Finding Class Version

Facing class mismatch error, I decided to find the JRE version and the compiler version used to compile the class file.

Its easy to get the details of the JRE version using System.getProperties(). A more difficult task was to find out the version of the class file. I could not find any API which did it :(. But I did find out that these details were available in the class file, so I wrote a small code to determine if the class will run on the current version of JRE.

import java.io.DataInputStream;
import java.io.FileInputStream;

public class ClassRuntimeVersion {
    public static void main(String[] args) throws Exception {

        FileInputStream fis = null;
        DataInputStream dis = null;

        try {
            if (args.length != 1) {
                throw new Exception("specify the class file");
            }

            fis = new FileInputStream(args[0]);
            dis = new DataInputStream(fis);

            if (dis.readInt() != 0xCAFEBABE) {
                throw new Exception("not a class file");
            }

            short minorVersion = dis.readShort();
            short majorVersion = dis.readShort();

            System.out.println("Java Class version info");
            System.out.println("1.6    50");
            System.out.println("1.5    49");
            System.out.println("1.4    48");
            System.out.println("1.3    47");
            System.out.println("-------------------------");

            float classVersion = Float.parseFloat(majorVersion + "."
                    + minorVersion);
            System.out.println("Class file compiled version : " + classVersion);

            float jreJavaVersion = Float.parseFloat(System
                    .getProperty("java.class.version"));

            System.out.println("JRE version : " + jreJavaVersion);
            if (jreJavaVersion < classVersion) {
                System.err.println("You r dead dude, versions did not match");
            }
        } finally {
            if (fis != null) {
                fis.close();
            }
            if (dis != null) {
                dis.close();
            }
        }
    }
}