JNI - How to callback from C++ or C to Java?

Question

I have Java application that invokes native C++/C code. The C++/C code needs to callback into Java. Could you give me some examples how to do this.

Answer 1

There are many valid ways to callback into Java from C/C++. I'm going to show you a technique using C (easy to adjust env for C++) that makes it fairly easy to pass data from native code to Java code. This example passes strings ( easy to modify for any data type ).

In native code, create the following:

// Globals static jmethodID midStr; static char * sigStr = "(Ljava/lang/String;ILjava/lang/String;)V";  // Init - One time to initialize the method id, (use an init() function) midStr = (*env)->GetMethodID(env, class, "javaDefineString", sigStr);  // Methods static void javaDefineString(JNIEnv * env, jobject o, char * name, jint index, char * value) {   jstring string = (*env)->NewStringUTF(env, name);   (*env)->CallVoidMethod(env, o, midStr, string, index, (*env)->NewStringUTF(env, value)); } 

In Java code create the following:

Map<String, String>  strings = new HashMap<String, String>();  // Never call this from Java void javaDefineString(String name, int index, String value) {   String key = name + "." + index;   strings.put(key, value); } 

Native usage to send data:

javaDefineString(env, o, "Greet", 0, "Hello from native code"); javaDefineString(env, o, "KeyTimeout", 0, "one second"); javaDefineString(env, o, "KeyTimeout", 1, "two second"); 

Java usage to receive data:

System.out.println(strings.get("Greet.0"); System.out.println(strings.get("KeyTimeout.0"); System.out.println(strings.get("KeyTimeout.1");