Question

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Tool to read and display Java .class versions

If I have a compiled Java class, is there a way to tell from just the class file what its target version compatibility is? Specifically, I have a number of class files, compiled to Java 6, which are running under Java 5 and giving the the "Unrecognized version" error. I want to be able to look at a class file and find what its target version compatibility is without running the JVM. Any ideas?

Answer 1

I’ve found this on the net and it works.

Every ‘.class’ file starts off with the following:

• Magic Number [4 bytes]
• Version Information [4 bytes]

A hexdump of a ‘.class’ file compiled with each of the following options reveals:

javac -target 1.1 ==> CA FE BA BE 00 03 00 2D
javac -target 1.2 ==> CA FE BA BE 00 00 00 2E
javac -target 1.3 ==> CA FE BA BE 00 00 00 2F
javac -target 1.4 ==> CA FE BA BE 00 00 00 30

Perhaps you could use this information to write your own ‘.class’ file version checking utility, using Java, or perhaps a scripting or shell language 😉 !

I hope this helps.

Anthony Borla

From: http://bytes.com/groups/java/16603-how-determine-java-bytecode-version