Dienstag, 16. Februar 2010

Java File Format Version

Once you're moving around for a while in Java based environments you might have seen an error like this

java.lang.UnsupportedClassVersionError: Bad version number in .class file


If you hit this kind of error than its time to realize that Java byte code might differ between JDK versions :-) You'll find a long list of explanations about the Java Class file format all over in the web (http://en.wikipedia.org/wiki/Class_%28file_format%29).
Here is a list of class file versions and the corresponding JDK

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).

Ok thanks for the list but how do I get the version out of an existing class file?
Answer:

 javap -v MyClass | more

this will print out something like this:

 class MyClass extends java.lang.Object
  minor version: 0
  major version: 50
....

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