encoding: 'binary' } fixed it for me > the code executed. But when compiling the code I get the following error: "unknown tag byte: 6f" (instead of 3f for some files also 54 and 3f). Any idea why this could be? P.s. I'm on node 10

mscdex Over a year ago

I'm not familiar with the Java .class format. Perhaps it stores strings preceded by a length value and you're making the strings larger or smaller but not updating the length value? So it ends up reading past or inside the string and finds an unexpected byte value?

Mdlc Over a year ago

JAVA is ISO 8859-1 with \uXXXX escape sequences, denoting Unicode BMP characters. Consecutive pairs of \uXXXX escape sequences in the surrogate range, as in UTF-16, denote Unicode characters outside the BMP. - github.com/bnoordhuis/node-iconv/blob/master/deps/libiconv/lib/…

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