How to use JNI to call JAVA method from C

Question

I want to use JNI (Java Native Interface) to call a specific java setter method, passing a short[] buffer into it as a parameter.

Java method implimentation looks as follows:

public void setPcmLeft(short[] data) { pcm_l = data; }

From inside my C function how can I call this method using JNI.

My code currently looks like this:

void Java_com_companyName_lame_LameActivity_lameDecode(JNIEnv *env, jobject jobj)
{
    jclass class = (*env)->GetObjectClass(env, jobj);

    if (class != NULL) {

        jmethodID setLeftDatatID = (*env)->GetMethodID(env, class, "<setPcmLeft>", "void(V)");
        if (setLeftDatatID == NULL) {
            LOGD("(Lame) No method setLeftData");
        }  
    } 
}

When I run this, the setLeftDataID is allays NULL.

Note that the jobj parameter is my object being passed in that contains the setPcmLeft implementation.

Andy Thomas · Accepted Answer · 2013-05-13 20:29:44Z

4

In the call to GetMethodID(), the method name does not need angle brackets, and the signature needs to match the Java method.

jmethodID setLeftDatatID = (*env)->GetMethodID(env, class, "setPcmLeft", "([S)V");

In general, the signature is of the form ( arg-types ) ret-type, encoded as specified in the link below. The argument is a short[], encoded as [S. The return type is V for void.

More information is available in Chapter 3 of the Oracle JNI guide.

answered May 13, 2013 at 20:29

Andy Thomas

86.8k12 gold badges111 silver badges160 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

smwikipedia Over a year ago

When do we need the angle brackets? I see some code like this jmethodID m_mid = env -> GetMethodID(m_cls,"<init>","()V");

Andy Thomas Over a year ago

@smwikipedia - You need them in the special name <init>, which GetMethodID() supports to allow reference to constructors. "To obtain the method ID of a constructor, supply <init> as the method name and void (V) as the return type." -- from the documentation for GetMethodID() in docs.oracle.com/javase/6/docs/technotes/guides/jni/spec/….

Amir Afghani · Accepted Answer · 2013-05-13 20:28:55Z

1

Try this:

   jmethodID midCallBack = (*env)->GetMethodID(env, class, "setPcmLeft", "([S)V");

answered May 13, 2013 at 20:28

Amir Afghani

38.7k17 gold badges87 silver badges128 bronze badges

4 Comments

Andy Thomas Over a year ago

You can use one of the CallVoidMethod*() functions.

Andy Thomas Over a year ago

He may have used your earlier version without "[S".

Sabobin Over a year ago

Sorry I removed that comment, I made a mistake in my code. I was missing [S. Thanks for your answer.

Amir Afghani Over a year ago

Ah, yes I missed the parameters and only noticed it after you posted yours.

Collectives™ on Stack Overflow

How to use JNI to call JAVA method from C

2 Answers 2

2 Comments

4 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

4 Comments

Your Answer

Sign up or log in

Post as a guest

Related