Question

I need to determine that whether a given file is a class file or not. Suppose I change the extension to .exe/.xml or some other, I need to determine whether that given file, if a class file will be parsed differently and if it would be some other, it’ll be parsed in that manner.

How can I read the class file format?

Answer 1

I suspect BCEL lets you do this. Checking now…

EDIT: Yes, it does. You can use the ClassParser class; the parse method will throw a ClassFormatException if the file isn’t really a Java class file.

EDIT: Okay, so you need to know from C#. Three options:

• Check whether the header is 0xcafebabe and leave it at that (it’ll be a pretty good heuristic)
• Port BCEL to C# (just the bits you need, obviously)
• Try using J# to compile BCEL to .NET directly – that will only work if it doesn’t use anything beyond Java 1.1.4, but you may find a really old version of BCEL works fine in that respect. Of course you’ll need to get hold of J# as well, which is now discontinued.

The CAFEBABE test is very easy:

public bool IsProbablyJava(string file)
{
    using (Stream stream = File.OpenRead(file))
    {
        return stream.ReadByte() == 0xca &&
               stream.ReadByte() == 0xfe &&
               stream.ReadByte() == 0xba &&
               stream.ReadByte() == 0xbe;
    }
}

It’s not exactly pretty… there are alternatives with BinaryReader etc as well.