JaCoP JNI: local refs from model conversion are never released (Invocation API) #283

Problem

The JaCoP solver embeds a JVM (JNI_CreateJavaVM in solvers/jacop/java.cc) and keeps JNIEnv for the process lifetime. Local references are not freed when C++ functions return. They live until DeleteLocalRef / PopLocalFrame / DestroyJavaVM.

Model conversion creates one local ref per Java IntVar / constraint and never deletes it. Cached jclass values from FindClass are also stored as members without NewGlobalRef.

Hot path

solvers/jacop/jacop.cc MPToJaCoPConverter::Convert:

for (int j = 0; j < num_vars; ++j) {
    jobject jvar = var_class_.NewObject(env_, store_, ...);
    vars_[j] = jvar;
    env_.SetObjectArrayElement(var_array_, j, jvar);
    // missing: DeleteLocalRef(jvar) after the array holds it,
    // or NewGlobalRef if vars_[j] must outlive the current frame
}

ClassBase::NewObject → Env::NewObjectV returns a fresh local ref on every call. The same pattern is used for constraints, NewObjectArray, and NewIntArray. Local-ref count scales with problem size.

SetObjectArrayElement / store.impose already keep the Java objects alive. The extra local refs are unnecessary.

Related issues in the JNI wrapper

solvers/jacop/java.h Class::DoInit:

class_ = env.FindClass(Info::name());  // local ref kept as a member

Init() runs once per Class<>, so this is a small fixed set, but it should still be a global ref. ClassBase::~ClassBase() does not release class_.

solvers/jacop/java.cc Env::NewObject(const char *class_name, ...) calls FindClass and never DeleteLocalRef on the jclass. Used a few times per solve (e.g. SimpleSelect), not in the variable loop.

The code already uses GlobalRef for search_ and obj_var, so the intended pattern exists; store_, vars_, and cached classes do not follow it.

Suggested fix

1. After SetObjectArrayElement / impose, DeleteLocalRef the temporary jobject (and temp arrays), or EnsureLocalCapacity / PushLocalFrame around Convert.
2. Promote long-lived refs (store_, vars_[i], cached class_) with NewGlobalRef; DeleteGlobalRef in the destructor.
3. In Env::NewObject(class_name, ...), DeleteLocalRef(cls) after NewObjectV.