Re: JNI generic type of jobject

Daniel Pitts <>
Tue, 04 Oct 2011 12:15:46 -0700
On 10/4/11 12:01 PM, Lew wrote:

Daniel Pitts wrote:

  Philipp Kraus wrote:

I use JNI calls for some Java classes. Some Java classes are generic
classes like:

class mytestclass<T> {

   public native void mymethod();


The stub shows:

JNIEXPORT void JNICALL Java_mytestclass_mymethod(JNIEnv* p_env, jobject

How can I get from the jobject which object type is the generic
parameter T? Because I would
like to create different codes if I do something like:

   mytestclass<int> x = new mytestclass<int>();

   mytestclass<String> x = new mytestclass<String>();

int is not a valid generic type parameter, as int is a primitive and
generic types must be Object types.

Also, generics are not the same as C++ templates. There isn't different
code created for each concrete usage. Its all exactly the same code.

If you are doing different behavior based on the compile time type, then
you need to do a little bit more work to implement the strategy pattern.

class MyTestClass<T> {
    private MyMethodStrategy<T> strategy;

    public mymethod() {

interface MyMethodStrategy<T> {
      void mymethod(MyTestClass<T> testClass);

class MyStringMethodStrategy implement MyMethodStrategy<String> {
     public native void mymethod(MyTestClass<String> testClass);

class MyIntegerMethodStrategy implement MyMethodStrategy<Integer> {
     public native void mymethod(MyTestClass<Integer> testClass);

Then you will have two different native methods to implement each strategy.

Kudos for a great answer!

This pattern or ones like it are frequent and very helpful in generics programming.

They also provide greater flexibility for pluggable (user-defined)
behavior. When applicable, I often replace protected methods with
strategy interfaces. Especially if any two protected methods are
independent of one another in the same class. Truth be told, I usually
don't create protected methods at all any more, unless they are in some
sort of adapter class for a "un-handled use-case" or some such.

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