How to pass java function pointer to c++ native code

Question

I want to call a native c++ method from java (android) code, and pass a java function as a parameter, so I will be able to save the function pointer in the c++ code, and activate it from the native code.

I chose to implement the function pointer in java using anonymous class, and I call the native function from java as following:

interface FunctionPtrHelper {
bool function(String param);

}

NativeFunc(param1,param2,new FunctionPtrHelper() {
public bool myFunction(String param) {
    //body of my function
}});

How can I make swig/jni know the 3rd param (which is actually a class) and translate it to a function pointer in c++ (that will contain'myFunction') ?
In case it is not possible, is there another way to pass a function pointer from java to c++?

I'm not sure JNI works that way. You're calling a native method with jobject as parameter. On that object you can call a method as usual, knowing its name and signature. — Violet Giraffe
– Violet Giraffe, Commented Nov 20, 2012 at 15:35
It's also possible to create easily callbacks with JavaCPP. Let me know if you want need more details! — Samuel Audet
– Samuel Audet, Commented Dec 1, 2012 at 5:13

japreiss · Accepted Answer · 2012-11-20 15:49:31Z

5

You can write a C++ interface and SWIG it as a "director" class. Then you can implement the interface in Java. Instantiate the implementation object in Java and pass it into a C++ method that takes a pointer or reference to the interface, and C++ will be able to call back into your Java class. For example:

// SWIGed C++
class IStringToBool
{
public:
    virtual bool call(std::string s) = 0;
}

class IStringToBoolUser
{
public:
    void setFunction(IStringToBool &function);
}

And then:

// Java
public class MyFunction implements IStringToBool {
    public bool call(String s) {
        // do something
        return true;
    }
}

Documentation: Cross language polymorphism using directors

answered Nov 20, 2012 at 15:49

japreiss

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user1680257 · Accepted Answer · 2012-11-20 15:36:43Z

1

With JNI you could pass a object to C++, then you could take function pointer from the C++ Object.

answered Nov 20, 2012 at 15:36

user1680257

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1 Comment

Flexo - Save the data dump Over a year ago

The problem with this is that JNI function pointer (e.g. CallMethodVoid etc.) almost certainly doesn't match the function pointer you're looking for in C++. You need to pass in at least JNIEnv, the jobject and jmethodID.

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