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.

score 3 · Accepted Answer · answered May 13 '13 at 20:29

3

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 '13 at 20:29

Andy Thomas

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When do we need the angle brackets? I see some code like this `jmethodID m_mid = env -> GetMethodID(m_cls,"","()V"); ` – smwikipedia Dec 05 '14 at 15:11
1

@smwikipedia - You need them in the special name ``, which `GetMethodID()` supports to allow reference to constructors. *"To obtain the method ID of a constructor, supply as the method name and void (V) as the return type."* -- from the documentation for `GetMethodID()` in http://docs.oracle.com/javase/6/docs/technotes/guides/jni/spec/functions.html. – Andy Thomas Dec 05 '14 at 16:14

score 1 · Answer 2 · answered May 13 '13 at 20:28

1

Try this:

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

answered May 13 '13 at 20:28

Amir Afghani

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You can use one of the CallVoidMethod*() functions. – Andy Thomas May 13 '13 at 20:34
He may have used your earlier version without "[S". – Andy Thomas May 13 '13 at 20:37
Sorry I removed that comment, I made a mistake in my code. I was missing [S. Thanks for your answer. – Sabobin May 13 '13 at 20:38
Ah, yes I missed the parameters and only noticed it after you posted yours. – Amir Afghani May 13 '13 at 20:38

How to use JNI to call JAVA method from C

2 Answers2