Harden JVM runtime correctness

[dlunch]

Summary

• implement reentrant object monitors, wait/notify, synchronized bytecode/methods, and reliable Thread.join cleanup
• serialize concurrent class initialization with owner/waiter state and same-thread recursion
• harden classfile parsing and map malformed, unsupported, and native linkage failures to Java errors
• complete runtime time/exit behavior, shallow clone, UTF-16 String contracts, and ClassLoader error handling

Why

Several Java 1.2 and CLDC execution paths could panic in Rust, lose monitor or class-initialization wakeups, or violate Java API contracts. This keeps failures in Java exception flow and makes concurrent behavior deterministic.

Validation

• cargo test --workspace --no-fail-fast
• cargo clippy --workspace --all-targets -- -D warnings
• cargo fmt --all -- --check
• monitor, class initialization, and compiled-class E2E tests repeated 5 times

[codecov]

Codecov Report

❌ Patch coverage is 90.25097% with 101 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.04%. Comparing base (af4f6f8) to head (d867095).

Files with missing lines	Patch %	Lines
classfile/src/opcode.rs	74.15%	23 Missing ⚠️
java_runtime/src/classes/java/lang/thread.rs	23.52%	13 Missing ⚠️
jvm/src/jvm.rs	87.95%	10 Missing ⚠️
classfile/src/validation.rs	94.73%	9 Missing ⚠️
java_runtime/src/classes/java/lang/string.rs	91.35%	7 Missing ⚠️
classfile/src/constant_pool.rs	89.28%	6 Missing ⚠️
...untime/src/classes/java/lang/class_format_error.rs	80.00%	5 Missing ⚠️
...me/src/classes/java/lang/unsatisfied_link_error.rs	80.00%	5 Missing ⚠️
...asses/java/lang/unsupported_class_version_error.rs	80.00%	5 Missing ⚠️
src/runtime.rs	50.00%	4 Missing ⚠️
... and 8 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #180      +/-   ##
==========================================
+ Coverage   86.60%   87.04%   +0.44%     
==========================================
  Files         191      199       +8     
  Lines       17055    17858     +803     
==========================================
+ Hits        14770    15544     +774     
- Misses       2285     2314      +29     

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[Copilot]

Pull request overview

This PR hardens JVM/runtime correctness by implementing proper object monitor semantics (reentrant enter/exit, wait/notify, and synchronized methods), serializing concurrent class initialization, and converting several previously-panicking paths (classfile parsing / native linkage) into Java-level exceptions. It also completes a few core Java API contracts around System, Thread, Object.clone, and UTF-16–based String operations.

Changes:

• Add reentrant monitors with wait/notify support and wire them into bytecode monitorenter/monitorexit, Object.wait/notify, and synchronized methods.
• Serialize class initialization with an owner/waiter model (including same-thread recursion) and broaden regression tests for initialization and monitor semantics.
• Harden classfile parsing and runtime linkage error handling (structured ClassFileError, translate parser errors to ClassFormatError / UnsupportedClassVersionError, and native calls to UnsatisfiedLinkError).

