Known Limitations

Known compiler bugs and sharp edges in what Juno compiles today, with workarounds.

The Feature Inventory lists what Juno supports. This page lists where reality is narrower than that inventory or where the generated code behaves differently from the JVM, so you can design around it. Anything outside the supported subset fails at link time with a diagnostic; a known bug would be the exception, because it compiles without one.

Known bugs

None open. Bugs the QEMU suite has found are fixed and kept as regression programs; open ones are listed in QemuRunIT.KNOWN_GAPS. Fixed so far: long and double parameters, results and fields lost their high word on the Thumb-2 path (for example factorial(20) printed 2192834560, the low 32 bits, with no diagnostic), and long[], double[] and float[] arrays were rejected by the backend although the inventory listed them.

Compile-time limits that are easy to hit

These fail with a CompileException; they are listed because they come up when writing ordinary Java.

  • Compound assignment on an array element (values[i] += x, values[i]++). javac compiles it with dup2, which is outside the accepted opcode subset; write values[i] = values[i] + x.
  • length of an array parameter. arraylength works only on a local array created once in the same method with a constant size. A method that receives an array must also receive its length, and an enhanced for over a parameter (which javac compiles to arraylength) is rejected. Use an indexed loop with a count.
  • Default and inherited interface methods. A reachable implementation must declare each interface method it is called through directly.
  • JDK functional interfaces such as IntBinaryOperator in signatures. Declare your own interface.
  • Object in signatures, which rules out equals(Object) on records and classes when it is reachable.
  • Concatenating arbitrary objects. Juno lowers StringConcatFactory for strings and primitive values, but not the general String.valueOf(Object)/toString() path.

Exceptions

See Feature Inventory for the supported subset. Beyond it:

  • An integer division by zero raises a catchable ArithmeticException, from a callee too, but only when some catch or finally in the program can receive it; otherwise it still panics (and costs no guard).
  • addSuppressed is accepted and dropped, so a close() failure while another exception unwinds is lost.
  • Exception causes and stack traces are not modelled.

How these are found

./mvnw -pl juno-compiler -am -Pqemu verify builds each program in juno-compiler/src/test/qemu/programs with the real generated assembly, runs it in QEMU (Cortex-M4) and requires output identical to the same source run on OpenJDK. Known bugs are listed in QemuRunIT.KNOWN_GAPS and skipped, so the suite stays green while the bug stays visible. The harness models no hardware, so it covers language and runtime behavior only.