JNI: Catching Init-Time Exceptions

Question

Okay, I'm all out of ideas on this one. Does anyone have any idea how I can hook into Java's exception pipeline in order to catch (and log to a text file) all exceptions that are occurring?

The situation is this: I have a library in a JAR file (A) which in turn depends on a second JAR file (B). A has no main class, as it's simply a class library, which I'm accessing and invoking through the JNI. The problem I'm having is this. When I attempt to initialise the JNI with A loaded, the JNI returns an unspecified error.

I strongly suspect that this error originates from an instantiation of Log4J's logger unit, which is occurring in static code (outside of a method) in B, which I believe is throwing an IOException as a result of permissions problems on the log file. I'm having issues finding out what's going on, however, as the exception (which I suspect is the cause of the problem) is being thrown during the linking stage (when A imports B) and so cannot be caught by a try-catch block. Also, since there is no main method there is no obvious place to put a try-catch block in order to catch this exception.

I would like some way of catching all exceptions that arise in either JAR and dumping them into a text file. I cannot (easily) modify B (I do not have the decompiled JAR). Any ideas?

Here is the C code which invokes the JNI with the specified libraries and options:

_DLL_EXPORT PyObject *initVM(PyObject *self, PyObject *args, PyObject *kwds)
{
    static char *kwnames[] = {
        "classpath", "initialheap", "maxheap", "maxstack",
        "vmargs", NULL
    };
    char *classpath = NULL;
    char *initialheap = NULL, *maxheap = NULL, *maxstack = NULL;
    char *vmargs = NULL;

    if (!PyArg_ParseTupleAndKeywords(args, kwds, "|zzzzz", kwnames,
                                     &classpath,
                                     &initialheap, &maxheap, &maxstack,
                                     &vmargs))
        return NULL;

    if (env->vm)
    {
        PyObject *module_cp = NULL;

        if (initialheap || maxheap || maxstack || vmargs)
        {
            PyErr_SetString(PyExc_ValueError,
                            "JVM is already running, options are ineffective");
            return NULL;
        }

        if (classpath == NULL && self != NULL)
        {
            module_cp = PyObject_GetAttrString(self, "CLASSPATH");
            if (module_cp != NULL)
                classpath = PyString_AsString(module_cp);
        }

        if (classpath && classpath[0])
            env->setClassPath(classpath);

        Py_XDECREF(module_cp);

        return getVMEnv(self);
    }
    else
    {
        JavaVMInitArgs vm_args;
        JavaVMOption vm_options[32];
        JNIEnv *vm_env;
        JavaVM *vm;
        unsigned int nOptions = 0;
        PyObject *module_cp = NULL;

        vm_args.version = JNI_VERSION_1_4;
        JNI_GetDefaultJavaVMInitArgs(&vm_args);

        if (classpath == NULL && self != NULL)
        {
            module_cp = PyObject_GetAttrString(self, "CLASSPATH");
            if (module_cp != NULL)
                classpath = PyString_AsString(module_cp);
        }

#ifdef _jcc_lib
        PyObject *jcc = PyImport_ImportModule("jcc");
        PyObject *cp = PyObject_GetAttrString(jcc, "CLASSPATH");

        if (classpath)
            add_paths("-Djava.class.path=", PyString_AsString(cp), classpath,
                      &vm_options[nOptions++]);
        else
            add_option("-Djava.class.path=", PyString_AsString(cp),
                       &vm_options[nOptions++]);

        Py_DECREF(cp);
        Py_DECREF(jcc);
#else
        if (classpath)
            add_option("-Djava.class.path=", classpath,
                       &vm_options[nOptions++]);
#endif

