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

On Linux, you can use the file command on a class file, which I found easiest for my use case. For example:

$ file Foo.class
Foo.class: compiled Java class data, version 50.0 (Java 1.6)

basszero's approach can be done via the UNIX command line, and the "od(1)" command:

% od -x HelloWorldJava.class |head -2
0000000 feca beba 0000 3100 dc00 0007 0102 2a00
0000020 6f63 2f6d 6e65 6564 6163 642f 6d65 2f6f

"feca beba" is the magic number. The "0000 3100" is 0x31, which represents J2SE 5.0.

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           5
50       0           6
51       0           7
52       0           8
53       0           9
54       0           10
55       0           11
56       0           12
57       0           13
58       0           14