How to find the version of given class file?

[dinesh Venkatesan]

Hi Friends,

I am having a classfile and i would like to findout the version of javac that compiled this class. How can i do that.

Thanks in advance,

regards,
dinesh Venkatesan.

[Ulf Dittmer]

You can't. You can only determine for which version of the JVM the code was compiled. Generally, javac will generate bytecode for the JVM version that it shipped with (i.e., javac in Java 5 will create Java 5 bytecode), but that can be changed using the -source and -target switches.

Bytes 5 through 8 of a class file contain the minor and major class file version (i.e., the one specified by the "-target" switch). Some more detail can be found here.

[Jesper de Jong]

Here's a small program that shows the class file version number. However, the class file version number is not the same as the JDK version number with which it is compiled.

For Java 1.4, you'll get 48.0, for Java 5 you'll get 49.0 and for Java 6 you'll get 50.0. But as Ulf already said, you can also create version 48.0 and 49.0 files with the compiler in JDK 6, if you use the -source and -target switches.

import java.io.FileInputStream;
import java.io.IOException;
 
public class ClassVersion
{
    public static void main(String[] args) throws IOException
    {
        byte[] buffer = new byte[8];
 
        FileInputStream in = new FileInputStream(args[0]);
        in.read(buffer, 0, 8);
        in.close();
 
        int majorVersion = buffer[6] << 8 | buffer[7];
        int minorVersion = buffer[4] << 8 | buffer[5];
 
        System.out.printf("Classfile version: %d.%d%n", majorVersion, minorVersion);
    }
}

[ August 16, 2007: Message edited by: Jesper Young ]

[dinesh Venkatesan]

Hi Ulf,

Thanks much for the help. But could you tell me how to get the information that represents the major version in the file. Should I go for BCEL?

regards,
dinesh V

[Ulf Dittmer]

What information are you looking for that Jesper's code doesn't provide?

BCEL can give you this information, but that would seem to be rather more effort than the above solution.

[dinesh Venkatesan]

Yes Ulf, Jasper's code gave me the info i wanted.
Thanks Jasper