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README.md
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bytecode.py
bytecode.py
 
 
common.py
common.py
 
 
constpool.py
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jxe.py
jxe.py
 
 
jxe2jar.py
jxe2jar.py
 
 
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README.md

src

Python implementation of JXE → JAR conversion for IBM J9/CDC (ROM class) images.

Overview

This tool:

  • Reads J9 rom.classes from a .jxe (ZIP) and reconstructs standard .class files
  • Rewrites J9/JIT-specific bytecodes into standard JVM bytecode
  • Rebuilds constant pools, fields, methods, and Code attributes

Entry Point

  • jxe2jar.py – CLI to convert a .jxe into a .jar

Usage

python3 src/jxe2jar.py input.jxe output.jar python3 src/jxe2jar.py input.jxe output.jar --skip-libs libs/ python3 src/jxe2jar.py input.jxe output.jar --skip-jdk /path/to/rt.jar python3 src/jxe2jar.py input.jxe output.jar --strip-synthetic

Flags

Flag Description
(default) JDK/JRE classes are skipped using src/rt.classes
--skip-jdk PATH Override the JDK/JRE skip list (e.g., rt.jar)
--skip-classes PATH Add classes from a JAR/JMOD/list file (or a directory of JARs)
--skip-libs DIR Skip classes present in JARs under DIR
--strip-synthetic Clear ACC_SYNTHETIC on classes/methods/fields (for strict javap)

Key Logic and Format Knowledge

J9 ROM Layout

Parsing implemented in jxe.py.

Modified UTF-8

Decoder in common.py, with validation and SRP fallbacks.

Constant Pool Reconstruction

Implemented in constpool.py:

  • Supports INTEGER/FLOAT/LONG/DOUBLE/STRING/CLASS/REF
  • Encodes UTF-8 with surrogatepass for emoji/surrogate sequences
  • Preserves method descriptors for invokeinterface argument counts

Field ConstantValue Recovery

Implemented in jxe2jar.py:

  • Supports static final primitives/String values via ROM field metadata
  • Handles empty string heuristics when ROM omits explicit empty string constant

Bytecode Transforms

Implemented in bytecode.py:

  • J9 wide opcodes → standard wide form
  • invokeinterface count byte computed from method descriptor
  • ldc upgraded to ldc_w if CP index > 255
  • Switch padding recalculated based on output position
  • J9 return opcodes mapped to standard returns

Output Classfile Notes

  • Classfile version is inferred from flags/opcodes with a minimum of 46 (no upper cap)
  • ACC_SUPER is enforced for non-interfaces
  • ACC_SYNTHETIC is preserved unless --strip-synthetic is used

Known Limitations

  • ROM metadata outside standard classfile fields is not preserved
  • Some J9-specific optimizations may be dropped during bytecode rewrite
  • For strict javap output on old classfile versions, --strip-synthetic may be required

Validation

  • Comprehensive edge-case suite in test/custom_edgecases/
  • Large LSD JXE (test/jxes/MU1316-lsd.jxe) converts fully with current code