Switching between bytecode versions for a Java class file

Question

Given a Java class file (ClassName.class) with bytecode version X is there a general way to convert this class file from being represented in bytecode version X to being represented in bytecode version Y?

Assumptions:

• The source code is not available. The class file is the only available representation of the class.
• The class file is heavily obfuscated, so decompiling the class with say jad or similar program and then recompiling it with "-target ..." does not work.

Updates after initial post:

• Update #1: Futhermore, assume that bytecode version X and bytecode version Y are sufficiently close so that all instructions used by the class (currently in bytecode version X) also exists in version Y.

Answer 1

You could use Apache BCEL

The Byte Code Engineering Library is intended to give users a convenient possibility to analyze, create, and manipulate (binary) Java class files

BCEL gives you the possibility of reading in a class file of a given version, manipulating it, generating a new class file stream, and then loading that into the VM using the low-level ClassLoader API. Very fiddly, no doubt, and I doubt this will let you downgrade the version as easily as you could prograde.

Answer 2

No. While later versions of Java will be able to execute that bytecode, you can't upgrade it: Later versions of the class files have a different format.

You can't downgrade it either because there is no way to replace the missing bytecodes by other constructs in older versions of Java.

[EDIT] IIRC, Sun added at least a single new bytecode instruction for every major version of Java. This is usually the reason for major Java releases: Changes to the bytecode.

That said, just try your luck and change the major version of the class file and see if your newer VM will load it. I doubt it will work for any complex example but you might be lucky.

Answer 3

For downgrading you can have a look at various methods to get Java 5/6 code running in Java 1.3/1.4. See my anwser to related question http://stackoverflow.com/questions/1011706/backport-java-5-6-features-to-java-1-4/1098923#1098923

Answer 4

Use retroweaver