Reviewed changes

Copilot reviewed 48 out of 54 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
test_utils/src/lib.rs	Implements runtime yield/exit, logs spawned task failures, and maps classfile parse errors to Java exceptions for tests.
test_utils/Cargo.toml	Adds tracing dependency needed for new error logging.
test_data/src/NativeMethod.java	New E2E fixture for missing native method linkage.
test_data/src/MonitorSemantics.java	New E2E fixture validating synchronized + join + exception paths.
test_data/NativeMethod.txt	Expected output for NativeMethod fixture.
test_data/MonitorSemantics.txt	Expected output for MonitorSemantics fixture.
src/runtime.rs	Implements sleep/yield/exit/now and maps classfile parse errors to Java exceptions in the main runtime.
jvm/src/type.rs	Adds try_parse and tightens descriptor validation rules.
jvm/src/monitor.rs	Introduces a reentrant monitor with waiters + timeout-safe notifications.
jvm/src/lib.rs	Registers the new monitor module and exports wait/timeout types.
jvm/src/jvm.rs	Integrates monitors into JVM operations, adds shallow clone support, and serializes class initialization.
jvm/src/class_loader.rs	Implements class initialization owner/waiter state with completion notification.
jvm/src/class_instance.rs	Extends the core instance trait with identity() and shallow_clone().
jvm/src/array_class_instance.rs	Plumbs identity() and shallow_clone() through array instances.
jvm_rust/src/method.rs	Maps missing bodies to UnsatisfiedLinkError / AbstractMethodError instead of panicking.
jvm_rust/src/lib.rs	Re-exports ClassFileError for downstream use.
jvm_rust/src/interpreter.rs	Implements monitorenter/monitorexit and NPE behavior for null monitors.
jvm_rust/src/class_instance.rs	Implements identity() and shallow cloning for object instances.
jvm_rust/src/class_definition.rs	Reworks classfile loading to return structured errors and validates more classfile invariants.
jvm_rust/src/array_class_instance.rs	Adds shallow cloning and identity for array instances.
java_runtime/tests/classes/java/lang/test_system.rs	Adds tests for System.currentTimeMillis, Thread.yield, and System.exit runtime contract.
java_runtime/tests/classes/java/lang/test_string.rs	Adds UTF-16 indexing, charset error, and trim contract tests.
java_runtime/tests/classes/java/lang/test_object.rs	Updates wait/notify tests for monitor-ownership requirements and adds clone semantics tests.
java_runtime/tests/classes/java/lang/test_class.rs	Adds tests for ClassLoader.findClass and defineClass error translation and bounds validation.
java_runtime/tests/classes/java/lang/test_class_initialization.rs	New concurrency tests for class initialization owner/waiter behavior and failure propagation.
java_runtime/tests/classes/java/lang/mod.rs	Wires new test modules into the test suite.
java_runtime/src/runtime.rs	Extends the runtime trait with exit(status).
java_runtime/src/loader.rs	Registers new runtime exception classes for linkage/format/version errors.
java_runtime/src/classes/java/lang/unsupported_class_version_error.rs	Adds UnsupportedClassVersionError runtime class implementation.
java_runtime/src/classes/java/lang/unsatisfied_link_error.rs	Adds UnsatisfiedLinkError runtime class implementation.
java_runtime/src/classes/java/lang/thread.rs	Makes start/join synchronized and improves join/cleanup behavior.
java_runtime/src/classes/java/lang/system.rs	Implements System.exit via the runtime exit hook.
java_runtime/src/classes/java/lang/string.rs	Aligns multiple string operations with UTF-16 indexing and adds proper charset error handling.
java_runtime/src/classes/java/lang/object.rs	Implements shallow clone and correct monitor-based wait/notify behavior with timeout safety.
java_runtime/src/classes/java/lang/class_loader.rs	Implements findClass throwing and validates defineClass byte ranges and nulls.
java_runtime/src/classes/java/lang/class_format_error.rs	Adds ClassFormatError runtime class implementation.
java_runtime/src/classes/java/lang.rs	Exports and wires the new java.lang error classes.
classfile/tests/test.rs	Adds structured error tests for malformed/unsupported class files.
classfile/src/opcode.rs	Rejects invalid opcodes/refs and tightens parsing to return errors instead of panicking.
classfile/src/method.rs	Makes parsing resilient to invalid access flags / constant pool lookups.
classfile/src/lib.rs	Exposes the new ClassFileError type.
classfile/src/interface.rs	Makes interface parsing resilient to invalid constant pool indices/types.
classfile/src/field.rs	Makes field parsing resilient to invalid access flags / constant pool lookups.
classfile/src/error.rs	Introduces ClassFileError (invalid format / unsupported version).
classfile/src/constant_pool.rs	Removes panics, validates counts/slot rules, and returns Option for typed constant pool lookups.
classfile/src/class.rs	Converts ClassInfo::parse to structured errors and enforces version bounds.
classfile/src/attribute.rs	Makes attribute parsing fail-fast on malformed constant pool references/opcodes.
Cargo.toml	Adds workspace tracing and enables Tokio time feature where needed.
Cargo.lock	Locks in tracing dependency additions.

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