JNI GetObjectClass with primitive type

Question

I have the following JNI/JVMTI code:

jfieldID field = ...;
jobject fieldValue = (*jni_env)->GetObjectField(jni_env, jObjectInstance, field);
jclass fieldClass = (*jni_env)->GetObjectClass(jni_env, fieldValue);
char* signature_ptr;
char* generic_ptr;
(*jvmti_env)->GetClassSignature(jvmti_env, fieldClass, &signature_ptr, &generic_ptr);

This code works as long field is a reference type (java.lang.Object and subtypes). If field is a primitive type, it will crash the JVM when it tries to execute GetObjectClass.

My question is: How can I use field and fieldValue to figure out if it is a primitive type and if it's a primitive type, which one (int, long, boolean etc.)?

I am new to JNI/JVMTI so if the answer is obvious, please bear with me.

Thanks!

How did you obtain `jfieldID field`? JNI `GetFieldID` function requires field name and signature, so usually, when getting `jfieldID`, you already know the type of the field. — apangin, Jun 03 '20 at 19:18
There is a method in JVMTI GetClassFields which returns all the fields for a jclass. — spreiter301, Jun 03 '20 at 20:06

score 3 · Accepted Answer · answered Jun 03 '20 at 20:06

It's illegal to call GetObjectField with jfieldID which is not an object type. For primitive fields, you need to call one of the other Get[Type]Field functions, where [Type] is Int/Long/Boolean/etc.

Since you already have jvmti_env, the easiest way to find jfieldID type is to call JVM TI GetFieldName function. Among other things, it returns the field signature, i.e. the type of the field.

The only problem is that jfieldID should always be accompanied with jclass - the holder of the field, since field IDs are relative to the particular class. Typically, when you get jfieldID, you already know its holder class. If not, in your case you have jObjectInstance - an object that has this field, so the holder class can be derived from the instance using JNI GetObjectClass function.

jclass fieldHolder = (*jni_env)->GetObjectClass(jni_env, jObjectInstance);

char* name;
char* signature;
if ((*jvmti_env)->GetFieldName(jvmti_env, fieldHolder, field, &name, &signature, NULL) == 0) {

    switch (signature[0]) {
        case 'I':
            // int
            break;
        case 'J':
            // long
            break;
        ...
        case 'L':
            // object
            break;
        case '[':
            // array
            break;
    }

    (*jvmti_env)->Deallocate(jvmti_env, signature);
    (*jvmti_env)->Deallocate(jvmti_env, name);
}

Thank you! I saw and tried to use GetFieldName, but I misunderstood the documentation of the jclass parameter which says *The class of the field to query*. I thought it was the class of the jfieldID and not the class of the instance that holds the field. — spreiter301, Jun 04 '20 at 04:40

JNI GetObjectClass with primitive type

1 Answers1