Tool to read and display Java .class versions

Question

Do any of you know of a tool that will search for .class files and then display their compiled versions?

I know you can look at them individually in a hex editor but I have a lot of class files to look over (something in my giant application is compiling to Java6 for some reason).

Answer 1

Use the javap tool that comes with the JDK. The -verbose option will print the version number of the class file.

> javap -verbose MyClass
Compiled from "MyClass.java"
public class MyClass
  SourceFile: "MyClass.java"
  minor version: 0
  major version: 46
...

To only show the version:

WINDOWS> javap -verbose MyClass | find "version"
LINUX  > javap -verbose MyClass | grep version

Answer 2

It is easy enough to read the class file signature and get these values without a 3rd party API. All you need to do is read the first 8 bytes.

ClassFile {
    u4 magic;
    u2 minor_version;
    u2 major_version;

For class file version 51.0 (Java 7), the opening bytes are:

CA FE BA BE 00 00 00 33

...where 0xCAFEBABE are the magic bytes, 0x0000 is the minor version and 0x0033 is the major version.

import java.io.*;

public class Demo {
  public static void main(String[] args) throws IOException {
    ClassLoader loader = Demo.class.getClassLoader();
    try (InputStream in = loader.getResourceAsStream("Demo.class");
        DataInputStream data = new DataInputStream(in)) {
      if (0xCAFEBABE != data.readInt()) {
        throw new IOException("invalid header");
      }
      int minor = data.readUnsignedShort();
      int major = data.readUnsignedShort();
      System.out.println(major + "." + minor);
    }
  }
}

Walking directories (File) and archives (JarFile) looking for class files is trivial.

Oracle's Joe Darcy's blog lists the class version to JDK version mappings up to Java 7:

Target   Major.minor Hex
1.1      45.3        0x2D
1.2      46.0        0x2E
1.3      47.0        0x2F
1.4      48.0        0x30
5 (1.5)  49.0        0x31
6 (1.6)  50.0        0x32
7 (1.7)  51.0        0x33
8 (1.8)  52.0        0x34
9        53.0        0x35

Answer 3

On Unix-like

file /path/to/Thing.class

Will give the file type and version as well. Here is what the output looks like:

compiled Java class data, version 49.0

Answer 4

If you are on a unix system you could just do a

find /target-folder -name \*.class | xargs file | grep "version 50\.0"

(my version of file says "compiled Java class data, version 50.0" for java6 classes).

Answer 5

Yet another java version check

od -t d -j 7 -N 1 ApplicationContextProvider.class | head -1 | awk '{print "Java", $2 - 44}'

Answer 6

In eclipse if you don't have sources attached. Mind the first line after the attach source button.

// Compiled from CDestinoLog.java (version 1.5 : 49.0, super bit)

Answer 7

Maybe this helps somebody, too. Looks there is more easy way to get JAVA version used to compile/build .class. This way is useful to application/class self check on JAVA version.

I have gone through JDK library and found this useful constant: com.sun.deploy.config.BuiltInProperties.CURRENT_VERSION. I do not know since when it is in JAVA JDK.

Trying this piece of code for several version constants I get result below:

src:

System.out.println("JAVA DEV       ver.: " + com.sun.deploy.config.BuiltInProperties.CURRENT_VERSION);
System.out.println("JAVA RUN     v. X.Y: " + System.getProperty("java.specification.version") );
System.out.println("JAVA RUN v. W.X.Y.Z: " + com.sun.deploy.config.Config.getJavaVersion() ); //_javaVersionProperty
System.out.println("JAVA RUN  full ver.: " + System.getProperty("java.runtime.version")  + " (may return unknown)" );
System.out.println("JAVA RUN       type: " + com.sun.deploy.config.Config.getJavaRuntimeNameProperty() );

output:

JAVA DEV       ver.: 1.8.0_77
JAVA RUN     v. X.Y: 1.8
JAVA RUN v. W.X.Y.Z: 1.8.0_91
JAVA RUN  full ver.: 1.8.0_91-b14 (may return unknown)
JAVA RUN       type: Java(TM) SE Runtime Environment

In class bytecode there is really stored constant - see red marked part of Main.call - constant stored in .class bytecode

Constant is in class used for checking if JAVA version is out of date (see How Java checks that is out of date)...