Java Language Java Native Interface Calling Java methods from C++ (callback)
Example
Calling a Java method from native code is a two-step process :
- obtain a method pointer with the
GetMethodIDJNI function, using the method name and descriptor ; - call one of the
Call*Methodfunctions listed here.
Java code
/*** com.example.jni.JNIJavaCallback.java ***/
package com.example.jni;
public class JNIJavaCallback {
static {
System.loadLibrary("libJNI_CPP");
}
public static void main(String[] args) {
new JNIJavaCallback().callback();
}
public native void callback();
public static void printNum(int i) {
System.out.println("Got int from C++: " + i);
}
public void printFloat(float i) {
System.out.println("Got float from C++: " + i);
}
}
C++ code
// com_example_jni_JNICppCallback.cpp
#include <iostream>
#include "com_example_jni_JNIJavaCallback.h"
using namespace std;
JNIEXPORT void JNICALL Java_com_example_jni_JNIJavaCallback_callback(JNIEnv *env, jobject jthis) {
jclass thisClass = env->GetObjectClass(jthis);
jmethodID printFloat = env->GetMethodID(thisClass, "printFloat", "(F)V");
if (NULL == printFloat)
return;
env->CallVoidMethod(jthis, printFloat, 5.221);
jmethodID staticPrintInt = env->GetStaticMethodID(thisClass, "printNum", "(I)V");
if (NULL == staticPrintInt)
return;
env->CallVoidMethod(jthis, staticPrintInt, 17);
}
Output
Got float from C++: 5.221
Got int from C++: 17
Getting the descriptor
Descriptors (or internal type signatures) are obtained using the javap program on the compiled .class file. Here is the output of javap -p -s com.example.jni.JNIJavaCallback :
Compiled from "JNIJavaCallback.java"
public class com.example.jni.JNIJavaCallback {
static {};
descriptor: ()V
public com.example.jni.JNIJavaCallback();
descriptor: ()V
public static void main(java.lang.String[]);
descriptor: ([Ljava/lang/String;)V
public native void callback();
descriptor: ()V
public static void printNum(int);
descriptor: (I)V // <---- Needed
public void printFloat(float);
descriptor: (F)V // <---- Needed
}
