Java API to find out the JDK version a class file is compiled for?

Question

Are there any Java APIs to find out the JDK version a class file is compiled for? Of course there is the javap tool to find out the major version as mentioned in here. However I want to do it programmatically so that that I could warn the user to compile it for the appropriate JDK

Answer 1

JavaP does have an API, but it's specific to the Sun JDK.

It's found in tools.jar, under sun/tools/javap/Main.class.

Answer 2

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 possible values are :

major  minor Java platform version 
45       3           1.0
45       3           1.1
46       0           1.2
47       0           1.3
48       0           1.4
49       0           1.5
50       0           1.6

Answer 3

This gets you the contents of the class file:

MysteryClass.class.getResourceAsStream("MysteryClass.class")

Then look at bytes 5-8 to get the minor and major version. A mapping between those numbers and the JDK releases can be found here.

Answer 4

Just read the class file directly. It's VERY easy to figure out the version. Check out the the spec and wiki and then try the code. Wrapping this code and making is more useful/pretty is left as an exercise. Alternatively, you could use a library like BCEL, ASM, or JavaAssist

import java.io.DataInputStream;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.IOException;

public class ClassVersionTest
{
    private static final int JAVA_CLASS_MAGIC = 0xCAFEBABE;

    public static void main(String[] args)
    {
     try
     {
      DataInputStream dis = new DataInputStream(new FileInputStream("Test.class"));
      int magic = dis.readInt();
      if(magic == JAVA_CLASS_MAGIC)
      {
       int minorVersion = dis.readUnsignedShort();
       int majorVersion = dis.readUnsignedShort();

       /**
        * majorVersion is ...
        * J2SE 6.0 = 50 (0x32 hex),
        * J2SE 5.0 = 49 (0x31 hex),
        * JDK 1.4 = 48 (0x30 hex),
        * JDK 1.3 = 47 (0x2F hex),
        * JDK 1.2 = 46 (0x2E hex),
        * JDK 1.1 = 45 (0x2D hex).
        */

       System.out.println("ClassVersionTest.main() " + majorVersion + "." + minorVersion);
      }
      else
      {
       // not a class file
      }
     }
     catch (FileNotFoundException e)
     {
      // TODO Auto-generated catch block
      e.printStackTrace();
     }
     catch (IOException e)
     {
      // TODO Auto-generated catch block
      e.printStackTrace();
     }
    }

}

Answer 5

As others have shown, it is easy enough to do by reading the first eight bytes of a class file. If you want a pre-built binary library, you can download one here.

Answer 6

Apache BCEL provides this API:

JavaClass c = Repository.lookupClass("com.x.MyClass")
c.getMinor();
c.getMajor();