How to determine the Java byte code version of the current class programatically?

Question

I have a situation where the deployment platform is Java 5 and the development happens with Eclipse under Java 6 where we have established a procedure of having a new workspace created when beginning work on a given project. One of the required steps is therefore setting the compiler level to Java 5, which is frequently forgotten.

We have a test machine running the deployment platform where we can run the code we build and do initial testing on our PC's, but if we forget to switch the compiler level the program cannot run. We have a build server for creating what goes to the customer, which works well, but this is for development where the build server is not needed and would add unnecessary waits.

The question is: CAN I programmatically determine the byte code version of the current class, so my code can print out a warning already while testing on my local PC?

EDIT: Please note the requirement was for the current class. Is this available through the classloadeer? Or must I locate the class file for the current class, and then investigate that?

Note that we have since migrated to maven, and the m2e plugin for Eclipse sets this according to the compilation settings in the pom. It is still an interesting academic problem, though. — Thorbjørn Ravn Andersen, Mar 01 '13 at 07:54

score 16 · Answer 1 · edited Nov 02 '11 at 06:14

16

Easy way to find this to run javap on class

For more details goto http://download.oracle.com/javase/1,5.0/docs/tooldocs/windows/javap.html

Example:

M:\Projects\Project-1\ant\javap -classpath M:\Projects\Project-1\build\WEB-INF\classes -verbose com.company.action.BaseAction

and look for following lines

minor version: 0
major version: 50

edited Nov 02 '11 at 06:14

Sathyajith Bhat

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answered Nov 01 '11 at 18:11

mm101

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I ended up doing this for a "what version do those jar files require"-report (without running the java program). – Thorbjørn Ravn Andersen Mar 01 '13 at 07:55

McDowell · Answer 2 · 2009-11-10T11:55:20.153

7

You could load the class file as a resource and parse the first eight bytes.

//TODO: error handling, stream closing, etc.
InputStream in = getClass().getClassLoader().getResourceAsStream(
    getClass().getName().replace('.', '/') + ".class");
DataInputStream data = new DataInputStream(in);
int magic = data.readInt();
if (magic != 0xCAFEBABE) {
  throw new IOException("Invalid Java class");
}
int minor = 0xFFFF & data.readShort();
int major = 0xFFFF & data.readShort();
System.out.println(major + "." + minor);

edited Nov 10 '09 at 11:55

answered Nov 10 '09 at 11:11

McDowell

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+1 for code getting the byte code as a resource, not a file. – Thorbjørn Ravn Andersen Nov 19 '09 at 12:27

score 3 · Answer 3 · edited Jun 20 '20 at 09:12

3

here is the Java Class File Format descriptor: Java Class File Format

and here the major version values:

public static final int JDK14_MAJOR_VERSION = 48;

public static final int JDK15_MAJOR_VERSION = 49;

public static final int JDK16_MAJOR_VERSION = 50;

Now, read the class file with Java code and check the major version to know which JVM generated it

edited Jun 20 '20 at 09:12

Community

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answered Nov 10 '09 at 11:12

dweeves

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score 3 · Accepted Answer · edited May 23 '17 at 10:30

3

Take a look at question: Java API to find out the JDK version a class file is compiled for?

edited May 23 '17 at 10:30

Community

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answered Nov 10 '09 at 11:12

Shimi Bandiel

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score 2 · Answer 5 · answered Nov 10 '09 at 11:10

2

Bytes 5 through 8 of a class file content is the version number in hex. You can use Java code (or any other language) to parse the version number.

answered Nov 10 '09 at 11:10

stepanian

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How to determine the Java byte code version of the current class programatically?

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