Calling Java Method from Objective C

PavlovOnline · Jun 14, 2015 6:38PM

I am trying to invoke a simple helloWorld java method from Objective C via JNI.

The JNI starts, but FindClass returns nil. No class is found, therefore, can't invoke the method.

Please suggest.

My setup:

Java class (located on desktop, compiled with javac)

public class HelloWorld { public static void main(String[] args) { System.out.println("Hello, World"); } }

Objective C

JNIEnv* jniEnv; JavaVM* javaVM; - (void)launch { JavaVMOption javaVMOptions[2]; javaVMOptions[0].optionString = "-Djava.awt.headless=true"; // directory path javaVMOptions[1].optionString = "-Djava.class.path=/Users/me/Desktop"; JavaVMInitArgs javaVMInitArgs; javaVMInitArgs.version = JNI_VERSION_1_6; javaVMInitArgs.options = javaVMOptions; javaVMInitArgs.nOptions = 1; javaVMInitArgs.ignoreUnrecognized = JNI_TRUE; int result = JNI_CreateJavaVM(&javaVM, (void**)&jniEnv, &javaVMInitArgs); NSLog(@"Java vm launched %i", result); invoke_class(jniEnv); } void invoke_class(JNIEnv* env) { jclass helloWorldClass; jmethodID mainMethod; jobjectArray applicationArgs; jstring applicationArg0; helloWorldClass = (*env)->FindClass(env, "HelloWorld"); mainMethod = (*env)->GetStaticMethodID(env, helloWorldClass, "main", "([Ljava/lang/String;)V"); applicationArgs = (*env)->NewObjectArray(env, 1, (*env)->FindClass(env, "java/lang/String"), NULL); applicationArg0 = (*env)->NewStringUTF(env, "From-C-program"); (*env)->SetObjectArrayElement(env, applicationArgs, 0, applicationArg0); (*env)->CallStaticVoidMethod(env, helloWorldClass, mainMethod, applicationArgs); }

jschellSomeoneStoleMyAlias · Jun 17, 2015 4:48PM

When it returns null it means an exception occurred. So get the exception and then look at what it says. JNI documentation has methods for getting that information.

And you should always check return types for java methods. Ignoring exceptions after they occur, which is what happens here, can result in an unstable VM.

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