Replace runtime panics with the matching Java exceptions

[dlunch]

Summary

An audit of every unwrap() in the runtime found panics reachable from ordinary Java code — starting from File.length() crashing on a file that simply doesn't exist. Each is replaced with the exception (or return value) the JDK produces; expected outputs for all new fixtures are generated by running them on a real JVM.

Fixes

File / streams

• File.length() returns 0 for a missing file (spec) instead of panicking; isDirectory/isFile drop their guard-then-unwrap shape.
• FileImpl in the native runtime panicked on any failing open, so the existing FileNotFoundException guards in FileInputStream/RandomAccessFile were unreachable. It now maps open/read/write/seek failures to IOError (new Io variant), FileOutputStream gains the same guard, and all file I/O operations (read, write, seek, available, length, setLength, getFilePointer) throw java.io.IOException on failure via a shared helper.

Class.forName

• Panicked for any class not already loaded (it only consulted the registry). It now resolves (loads) the class and throws ClassNotFoundException (new runtime class, extends Exception per the 1.2 hierarchy) when resolution fails.

Unpaired surrogates

• StringBuffer.append(char) / append(char[],int,int) panicked on an unpaired surrogate. The internal append is now UTF-16-based, so building a surrogate pair char by char produces the correct string; String.valueOf(char) constructs through [C for the same reason; PrintStream.println(char) replaces an unpaired surrogate with ? matching the JDK charset encoder.

Zip

• new ZipFile(...) on a non-zip file panicked in ZipArchive::new. The constructor now validates the archive (matching the JDK's timing) and throws java.util.zip.ZipException (new runtime class, extends IOException); getInputStream returns null for an entry not in the archive.

Not converted (audited, intentionally left)

• Interpreter stack pops, thread-attach lookups, guarded unwraps — invariants.
• Uncaught-exception stack-trace printing in Thread — error path of the error path, inside a spawn closure.
• GregorianCalendar on out-of-range fields — the correct behavior is lenient normalization (a feature), not an exception.
• ClassInfo::parse on a malformed classfile — needs ClassFormatError plumbing through jvm-less sync code (existing TODO).

Test plan

E2E fixtures with real-JVM-generated expected output, each verified to fail before its fix: FileLength (missing-file length/exists), FileErrors (FileNotFoundException from FileInputStream/FileOutputStream/RandomAccessFile), ForName (loads java.util.Vector, ClassNotFoundException for an unknown name), SurrogateChars (pair building, lone-surrogate length/println), ZipCorrupt (ZipException from the constructor). cargo test --workspace green, fmt clean, clippy no new warnings.

[codecov]

Codecov Report

❌ Patch coverage is 66.94215% with 40 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.89%. Comparing base (62cf0c6) to head (760e734).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
..._runtime/src/classes/java/io/random_access_file.rs	5.88%	16 Missing ⚠️
..._runtime/src/classes/java/io/file_output_stream.rs	50.00%	6 Missing ⚠️
...src/classes/java/lang/class_not_found_exception.rs	80.00%	5 Missing ⚠️
...runtime/src/classes/java/util/zip/zip_exception.rs	80.00%	5 Missing ⚠️
...a_runtime/src/classes/java/io/file_input_stream.rs	50.00%	4 Missing ⚠️
java_runtime/src/classes/java/io/file.rs	33.33%	2 Missing ⚠️
java_runtime/src/classes/java/util/zip/zip_file.rs	83.33%	2 Missing ⚠️

Additional details and impacted files

@@ Coverage Diff @@## main #174 +/- ##
==========================================
+ Coverage 83.71% 83.89% +0.18% 
==========================================
Files 171 173 +2 Lines 12867 12939 +72 ==========================================
+ Hits 10771 10855 +84 + Misses 2096 2084 -12 

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[Copilot]

Pull request overview

This PR hardens the RustJava runtime by removing panic paths reachable from ordinary Java code and mapping failures to the Java exceptions/behaviors that the JDK produces, with new E2E fixtures whose expected outputs were generated from a reference JVM.

