jxe2jar

Tools and notes for converting IBM J9/CDC JXE (rom.classes) images back to standard Java .class files / .jar archives.

Repository Structure

Directory	Description
src/	Python implementation of JXE → JAR conversion
test/custom_edgecases/	Exhaustive edge-case suite (Java 1.2) to validate the converter
out/	Conversion outputs and logs
vms/	Virtualized environments for legacy tooling (XP VM → WM5 emulator → jar2jxe)

The converter is validated through edge-case tests and a JAR → JXE → JAR round-trip pipeline.

Conversion Logic

Classfile Version

• Before: Wrote ROM class version as-is (can be non-standard).
• Now:
  • Infers minimal version from flags/opcodes (minimum 46, no upper cap).
  • Bumps to 49 when synthetic/enum/annotation flags are present so javac/javap accept output.

Field Parsing and Constants

• Before: Ignored ROM field constant values.
• Now:
  • J9ROMField captures constant slots (const_value, const_value2, const_value3).
  • jxe2jar.py emits ConstantValue for static final fields (int/float/long/double/String).
  • Recovers constants from ROM metadata even when the constant pool slot is placeholder/omitted.

Constant Pool Handling

• Before: Limited constant types and minimal UTF-8 handling.
• Now:
  • Supports INTEGER/FLOAT/LONG/DOUBLE/STRING/REF and preserves descriptors.
  • Encodes UTF-8 with surrogatepass to keep odd ROM strings stable.
  • Decodes J9 ROM "LONG" slots into proper field/method refs using ROM base offsets.

Bytecode Translation

• Before: Basic mapping, missing J9 wide opcodes and invokeinterface handling.
• Now:
  • Wide opcodes are normalized to JVM wide.
  • invokeinterface count is computed from the descriptor.
  • ldc is promoted to ldc_w when CP index > 255.
  • J9 prefix opcodes are expanded correctly (e.g., implicit aload_0 prefix).
  • Branch and switch offsets are rewritten using output offset maps to avoid javap errors.
  • Switch padding uses output offset so alignment is correct.
  • Invalid/missing CP refs are handled defensively instead of crashing.

Code Attribute Layout

• Before: Conditional 45.3 layout (u8/u8/u16 for stack/locals/code_len).
• Now:
  • Always JVM-standard u16/u16/u32 layout.
  • Exception table offsets are rewritten after bytecode expansion.

Flag Cleanup

• Before: Synthetic flags passed through blindly.
• Now: --strip-synthetic option clears ACC_SYNTHETIC on classes/methods/fields for strict tooling.

CLI Behavior

• Before: Minimal python JXE2JAR.py input.jxe output.jar.
• Now: argparse with options: --skip-jdk, --skip-classes, --skip-libs, --strip-synthetic.

Testing Workflow

1. Build edge-case JAR:

   sh test/custom_edgecases/build.sh

2. Convert JAR → JXE with jar2jxe.exe (see vms/xp/README.md).
3. Convert JXE → JAR with Python:

   python3 src/jxe2jar.py input.jxe output.jar

Usage Notes

• JDK/JRE classes are skipped by default using src/rt.classes.
• Use --skip-jdk /path/to/rt.jar to override the JDK/JRE skip list.
• Use --skip-classes to provide additional JAR/JMOD/list files or a directory.
• The converter preserves ACC_SYNTHETIC by default. Use --strip-synthetic if you need strict javap output for 45.0 classes.
• Classfile versions are inferred from flags/opcodes with a minimum of 46 (no upper cap).
• Some large binaries/ISOs are referenced via .url files pointing to original archives:
  • vms/xp/en_vs_2005_pro_dvd.iso.url
  • vms/xp/en_windows_xp_professional_with_service_pack_3_x86_cd_vl_x14-73974.iso.url

See Also

• src/README.md – Format knowledge and implementation details
• test/custom_edgecases/README.md – Test coverage list
• vms/xp/README.md – WM5 emulator + XP VM instructions

Credits

Thanks to the original repo, forks, and contributors:

• https://github.com/moradek/jxe2jar
• https://github.com/andrewleech/jxe2jar