make native code access java methods and data members

Question

I am working on an android project with native code that is supposed to update a List and some other boolean variables in an object.

consider the following code

some java class in my code looks like:

 class ReturnObject
{
    boolean a, b;
    public List<String[]> listA;

}

public class foo
{
    public native void someFunction(ReturnObject returnObject);
}

and the native code looks like:

JNIEXPORT void JNICALL Java_com_example_androidtest_TestActivity_someFunction
  (JNIEnv * env, jobject jObj, jobject returnObject) {
    std::string f = "foo";
    // missing code here
    // returnObject.add(f) // add like in java
}

How can I set the values of the boolean variables?

and for the the List (knowing my data in the native code are stored as std::string), how can I invoke add(String[] string) method of the List class from native code?

Answer 1

Here is the sample code to set the boolean values, follow the same guideline to call the add method of the list.

#include <stdio.h>
#include <jni.h>
#include "com_example_Foo.h"

JNIEXPORT void JNICALL Java_com_example_Foo_someFunction
(JNIEnv *env, jobject object, jobject returnObject)
{
    jclass clzReturnObject = env->FindClass("com/example/ReturnObject");
    jfieldID fieldA = env->GetFieldID(clzReturnObject, "a", "Z");
    jfieldID fieldB = env->GetFieldID(clzReturnObject, "b", "Z");

    env->SetBooleanField(returnObject, fieldA, true);
    if (env->ExceptionCheck()) {
        fprintf(stderr, "error set boolean for a");
        env->ExceptionDescribe();
    }
    env->SetBooleanField(returnObject, fieldB, false);
    if (env->ExceptionCheck()) {
        fprintf(stderr, "error set boolean for b");
        env->ExceptionDescribe();
    }

    jfieldID fieldListID = env->GetFieldID(clzReturnObject, "list",
            "Ljava/util/List;");
    jobject listObject = env->GetObjectField(returnObject, fieldListID);

    jclass clzList = env->FindClass("java/util/List");
    jmethodID addMethodID = env->GetMethodID(clzList, "add",
            "(Ljava/lang/Object;)Z");

    jclass clzString = env->FindClass("java/lang/String");

    jstring initElement = env->NewStringUTF("0000");

    jobjectArray toAdd = env->NewObjectArray(10, clzString, initElement);

    if (env->ExceptionCheck()) {
        fprintf(stderr, "error create string array");
        env->ExceptionDescribe();
    }

    env->CallBooleanMethod(listObject, addMethodID, toAdd);
    env->DeleteLocalRef(initElement);
}

Following is the java code

public class Foo {

static {
    System.loadLibrary("Foo");
}

public static void main(String[] args) {

    ReturnObject returnObject = new ReturnObject();
    returnObject.list = new ArrayList<>();

    Foo foo = new Foo();
    foo.someFunction(returnObject);
    System.out.println("size is " + returnObject.list.size());

    foo.someFunction(returnObject);
    System.out.println("size is " + returnObject.list.size());

    foo.someFunction(returnObject);
    System.out.println("size is " + returnObject.list.size());

    // we have three element added from JNI
    for (String string : returnObject.list.get(2)) {
        System.out.println(string);
    }
}

public native void someFunction(ReturnObject returnObject);
}