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

The answers that involve reading bytes from a file seem quite uncivilized to me. There ought to be a way to get this information using Reflection, such as Foobar.class.getTargetVersion(). — John Henckel
– John Henckel, Commented Oct 1, 2014 at 17:53

RealHowTo · Accepted Answer · 2025-08-28 15:14:59Z

61

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
59       0           15
60       0           16
61       0           17
62       0           18
63       0           19
64       0           20
65       0           21
66       0           22
67       0           23
68       0           24

ref : The Java Virtual Machine Specification, Java SE 24 Edition

edited Aug 28 at 15:14

answered Aug 18, 2009 at 11:48

RealHowTo

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2 Comments

Christoph S. Over a year ago

Would it be possible to update this table or link to the official website where someone can find the versions past 14? Thank you

RealHowTo Over a year ago

@ChristophSchreiber, the answer is updated

rickumali · Accepted Answer · 2011-04-06 18:43:18Z

22

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.

answered Apr 6, 2011 at 18:43

rickumali

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Comments

Daniel Wille · Accepted Answer · 2016-08-09 17:31:56Z

19

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)

answered Aug 9, 2016 at 17:31

Daniel Wille

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1 Comment

Mark Edington Over a year ago

This works perfectly and is so much simpler than any of the other answers if you are working in Linux.

skaffman · Accepted Answer · 2009-08-18 12:53:16Z

10

Apache BCEL provides this API:

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

answered Aug 18, 2009 at 12:53

skaffman

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Comments

Bill the Lizard · Accepted Answer · 2012-07-29 01:19:15Z

9

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.

edited Jul 29, 2012 at 1:19

Bill the Lizard

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answered Aug 18, 2009 at 11:49

Michael Borgwardt

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2 Comments

Thorbjørn Ravn Andersen Over a year ago

Would the string argument need to be fully qualified? getResourceAsStream("org/apache/foobar/Whatever.class")?

Michael Borgwardt Over a year ago

No, since you are using the class itself as base for getting the resource. You need the fully qualified name if you you can getResource methods of the ClassLoader instead of a class.

basszero · Accepted Answer · 2009-08-18 12:03:05Z

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();
        }
    }

}

McDowell · Accepted Answer · 2009-08-18 12:45:49Z

2

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.

answered Aug 18, 2009 at 12:45

McDowell

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Comments

Robert Munteanu · Accepted Answer · 2009-08-18 11:43:51Z

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.

answered Aug 18, 2009 at 11:43

Robert Munteanu

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Comments

Beezer · Accepted Answer · 2020-12-26 13:24:33Z

1

I use this script in a batch file on windows, redirect the output and grep for the major number I am verifying for:

for /R %%f in (*.class) do (
"C:\Program Files\Java\JDK8\bin\javap" -v "%%f" | findstr major )

I run it as such:

verify.bat > versionResults.txt

answered Dec 26, 2020 at 13:24

Beezer

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