Wednesday, 23rd June 2010
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Check the class version

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This tutorial will show how to get the version number of a compiled .class file

Contents

the article

The first 4 bytes are a magic number, 0xCAFEBABe, to identify a valid class file then the next 2 bytes identify the class format version (major and minor).

Possible major/minor value :

ClassVersionChecker.java 

import java.io.*;
 
public class ClassVersionChecker {
    public static void main(String[] args) throws IOException {
        for (int i = 0; i < args.length; i++)
            checkClassVersion(args[i]);
    }
 
    private static void checkClassVersion(String filename)
        throws IOException
    {
        DataInputStream in = new DataInputStream
         (new FileInputStream(filename));
 
        int magic = in.readInt();
        if(magic != 0xcafebabe) {
          System.out.println(filename + " is not a valid class!");;
        }
        int minor = in.readUnsignedShort();
        int major = in.readUnsignedShort();
        System.out.println(filename + ": " + major + " . " + minor);
        in.close();
    }
}

the output 

> java ClassVersionChecker ClassVersionChecker.class
ClassVersionChecker.class: 49 . 0

See Also

The Java Virtual Machine Specification, Real's Java How To

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