Changes:

• Replace unwrap()-based runtime panics in file I/O, class loading, surrogate handling, and zip handling with JDK-aligned exceptions/return values.
• Add runtime classes java.lang.ClassNotFoundException and java.util.zip.ZipException and wire them into the runtime loader.
• Add new Java fixtures and golden outputs covering missing files, I/O errors, Class.forName, surrogate chars, and corrupt zip behavior.

Reviewed changes

Copilot reviewed 28 out of 33 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
test_data/ZipCorrupt.txt	Golden output for corrupt/non-zip ZipFile constructor behavior.
test_data/SurrogateChars.txt	Golden output for surrogate-pair and lone-surrogate behaviors.
test_data/src/ZipCorrupt.java	Fixture exercising ZipFile constructor throwing ZipException.
test_data/src/SurrogateChars.java	Fixture exercising surrogate pair construction and lone surrogate printing.
test_data/src/ForName.java	Fixture exercising Class.forName success + ClassNotFoundException.
test_data/src/FileLength.java	Fixture for File.exists() + File.length() on missing file.
test_data/src/FileErrors.java	Fixture for FileNotFoundException from common file I/O constructors.
test_data/ForName.txt	Golden output for Class.forName fixture.
test_data/FileLength.txt	Golden output for missing-file File.length() returning 0.
test_data/FileErrors.txt	Golden output for missing-file/dir constructor failures.
src/runtime/io.rs	Replace several I/O unwrap()s with IOError propagation in the native runtime file impls.
src/runtime.rs	Change runtime open() to return an IOError instead of panicking on open failure.
java_runtime/src/runtime/io.rs	Add IOError::Io variant for non-NotFound I/O failures.
java_runtime/src/loader.rs	Register new runtime classes (ClassNotFoundException, ZipException).
java_runtime/src/classes/java/util/zip/zip_file.rs	Convert zip parsing/entry access panics into ZipException and JDK-like null returns.
java_runtime/src/classes/java/util/zip/zip_exception.rs	Add java.util.zip.ZipException runtime class (extends IOException).
java_runtime/src/classes/java/util/zip.rs	Export ZipException module/type from java.util.zip.
java_runtime/src/classes/java/lang/string.rs	Preserve unpaired surrogates in String.valueOf(char) by constructing via [C.
java_runtime/src/classes/java/lang/string_buffer.rs	Switch internal appends to UTF-16-based append to preserve unpaired surrogates.
java_runtime/src/classes/java/lang/class.rs	Make Class.forName resolve/load and throw ClassNotFoundException instead of panicking.
java_runtime/src/classes/java/lang/class_not_found_exception.rs	Add java.lang.ClassNotFoundException runtime class (extends Exception).
java_runtime/src/classes/java/lang.rs	Export ClassNotFoundException from java.lang module.
java_runtime/src/classes/java/io/random_access_file.rs	Route file op failures through shared checked() helper instead of unwrap().
java_runtime/src/classes/java/io/print_stream.rs	Avoid panicking on surrogate char printing; replace with ?.
java_runtime/src/classes/java/io/input_stream_reader.rs	Propagate array-buffer read failures instead of unwrapping.
java_runtime/src/classes/java/io/file.rs	Make File.isDirectory/isFile/length avoid unwraps and match JDK missing-file behavior.
java_runtime/src/classes/java/io/file_output_stream.rs	Add open guard for FileOutputStream and propagate write failures as IOException.
java_runtime/src/classes/java/io/file_input_stream.rs	Propagate read/available failures as IOException via checked().
java_runtime/src/classes/java/io.rs	Add shared checked() helper to convert runtime I/O failures into java.io.IOException.

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Preserve linkage errors from Class.forName

When the requested class file is present but resolving one of its superclasses or interfaces fails, resolve_class propagates that Java error (for example NoClassDefFoundError from jvm/src/jvm.rs when a dependency is missing). This blanket Err(_) converts those linkage failures into ClassNotFoundException, so Java code can incorrectly catch them as CNFE instead of seeing the linkage error that Class.forName should propagate. Only the actual “requested class not found” case should be remapped here; other resolve_class errors should be returned unchanged.

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