Phase 05: Wrapper synthesiser

Status: Planned

Deliverables

1. package3/kotlin/wrapper/synth.go — Synthesize(surface *APIObject, outputDir string) error — emit Java source files for the JNI wrapper.
2. package3/kotlin/wrapper/jni_types.go — Kotlin→JNI type conversion code generators: jstring (UTF-16↔UTF-8), jobjectArray (list copy), jlongArray (long[] copy), jobject (data class accessor).
3. package3/kotlin/wrapper/handle_registry.go — generate the MochiHandleRegistry.java class (for coroutines event-loop mode, Phase 13 prerequisite).
4. package3/kotlin/wrapper/compile.go — CompileJava(srcDir, classpath, outputDir string) error — invoke javac to compile the generated Java source to .class files, then jar to pack into a JAR.
5. package3/kotlin/wrapper/reflect_config.go — generate reflect-config.json and resource-config.json for GraalVM from the type-mapped surface.

Generated wrapper pattern

For each public Kotlin function fun Foo.bar(x: Int, y: String): List<Long>, the synthesiser emits:

// com/example/foo/MochiBridge.java (generated)
package com.example.foo;
import org.graalvm.nativeimage.IsolateThread;
import org.graalvm.nativeimage.c.function.CEntryPoint;
import com.oracle.svm.core.c.CTypedef;

public class MochiBridge {
    @CEntryPoint(name = "com_example_foo_bar")
    public static long[] com_example_foo_bar(
            IsolateThread thread,
            long fooHandle,
            int x,
            @CTypedef(name = "jstring") long y_ptr,
            int y_len
    ) {
        Foo foo = (Foo) MochiHandleRegistry.get(fooHandle);
        String y = new String(MochiJNI.utf8Bytes(y_ptr, y_len), java.nio.charset.StandardCharsets.UTF_8);
        java.util.List<Long> result = foo.bar(x, y);
        return MochiJNI.longListToArray(result);
    }
}

The actual generated code uses @CEntryPoint (GraalVM annotation) for native image entry points, not traditional JNI JNIEXPORT signatures. This produces cleaner C headers.

Gate

Synthesise wrappers for [email protected] and [email protected]. Validate:

1. Generated Java source compiles with javac --release 21 without errors.
2. The wrapper JAR is produced under kotlin_wrap/okhttp/wrapper.jar.
3. reflect-config.json contains all data class types from the API surface.
4. resource-config.json is non-empty for ktor-client-core (which bundles resource files).
5. No javac warnings for unchecked casts (all casts are checked and annotated with @SuppressWarnings("unchecked")).