Android Tutorial => How to call functions in a native library via...

Example

The Java Native Interface (JNI) allows you to call native functions from Java code, and vice versa. This example shows how to load and call a native function via JNI, it does not go into accessing Java methods and fields from native code using JNI functions.

Suppose you have a native library named libjniexample.so in the project/libs/<architecture> folder, and you want to call a function from the JNITestJava class inside the com.example.jniexample package.

In the JNITest class, declare the function like this:

public native int testJNIfunction(int a, int b);

In your native code, define the function like this:

#include <jni.h>

JNIEXPORT jint JNICALL Java_com_example_jniexample_JNITest_testJNIfunction(JNIEnv *pEnv, jobject thiz, jint a, jint b)
{
    return a + b;
}

The pEnv argument is a pointer to the JNI environment that you can pass to JNI functions to access methods and fields of Java objects and classes. The thiz pointer is a jobject reference to the Java object that the native method was called on (or the class if it is a static method).

In your Java code, in JNITest, load the library like this:

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

Note the lib at the start, and the .so at the end of the filename are omitted.

Call the native function from Java like this:

JNITest test = new JNITest();
int c = test.testJNIfunction(3, 4);

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Android Android Java Native Interface (JNI) How to call functions in a native library via the JNI interface

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