        Py_XDECREF(module_cp);

        if (initialheap)
            add_option("-Xms", initialheap, &vm_options[nOptions++]);
        if (maxheap)
            add_option("-Xmx", maxheap, &vm_options[nOptions++]);
        if (maxstack)
            add_option("-Xss", maxstack, &vm_options[nOptions++]);

        if (vmargs)
        {
#ifdef _MSC_VER
            char *buf = _strdup(vmargs);
#else
            char *buf = strdup(vmargs);
#endif
            char *sep = ",";
            char *option;

            for (option = strtok(buf, sep); option; option = strtok(NULL, sep))
            {
                if (nOptions < sizeof(vm_options) / sizeof(JavaVMOption))
                    add_option("", option, &vm_options[nOptions++]);
                else
                {
                    free(buf);
                    for (unsigned int i = 0; i < nOptions; i++)
                        delete vm_options[i].optionString;
                    PyErr_Format(PyExc_ValueError, "Too many options (> %d)",
                                 nOptions);
                    return NULL;
                }
            }
            free(buf);
        }

        //vm_options[nOptions++].optionString = "-verbose:gc";
        //vm_options[nOptions++].optionString = "-Xcheck:jni";

        vm_args.nOptions = nOptions;
        vm_args.ignoreUnrecognized = JNI_FALSE;
        vm_args.options = vm_options;
 if (JNI_CreateJavaVM(&vm, (void **) &vm_env, &vm_args) < 0)
        {
            for (unsigned int i = 0; i < nOptions; i++)
                delete vm_options[i].optionString;

            PyErr_Format(PyExc_ValueError,
                         "An error occurred while creating Java VM");
            return NULL;
        }

        env->set_vm(vm, vm_env);

        for (unsigned int i = 0; i < nOptions; i++)
            delete vm_options[i].optionString;

        t_jccenv *jccenv = (t_jccenv *) PY_TYPE(JCCEnv).tp_alloc(&PY_TYPE(JCCEnv), 0);
        jccenv->env = env;

#ifdef _jcc_lib
        registerNatives(vm_env);
#endif

        return (PyObject *) jccenv;
    }
}

Answer 1

Okay, so I've got the solution I was after. The solution is an update to the following segment of the code listed in the question:

    if (JNI_CreateJavaVM(&vm, (void **) &vm_env, &vm_args) < 0)
    {
        for (unsigned int i = 0; i < nOptions; i++)
            delete vm_options[i].optionString;

        PyErr_Format(PyExc_ValueError,
                     "An error occurred while creating Java VM");
        return NULL;
    }

The adaptation supports the construction of a more detailed error message which adds two specific pieces of information:

1. The error code (if any) which is returned by the JNI_CreateJavaVM method;
2. The detailed Java exception which occurs in the event that such an error code arises.

The above snippet from the original code was replaced with the following:

    vmInitSuccess = JNI_CreateJavaVM(&vm, (void **) &vm_env, &vm_args);
    if (vmInitSuccess < 0)
    {
        for (unsigned int i = 0; i < nOptions; i++)
            delete vm_options[i].optionString;

        //Set up basic error message
        sprintf(strVMInitSuccess, "%d", vmInitSuccess);
        strcpy(strVMError, "An error occurred while creating Java VM (No Exception): ");
        strcat(strVMError, strVMInitSuccess);

        //Get exception if there is one
        if((exc = vm_env->ExceptionOccurred()))
        {
            //Clear the exception since we have it now
            vm_env->ExceptionClear();
            //Get the getMessage() method
            if ((java_class = vm_env->FindClass ("java/lang/Throwable")))
            {
                if ((method = vm_env->GetMethodID(java_class, "getMessage", "()Ljava/lang/String;")))
                {
                    int size;
                    strExc = static_cast<jstring>(vm_env->CallObjectMethod(exc, method)); 
                    charExc = vm_env->GetStringUTFChars(strExc, NULL);
                    size = sizeof(strVMError) + sizeof(charExc);
                    char strVMException[size];
                    strcpy(strVMException, "An error occurred while creating Java VM (Exception): ");
                    strcat(strVMException, charExc);
                    PyErr_Format(PyExc_ValueError, strVMException);
                    return NULL;
                }
            }
        }
        PyErr_Format(PyExc_ValueError, strVMError);
        return NULL;
    }

Thanks to @Parsifal for help with this solution.